# Boeing Alert Requirements Bulletin 737-53A1421 RB: No revision named

- Rights: © The Boeing Company. Filed by the FAA in regulations.gov docket FAA-2026-3868, the docket of a proposed AD (Federal Register 2026-08154). Not covered by OpenAirDocs’ CC0 dedication.
- Document: [Boeing Alert Requirements Bulletin 737-53A1421 RB](https://openairdocs.org/service-documents/boeing/737-53a1421-rb.md)
- Title as filed: 737-53A1421 RB
- Pages: 35
- PDF: https://openairdocs.org/files/sha256/97/b4/97b4d8a7a49440a30db5715d61c9282ed2f3fba40db1cb2258e199e528d73bf7
- Page: https://openairdocs.org/service-documents/boeing/737-53a1421-rb/file/97b4d8a7a494

## Text

Some pages are OCR from a scan. OCR can misread letters and numbers. Each page's text from the PDF; open the PDF at a page with #page=N.

### Page 1

Commercial 
Airplanes
737
Requirements Bulletin
ALERT
Number: 737-53A1421 RB
Original Issue: March 20, 2025
ATA System: 5331
SUBJECT: FUSELAGE - Skin - Skin and Structure Common to the Aft Drain Mast - Inspection
EXPORT CONTROLLED - This technology or software is subject to the U.S. Export Administration Regulations
(EAR), (15 C.F.R. Parts 730-774). No authorization from the U.S. Department of Commerce is required for
export, re-export, in-country transfer, or access EXCEPT to country group E:1 or E:2 countries/persons per
Supp.1 to Part 740 of the EAR.
9E991ECCN:
Copyright © 2025 The Boeing Company
Treatment of this document and its content is governed by contract with Boeing. Rights to use this document
are licensed on a per aircraft basis and such rights may not be transferable. For more information, contact The
Boeing Company.
Boeing, the Boeing signature, the Boeing symbol, 707, 717, 727, 737, 747, 757, 767, 777, 787, Dreamliner,
BBJ, DC-8, DC-9, DC-10, KC-10, KC-46, KDC-10, MD-10, MD-11, MD-80, MD-88, MD-90, P-8, Poseidon and
Boeing liveries are all trademarks owned by The Boeing Company; and no trademark license is granted in
connection with this document unless provided in writing by Boeing.
Original Issue: March 20, 2025
737-53A1421 RB
1 of 35
4.0

### Page 2

Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
2 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT
BLANK PAGE

### Page 3

Table of Contents
1. EFFECTIVITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
A. Airplanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
B. Spares Affected . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2. CONCURRENT REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3. COMPLIANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4. APPROVAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5. ACCOMPLISHMENT INSTRUCTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
A. General Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
B. Work Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
FIGURE 1: FUSELAGE SKIN COMMON TO THE AFT DRAIN MAST - EXTERNAL GENERAL VISUAL
INSPECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
FIGURE 2: STRUCTURE COMMON TO THE AFT DRAIN MAST - INTERNAL DETAILED INSPECTION
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
FIGURE 3: FUSELAGE SKIN COMMON TO THE AFT DRAIN MAST - EXTERNAL DETAILED INSPECTION
AND EXTERNAL SURFACE HIGH FREQUENCY EDDY CURRENT INSPECTION . . . . 33
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
3 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 4

Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
4 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT
BLANK PAGE

### Page 5

Commercial 
Airplanes
737
Requirements Bulletin
ALERT
Number: 737-53A1421 RB
Original Issue: March 20, 2025
ATA System: 5331
SUBJECT: FUSELAGE - Skin - Skin and Structure Common to the Aft Drain Mast - Inspection
THIS DOCUMENT IS SENT TO THE OPERATORS OF RECORD OF THE AIRPLANES SHOWN IN
EFFECTIVITY. IF AN AIRPLANE HAS BEEN LEASED OR SOLD, SEND THIS DOCUMENT TO THE NEW
OPERATOR. IF APPLICABLE SPARES HAVE BEEN SOLD, SEND THIS DOCUMENT TO THE NEW OWNER.
THIS IS AN ENHANCED REQUIRED FOR COMPLIANCE DOCUMENT. THE BOEING PROCESSES USED
TO CREATE THE SERVICE BULLETIN (SB) AND THE REQUIREMENTS BULLETIN (RB) ENSURE THAT
THE INFORMATION BETWEEN "RC START" AND "RC END" IN THE SB ARE IDENTICAL TO THE
INFORMATION IN THE SAME SECTIONS (COMPLIANCE, WORK INSTRUCTIONS…) OF THE RB WHICH
THE SB REFERS TO AS THE RELATED DOCUMENT.
1. Effectivity
A. Airplanes
This bulletin is applicable to 737-600, 737-700, 737-700C, 737-800, 737-900, 737-900ER
Airplane(s), line number(s) 1-2598, 2600-2721, 2723-2813, 2815-2930, 2932-3068, 3070-3323,
3325-3425, 3427-3521, 3523-3611, 3613-3701, 3703-3791, 3793-3878, 3880-3915, 3917-3968,
3970-4013, 4015-4054, 4056-4098, 4100-4140, 4142-4180, 4182-4218, 4220-4255, 4257-4293,
4295-4330, 4332-4365, 4367-4398, 4400-4444, 4446-4480, 4482-4546, 4548-4608, 4610-4672,
4674-4727, 4729-4767, 4769-4812, 4814-4858, 4860-4897, 4899-4937, 4939-4977, 4979-5019,
5021-5057, 5059-5097, 5099-5137, 5139-5177, 5179-5217, 5219-5261, 5263-5297, 5299-5337,
5339-5367, 5369-5397, 5399-5427, 5429-5457, 5459-5477, 5479-5509, 5511-5541, 5543-5566,
5568-5586, 5588-5601, 5603-5622, 5624-5655, 5657-5667, 5669-5679, 5681-5711, 5713-5727,
5729-5739, 5741-5764, 5766-5787, 5789-5795, 5797-5822, 5824-5850, 5853-5879, 5881-5901,
5904-5935, 5937-5947, 5949-5962, 5964-5991, 5994-6019, 6021-6033, 6035-6047, 6049-6073,
6075, 6077-6103, 6105-6113, 6115-6131, 6133-6153, 6155-6159, 6161-6187, 6190-6215,
6217-6221, 6223-6243, 6245-6249, 6251-6271, 6274-6289, 6291-6299, 6301-6307, 6309-6325,
6327, 6329-6343, 6345-6355, 6357-6359, 6361-6371, 6373-6383, 6386-6395, 6397-6407,
6409-6411, 6413-6419, 6421-6429, 6431-6439, 6442-6447, 6449-6455, 6457-6463, 6465-6467,
6469-6471, 6473-6480, 6482-6489, 6491, 6493-6498, 6500-6507, 6509-6514, 6516-6520,
6522-6527, 6529-6531, 6533-6534, 6536-6541, 6543-6547, 6549-6554, 6556-6559, 6561,
6563-6568, 6570-6574, 6576-6581, 6583-6588, 6590-6593, 6596-6597, 6599-6602, 6604-6606,
6608-6611, 6613-6615, 6617-6620, 6622, 6624, 6626-6629, 6631-6633, 6635-6638, 6640-6642,
6644-6647, 6649-6651, 6653-6656, 6659-6660, 6662-6665, 6667-6671, 6673-6676, 6678-6680,
6682-6685, 6688-6689, 6691-6694, 6696-6698, 6700-6703, 6705-6707, 6709-6712, 6714-6716,
6718-6719, 6721, 6723-6725, 6727-6730, 6732-6734, 6736-6739, 6741-6743, 6745-6748,
6751-6752, 6754-6757, 6759-6761, 6763-6766, 6768-6770, 6772-6775, 6777-6779, 6781-6782,
6786-6788, 6790-6791, 6793, 6795, 6797, 6799-6802, 6804, 6806, 6808-6809, 6811, 6813, 6815,
6817-6818, 6820, 6822-6824, 6826-6827, 6829, 6831-6833, 6835-6836, 6838, 6840, 6842,
6844-6845, 6847, 6849, 6851, 6853-6854, 6856-6858, 6860, 6862-6863, 6865, 6867, 6869-6871,
6873-6874, 6878-6880, 6882-6883, 6885, 6887-6889, 6891-6892, 6894, 6896, 6898, 6900-6903,
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
5 of 35

### Page 6

6905, 6907, 6910, 6912-6914, 6916, 6918-6919, 6921, 6923-6925, 6927-6928, 6930, 6932-6934,
6936-6937, 6939, 6943, 6945-6948, 6950, 6952, 6954-6955, 6957-6959, 6961-6962, 6964,
6966-6967, 6969, 6971-6972, 6977, 6979, 6981-6982, 6984, 6986-6987, 6989, 6992, 6994,
6996-6997, 6999, 7001-7002, 7007, 7009, 7011-7012, 7014, 7016-7017, 7019, 7021-7022, 7024,
7026, 7029, 7031-7032, 7034, 7036-7037, 7039, 7041, 7046-7047, 7049, 7051-7052, 7054,
7056-7057, 7059, 7061, 7063, 7066-7067, 7069, 7071-7072, 7076, 7078, 7081-7082, 7084,
7086-7087, 7089, 7091-7092, 7094, 7097, 7099, 7101-7102, 7104, 7106-7107, 7109, 7112,
7116-7117, 7119, 7121-7122, 7124, 7126-7127, 7129, 7131, 7134, 7136-7137, 7139, 7141-7142,
7146, 7149, 7151-7152, 7154, 7156-7157, 7159, 7161-7162, 7164, 7167, 7169, 7171-7172, 7174,
7176-7177, 7179, 7182, 7187, 7189, 7191, 7194, 7197, 7199, 7202, 7204, 7206, 7209, 7212,
7214, 7217, 7221, 7224, 7227, 7229, 7232, 7234-7235, 7239, 7242, 7244, 7247, 7249, 7251,
7257, 7259, 7262, 7264, 7266, 7269, 7272, 7274, 7277, 7279-7280, 7284, 7287, 7292, 7294,
7297, 7299, 7302, 7304, 7307, 7309, 7312, 7314, 7317, 7319, 7322, 7327, 7329, 7332, 7334,
7337, 7339, 7342, 7344, 7347, 7349, 7352, 7354, 7357, 7362, 7364, 7367, 7369, 7372, 7374,
7377, 7379, 7382, 7387, 7397, 7402, 7407, 7412, 7417, 7422, 7432, 7437, 7442, 7447, 7452,
7457, 7467, 7472, 7477, 7482, 7487, 7492, 7502, 7507, 7512, 7517, 7522, 7527, 7537, 7547,
7552, 7557, 7562, 7572, 7577, 7582, 7587, 7592, 7597, 8592, 9103, 9274, 9430-99999 in 1
Group(s). Where the effectivity is presented with hyphens between line numbers, the airplane
applicability means "through" and "inclusive", e.g. line numbers 1-9 means line numbers 1 through
9 inclusive.
NOTE: The Variable Number table below includes affected airplanes available as of the issue
date of this bulletin. Affected airplane Variable Numbers added to MyBoeingFleet.com
after the issue date of this bulletin will be shown using the Maintenance Performance
Toolbox EFFECTIVITY search function result.
DESCRIPTIONCONFIGURATIONGROUP
All 737-600/-700/-700C/-800/-900/-900ER Airplanes-1
Airplane Models:
737-600, 737-700, 737-700C, 737-800, 737-900, 737-900ER
GroupVariable Number
1YA001 - YA099
1YA101 - YA199
1YA201 - YA211
1YA221
1YA231 - YA242
1YA251 - YA256
1YA271 - YA272
1YA291
1YA301 - YA302
GroupVariable Number
1YA311 - YA314
1YA321 - YA323
1YA336 - YA345
1YA351
1YA356 - YA357
1YA366 - YA367
1YA371 - YA377
1YA501 - YA536
1YA541 - YA552
GroupVariable Number
1YA571 - YA578
1YA601 - YA615
1YA621 - YA622
1YA626 - YA631
1YA635 - YA636
1YA641 - YA642
1YA645 - YA650
1YA656 - YA659
1YA666 - YA667
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
6 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 7

GroupVariable Number
1YA671 - YA672
1YA681 - YA691
1YA701 - YA710
1YA721 - YA722
1YA731 - YA734
1YA751 - YA756
1YA801 - YA803
1YA809
1YA811 - YA814
1YA831 - YA835
1YA841 - YA862
1YA881 - YA882
1YA891 - YA892
1YA961 - YA978
1YB001 - YB006
1YB101 - YB132
1YB151 - YB153
1YB156 - YB158
1YB161 - YB164
1YB171 - YB172
1YB181 - YB184
1YB201 - YB208
1YB271
1YB276
1YB301 - YB310
1YB371 - YB392
1YB501 - YB502
1YB521 - YB526
1YB541 - YB544
1YB551
1YB561 - YB598
1YB601 - YB636
1YB651
GroupVariable Number
1YB656
1YB671 - YB672
1YB851 - YB863
1YB871 - YB874
1YB881 - YB893
1YB901 - YB903
1YB911
1YB961 - YB975
1YB981
1YB986 - YB998
1YC001 - YC030
1YC051 - YC084
1YC091 - YC095
1YC101 - YC104
1YC111 - YC113
1YC121
1YC126 - YC127
1YC136 - YC141
1YC146 - YC147
1YC151 - YC156
1YC166 - YC171
1YC176 - YC179
1YC186 - YC190
1YC201 - YC203
1YC206 - YC207
1YC301 - YC305
1YC321 - YC387
1YC391 - YC394
1YC396
1YC401 - YC417
1YC421 - YC422
1YC426 - YC428
1YC436 - YC439
GroupVariable Number
1YC441 - YC448
1YC451 - YC455
1YC459
1YC461 - YC467
1YC471 - YC499
1YC501 - YC526
1YC571 - YC583
1YC587 - YC589
1YC591 - YC593
1YC601 - YC655
1YC681 - YC696
1YC701 - YC715
1YC720 - YC725
1YC727 - YC752
1YC761 - YC770
1YC781 - YC793
1YC801 - YC892
1YC901 - YC907
1YC921 - YC922
1YC931 - YC932
1YC941 - YC951
1YC971 - YC978
1YC981 - YC983
1YD001 - YD007
1YD021 - YD025
1YD041 - YD057
1YD081 - YD084
1YD101 - YD117
1YD121 - YD126
1YD151 - YD159
1YD171 - YD172
1YD201 - YD203
1YD206 - YD209
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
7 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 8

GroupVariable Number
1YD216 - YD219
1YD251 - YD254
1YD256 - YD257
1YD261
1YD301 - YD334
1YD391
1YD401 - YD410
1YD412 - YD424
1YD481 - YD485
1YD491 - YD499
1YD501 - YD512
1YD531 - YD535
1YD541 - YD547
1YD561 - YD564
1YD571 - YD572
1YD591 - YD595
1YD601 - YD612
1YD651 - YD655
1YE001 - YE020
1YE051
1YE101 - YE109
1YE151 - YE157
1YE171 - YE172
1YE201 - YE206
1YE301 - YE305
1YE321 - YE326
1YE371 - YE383
1YF001 - YF011
1YF021 - YF049
1YF051 - YF080
1YF086 - YF087
1YF106 - YF107
1YF111 - YF114
GroupVariable Number
1YF116 - YF119
1YF121 - YF132
1YF176 - YF178
1YF191 - YF192
1YF201 - YF276
1YF401 - YF402
1YF431 - YF432
1YF436 - YF437
1YF441 - YF442
1YF451 - YF460
1YF501 - YF599
1YF601 - YF685
1YF691 - YF695
1YF701 - YF707
1YF801 - YF805
1YF831 - YF832
1YF836 - YF838
1YF871 - YF872
1YF901 - YF905
1YF921 - YF928
1YF951 - YF954
1YG001 - YG044
1YG061 - YG064
1YG066 - YG089
1YG091 - YG099
1YG101 - YG104
1YG111 - YG114
1YG116 - YG149
1YG151 - YG152
1YG201 - YG205
1YG211 - YG214
1YG221 - YG224
1YG251 - YG252
GroupVariable Number
1YG501 - YG517
1YG601 - YG602
1YG701
1YG711
1YG721 - YG722
1YH001 - YH012
1YH031 - YH055
1YH071 - YH080
1YH101 - YH132
1YH201 - YH202
1YH206 - YH207
1YH301 - YH307
1YH321 - YH322
1YH501 - YH542
1YH551 - YH594
1YH601 - YH606
1YH611 - YH620
1YH631 - YH632
1YH636 - YH637
1YH641
1YH681 - YH686
1YH701 - YH755
1YH801 - YH899
1YH901 - YH908
1YH911 - YH915
1YH951 - YH952
1YJ001 - YJ013
1YJ021
1YJ471 - YJ480
1YJ501 - YJ517
1YJ531 - YJ579
1YJ591 - YJ599
1YJ631 - YJ632
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
8 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 9

GroupVariable Number
1YJ671 - YJ694
1YJ801 - YJ867
1YJ871 - YJ880
1YJ901 - YJ904
1YJ908 - YJ929
1YJ931 - YJ937
1YJ941 - YJ956
1YJ961
1YJ976 - YJ977
1YK001 - YK007
1YK101 - YK104
1YK111 - YK112
1YK121 - YK122
1YK131 - YK133
1YK136 - YK139
1YK141 - YK142
1YK146 - YK148
1YK151 - YK154
1YK161 - YK167
1YK171
1YK176 - YK177
1YK186
1YK191
1YK196
1YK201 - YK202
1YK301 - YK314
1YK321 - YK340
1YK361 - YK378
1YK401 - YK406
1YK426 - YK427
1YK431 - YK450
1YK456 - YK467
1YK471 - YK473
GroupVariable Number
1YK476
1YK478
1YK480 - YK487
1YK491 - YK493
1YK495 - YK499
1YK511 - YK516
1YK521 - YK530
1YK551 - YK571
1YK576 - YK577
1YK580 - YK585
1YK601 - YK602
1YK606 - YK607
1YK611 - YK616
1YK621 - YK634
1YK641 - YK644
1YK651 - YK665
1YK671 - YK672
1YK676 - YK677
1YK681 - YK686
1YK691
1YK695 - YK699
1YK701 - YK713
1YK716 - YK719
1YK721 - YK741
1YK751 - YK770
1YK776 - YK778
1YK781 - YK785
1YK791 - YK792
1YK796 - YK797
1YK801 - YK899
1YK901 - YK912
1YK918 - YK919
1YK921 - YK929
GroupVariable Number
1YK941 - YK999
1YL001 - YL005
1YL011 - YL016
1YL021 - YL024
1YL051 - YL052
1YL056 - YL057
1YL061
1YL066 - YL069
1YL076 - YL077
1YL101 - YL137
1YL201 - YL237
1YL241 - YL242
1YL271 - YL272
1YL281 - YL284
1YL301 - YL303
1YL311 - YL314
1YL316 - YL321
1YL351 - YL353
1YL371 - YL374
1YL401
1YL421 - YL429
1YL431 - YL444
1YL461 - YL478
1YL501 - YL509
1YL531 - YL534
1YL541 - YL551
1YL561 - YL579
1YL591 - YL598
1YL601 - YL606
1YL611 - YL612
1YL616 - YL617
1YL621
1YL626
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
9 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 10

GroupVariable Number
1YL631 - YL634
1YL636 - YL637
1YL661 - YL664
1YL676 - YL691
1YL696 - YL697
1YL701 - YL707
1YL731
1YL751 - YL752
1YL756 - YL757
1YL761 - YL780
1YL796 - YL797
1YL801 - YL815
1YL901 - YL910
1YL921 - YL940
1YL951 - YL968
1YM101 - YM103
1YM201 - YM244
1YM251 - YM299
1YM301 - YM399
1YM401 - YM415
1YM471
1YM481 - YM484
1YM501 - YM515
1YM521 - YM522
1YM541
1YM551 - YM553
1YM571 - YM572
1YM581 - YM582
1YM591 - YM599
1YM631 - YM634
1YM641 - YM652
1YM671 - YM674
1YM691 - YM699
GroupVariable Number
1YM701 - YM710
1YM761 - YM765
1YM771 - YM779
1YN001 - YN016
1YN021 - YN024
1YN061
1YN071 - YN082
1YN091 - YN092
1YN101 - YN112
1YN121
1YN126
1YN201 - YN202
1YN211 - YN219
1YN231 - YN236
1YN261
1YN501
1YN531 - YN534
1YN551 - YN566
1YN581 - YN585
1YN591 - YN593
1YN601 - YN606
1YN651 - YN653
1YN661 - YN663
1YN666 - YN667
1YN671 - YN674
1YN701 - YN702
1YN741 - YN746
1YN756 - YN758
1YQ001 - YQ003
1YQ011 - YQ021
1YQ026 - YQ029
1YQ036 - YQ045
1YQ201 - YQ207
GroupVariable Number
1YQ231
1YQ241 - YQ251
1YQ281 - YQ284
1YQ291 - YQ296
1YQ301 - YQ302
1YQ321 - YQ324
1YQ331 - YQ334
1YQ346 - YQ347
1YQ351 - YQ367
1YQ371
1YQ401 - YQ403
1YQ451 - YQ454
1YQ456
1YQ461
1YQ471
1YQ501 - YQ520
1YQ601 - YQ606
1YQ610 - YQ614
1YR001 - YR098
1YR101 - YR105
1YR111 - YR124
1YR126 - YR144
1YR166 - YR182
1YR186 - YR198
1YR201 - YR215
1YR231 - YR294
1YR296 - YR297
1YR301 - YR334
1YR338 - YR341
1YR361 - YR366
1YR371 - YR373
1YR376 - YR377
1YR381 - YR382
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
10 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 11

GroupVariable Number
1YR386 - YR388
1YR391
1YR396 - YR399
1YR401 - YR405
1YR411 - YR413
1YR416 - YR418
1YR421 - YR424
1YR426 - YR455
1YR457 - YR499
1YR501 - YR506
1YR511 - YR520
1YR526 - YR544
1YR546 - YR551
1YR556 - YR562
1YR571 - YR582
1YR591 - YR592
1YR601 - YR605
1YR616 - YR658
1YR661 - YR663
1YR671 - YR673
1YR676 - YR678
1YR681 - YR682
1YR701 - YR769
1YR771 - YR799
1YR801 - YR827
1YR831 - YR838
1YR841 - YR899
1YR901 - YR903
1YR931 - YR934
1YR938 - YR944
1YR951 - YR998
1YS001 - YS002
1YS006 - YS009
GroupVariable Number
1YS031 - YS032
1YS051 - YS073
1YS101 - YS120
1YS126 - YS142
1YS146 - YS160
1YS166 - YS194
1YS196 - YS197
1YS201 - YS209
1YS221 - YS225
1YS231 - YS236
1YS271 - YS288
1YS295 - YS296
1YS301 - YS348
1YS351 - YS378
1YS381 - YS386
1YS393 - YS395
1YS401
1YS451
1YS456 - YS457
1YS462 - YS465
1YS467
1YS501 - YS502
1YS506
1YS511 - YS518
1YS531
1YS536 - YS538
1YS541 - YS542
1YS546
1YS551 - YS556
1YS561 - YS578
1YS581 - YS595
1YS601 - YS690
1YS701 - YS716
GroupVariable Number
1YS721 - YS762
1YS771 - YS780
1YS806 - YS810
1YS816 - YS818
1YS823 - YS825
1YS851 - YS863
1YS876
1YS881 - YS884
1YS891 - YS894
1YS896 - YS897
1YS901 - YS909
1YS911 - YS957
1YS961 - YS974
1YS976 - YS977
1YS981 - YS987
1YS991 - YS998
1YT001 - YT004
1YT006
1YT026 - YT027
1YT031 - YT032
1YT036 - YT038
1YT061 - YT068
1YT101 - YT138
1YT176 - YT185
1YT191
1YT196 - YT199
1YT201
1YT206 - YT207
1YT211 - YT213
1YT251 - YT299
1YT301 - YT307
1YT311 - YT312
1YT321 - YT355
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
11 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 12

GroupVariable Number
1YT371 - YT372
1YT381
1YT391 - YT396
1YT401 - YT403
1YT411 - YT417
1YT427
1YT461 - YT477
1YT491 - YT492
1YT501 - YT521
1YT551 - YT552
1YT556 - YT563
1YT565
1YT571 - YT578
1YT601 - YT606
1YT701 - YT708
1YT723 - YT726
1YT761 - YT781
1YT791 - YT794
1YT796 - YT797
1YT801 - YT804
1YT811 - YT813
1YT826 - YT827
1YT831 - YT832
1YT841 - YT852
1YT856 - YT863
1YT885 - YT887
1YT891 - YT895
1YT901 - YT998
1YU001 - YU060
1YU201 - YU218
1YU301 - YU329
1YU901 - YU931
1YV001 - YV012
GroupVariable Number
1YV021 - YV023
1YV030 - YV034
1YV037 - YV042
1YV045 - YV047
1YV051 - YV059
1YV081
1YV083 - YV084
1YV097 - YV099
1YV101 - YV154
1YV201 - YV203
1YV251 - YV252
1YV256 - YV257
1YV301 - YV312
1YV351 - YV368
1YV381 - YV388
1YV391 - YV394
1YV401
1YV411 - YV413
1YV431 - YV433
1YV441 - YV454
1YV461 - YV464
1YV467 - YV470
1YV501 - YV512
1YV521
1YV571 - YV586
1YV596
1YV601 - YV605
1YV651 - YV674
1YV676
1YV695 - YV696
1YV701 - YV702
1YV711
1YV722 - YV731
GroupVariable Number
1YV741 - YV794
1YV801 - YV820
1YV831 - YV848
1YV881 - YV883
1YV901 - YV915
1YV923 - YV926
1YV971 - YV983
1YW001
1YW011 - YW099
1YW101 - YW164
1YW201 - YW212
1YW226 - YW240
1YW245 - YW261
1YW263
1YW267 - YW281
1YW283 - YW285
1YW292 - YW299
1YW351
1YW356 - YW358
1YW361 - YW366
1YW371 - YW399
1YW401 - YW430
1YW441 - YW445
1YW451 - YW452
1YW471 - YW472
1YW491 - YW498
1YW531 - YW536
1YW541 - YW545
1YW551 - YW580
1YW585 - YW591
1YW595 - YW596
1YW601
1YW610 - YW613
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
12 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 13

GroupVariable Number
1YW616 - YW623
1YW666
1YW696 - YW699
1YW701 - YW715
1YW726 - YW731
1YW766 - YW775
GroupVariable Number
1YW801 - YW803
1YW811 - YW813
1YW821
1YW831
1YW881
1YW891 - YW893
GroupVariable Number
1YW906 - YW910
1YW916 - YW918
1YW921 - YW925
1YW971 - YW976
--
--
B. Spares Affected
None.
2. CONCURRENT REQUIREMENTS
None.
3. COMPLIANCE
The Federal Aviation Administration (FAA) will possibly release an Airworthiness Directive related to
Requirements Bulletin 737-53A1421 RB. The Airworthiness Directive will make the compliance tasks and
times given in Requirements Bulletin 737-53A1421 RB mandatory.
For airplanes which have incorporated Boeing Business Jet (BBJ) Lower Cabin Altitude Supplemental Type
Certificate (STC) ST01697SE (6500 feet maximum cabin altitude in lieu of 8000 feet) as referenced in
Boeing SB 737-21-1149, the flight cycle related compliance times are different from those specified in this
bulletin. All initial compliance times (thresholds) specified in flight cycles must be reduced to half of those
specified in this bulletin. All repeat interval compliance times specified in flight cycles must be reduced to
one-quarter of those specified in this bulletin.
Accomplish the required actions, based on the applicable conditions in Table 1, in accordance with
Accomplishment Instructions.
When more than one OPTION is given for a CONDITION, do only one of the OPTION numbers. When
more than one ACTION is given for a CONDITION or an OPTION, do all of the ACTION numbers for that
CONDITION or OPTION.
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
13 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 14

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Repeat Interval
(Not to Exceed)
Boeing Recommended
Compliance Time (Whichever
Occurs Later)
ActionCondition
-Whichever oc-
curs first;
Before 37,000
total flight cy-
cles.
Do an external general
visual inspection of the aft
drain mast area and sur-
rounding fuselage skin for
any repair.
All Airplanes.
1. Within 3,000
flight cycles af-
ter the Original
Issue date of
Requirements
Bulletin 737-
53A1421 RB.
2. Within 24
months after the
Original Issue
date of Require-
ments Bulletin
737-53A1421
RB.
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
14 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 15

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Repeat Interval
(Not to Exceed)
Boeing Recommended
Compliance Time (Whichever
Occurs Later)
ActionCondition
-Whichever oc-
curs first;
Before 37,000
total flight cy-
cles.
CONDITION 1 (ACTION
1): Contact the Boeing
Company for alternative
inspection(s) and correc-
tive action instruction(s),
and do the alternative in-
spection(s) and applica-
ble on condition correc-
tive action(s).
CONDITION 1: ANY RE-
PAIR FOUND
1. Within 3,000
flight cycles af-
ter the Original
Issue date of
Requirements
Bulletin 737-
53A1421 RB.
2. Within 24
months after the
Original Issue
date of Require-
ments Bulletin
737-53A1421
RB.
CONDITION 1 (ACTION
2): Do an internal detailed
inspection of the aft drain
mast fitting, surrounding
fuselage skin, and struc-
ture for any crack and
any corrosion at the unre-
paired areas.
CONDITION 1 (ACTION
3): Do an external de-
tailed inspection of the
fuselage skin common to
the area where the aft
drain mast was removed
for any crack and any
corrosion, and external
surface High Frequency
Eddy Current (HFEC) in-
spection on the fuselage
skin around the fastener
holes and cutout for any
crack at the unrepaired
areas.
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
15 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 16

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Repeat Interval
(Not to Exceed)
Boeing Recommended
Compliance Time (Whichever
Occurs Later)
ActionCondition
15,000 flight cycles
or 8 years,
whichever occurs
first.
-CONDITION 1.1 (AC-
TION 1): Repeat the inter-
nal detailed inspection of
the aft drain mast fitting,
surrounding fuselage
skin, and structure for any
crack and any corrosion.
(a)
CONDITION 1.1: NO
CRACK AND NO CORRO-
SION FOUND
15,000 flight cycles
or 8 years,
whichever occurs
first.
-CONDITION 1.1 (AC-
TION 2): Repeat the exter-
nal detailed inspection of
the fuselage skin com-
mon to the area where
the aft drain mast was re-
moved for any crack and
any corrosion, and exter-
nal surface HFEC inspec-
tion on the fuselage skin
around the fastener holes
and cutout for any crack.
(a)
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
16 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 17

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Repeat Interval
(Not to Exceed)
Boeing Recommended
Compliance Time (Whichever
Occurs Later)
ActionCondition
-Before further flight.CONDITION 1.2 (AC-
TION 1): Contact The
Boeing Company for re-
pair instructions and do
the repair.
CONDITION 1.2: ANY
CRACK OR ANY CORRO-
SION FOUND
15,000 flight cycles
or 8 years,
whichever occurs
first.
-CONDITION 1.2 (AC-
TION 2): Repeat the inter-
nal detailed inspection of
the aft drain mast fitting,
surrounding fuselage
skin, and structure for any
crack and any corrosion.
(a)
15,000 flight cycles
or 8 years,
whichever occurs
first.
-CONDITION 1.2 (AC-
TION 3): Repeat the exter-
nal detailed inspection of
the fuselage skin com-
mon to the area where
the aft drain mast was re-
moved for any crack and
any corrosion, and exter-
nal surface HFEC inspec-
tion on the fuselage skin
around the fastener holes
and cutout for any crack.
(a)
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
17 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 18

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Repeat Interval
(Not to Exceed)
Boeing Recommended
Compliance Time (Whichever
Occurs Later)
ActionCondition
-Whichever oc-
curs first;
Before 37,000
total flight cy-
cles.
CONDITION 2 (ACTION
1): Do an internal detailed
inspection of the aft drain
mast fitting, surrounding
fuselage skin, and struc-
ture for any crack and
any corrosion.
CONDITION 2: NO RE-
PAIR FOUND
1. Within 3,000
flight cycles af-
ter the Original
Issue date of
Requirements
Bulletin 737-
53A1421 RB.CONDITION 2 (ACTION2): Do an external de-
tailed inspection of the
fuselage skin common to
the area where the aft
drain mast was removed
for any crack and any
corrosion, and external
surface HFEC inspection
on the fuselage skin
around the fastener holes
and cutout for any crack.
2. Within 24
months after the
Original Issue
date of Require-
ments Bulletin
737-53A1421
RB.
15,000 flight cycles
or 8 years,
whichever occurs
first.
-CONDITION 2.1 (AC-
TION 1): Repeat the inter-
nal detailed inspection of
the aft drain mast fitting,
surrounding fuselage
skin, and structure for any
crack and any corrosion.
(a)
CONDITION 2.1: NO
CRACK AND NO CORRO-
SION FOUND
15,000 flight cycles
or 8 years,
whichever occurs
first.
-CONDITION 2.1 (AC-
TION 2): Repeat the exter-
nal detailed inspection of
the fuselage skin com-
mon to the area where
the aft drain mast was re-
moved for any crack and
any corrosion, and exter-
nal surface HFEC inspec-
tion on the fuselage skin
around the fastener holes
and cutout for any crack.
(a)
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
18 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 19

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Repeat Interval
(Not to Exceed)
Boeing Recommended
Compliance Time (Whichever
Occurs Later)
ActionCondition
-Before further flight.CONDITION 2.2 (AC-
TION 1): Contact The
Boeing Company for re-
pair instructions and do
the repair.
CONDITION 2.2: ANY
CRACK OR ANY CORRO-
SION FOUND
15,000 flight cycles
or 8 years,
whichever occurs
first.
-CONDITION 2.2 (AC-
TION 2): Repeat the inter-
nal detailed inspection of
the aft drain mast fitting,
surrounding fuselage
skin, and structure for any
crack and any corrosion.
(a)
15,000 flight cycles
or 8 years,
whichever occurs
first.
-CONDITION 2.2 (AC-
TION 3): Repeat the exter-
nal detailed inspection of
the fuselage skin com-
mon to the area where
the aft drain mast was re-
moved for any crack and
any corrosion, and exter-
nal surface HFEC inspec-
tion on the fuselage skin
around the fastener holes
and cutout for any crack.
(a)
(a) It is not required to do the internal and external detailed inspection, and external surface HFEC inspection
in areas where a repair covers the affected inspection zone if the repair meets the conditions that follow:
1. The repair is the corrective action to the aft drain mast crack or corrosion condition identified in
Requirements Bulletin 737-53A1421 RB and installed after the Original Issue date of Require-
ments Bulletin 737-53A1421 RB AND
2. The installed repair was approved by the Boeing Organization Designation Authorization (ODA)
via a FAA Form 8100-9.
4. APPROVAL
This Requirements Bulletin was examined by the Federal Aviation Administration (FAA). The changes
specified in this document comply with the applicable regulations and are FAA approved, as well as European
Union Aviation Safety Agency (EASA)/Joint Aviation Authorities (JAA) approved for all EASA/JAA approved
airplanes listed in this Requirements Bulletin effectivity. This document and its approval were based on the
airplane in its original Boeing delivery configuration or as modified by other Boeing changes.
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
19 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 20

If an airplane has a non-Boeing modification or repair that affects a component or system also affected by
this Requirements Bulletin, the operator is responsible for obtaining appropriate regulatory agency approval
before incorporating this Requirements Bulletin.
This requirements bulletin is also approved for airplanes converted into 737-800BCF under project number
TS14-0042 that have incorporated Boeing drawing 800A0003 specified in the Type Certification Data Sheet
A16WE.
This requirements bulletin is also approved for airplanes having FAA Aviation Partners Boeing (APB)
Supplemental Type Certificate (STC) number ST00830SE installed (not including any areas affected by
the split scimitar winglet configuration).
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
20 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 21

5.    ACCOMPLISHMENT INSTRUCTIONS
A. GENERAL INFORMATION
NOTE: 1. This is an Enhanced Required for Compliance document. There will be a separate
Service Bulletin (SB) and a separate Requirements Bulletin (RB). If an RB is mandated
by an Airworthiness Directive (AD), then all applicable requirements specified in the
RB must be done. In the RB, the "Refer to the listed procedures..." column of the
table(s) in the Work Instructions is for reference only. The procedures in that column
are not required by the AD and are for guidance only. For the related SB, some
locations are marked RC Start and RC End to identify the requirements that are
restated from the RB. For the related SB, the areas between RC Start and RC End,
including sub-steps and any figures identified between RC Start and RC End, must
be done to comply with the AD. Therefore, an Alternative Method of Compliance
(AMOC) is required for any deviations to steps between RC Start and RC End,
including substeps and identified figures. For the related SB, the steps not between
RC Start and RC End may be deviated from using accepted methods in accordance
with the operator's maintenance or inspection program without obtaining approval
of an AMOC; this is provided that the steps between RC Start and RC End, including
sub-steps and identified figures, can still be done as specified, and the airplane can
be put back in an airworthy condition. Figures not required for compliance are omitted
from the RB.
2. This is an Enhanced Required for Compliance document. The Boeing processes
used to create the Service Bulletin (SB) and the Requirements Bulletin (RB) ensure
that the information between "RC Start" and "RC End" in the SB are identical to the
information in the same sections (Compliance, Work Instructions...) of the RB which
the SB refers to as the related document.
3. Manual titles are referred to by acronyms. Refer to Paragraph 1.J., References, for
definition of the acronyms.
4. Obey all of the warnings and cautions given in the specified manual sections.
5. Unless shown differently, these dimensions and tolerances are used:
– Linear dimensions are in inches
– Tolerance on linear dimensions, other than rivet and bolt edge margins, is plus
or minus 0.03 inch
– Tolerance on rivet and bolt edge margin is plus or minus 0.05 inch
– Angular tolerance is plus or minus 2 degrees
– Hole dimensions for standard solid rivets and fasteners are in Structural Repair
Manual (SRM) Chapter 51
– Torque Values:
— Values for structural fasteners are given in 737 Structural Repair Manual,
Chapter 51.
— Values for airframe maintenance tasks are included in Chapter 20 of 737
Aircraft Maintenance Manual (AMM).
6. Use the approved fastener, process and material substitutions in accordance with
SRM Chapter 51.
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
21 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 22

7. If the length of any fastener specified in this service bulletin does not meet installation
standards given in SRM Chapter 51, then a fastener of the same specification, or
an approved substitute, with a length which meets the installation standards given
in SRM Chapter 51 may be used. In addition, washers may be installed for fastener
grip length in accordance with SRM Chapter 51. Refer to SOPM 20-50-01 for alternate
full threaded fasteners (screws) needed for installation in this service bulletin.
8. A Detailed Inspection is defined as: An intensive examination of a specific item,
installation, or assembly to detect damage, failure, or irregularity. This could include
tactile assessment in which a component or assembly can be checked for
tightness/security. Available lighting is normally supplemented with a direct source
of good lighting at an intensity deemed appropriate. Inspection aids such as mirrors,
magnifying lenses, etc. may be necessary. Surface cleaning and elaborate procedures
may be required.
9. These work instructions refer to procedures included in other Boeing documents.
When the words "refer to" are used and the operator has an accepted alternative
procedure, the accepted alternative procedure can be used. When the words "in
accordance with" are included in the instruction, the procedure in the Boeing document
must be used.
10. Boeing Service Letter 737-SL-51-042, Damage Reporting and Repair Plan/Design
Guidelines, is an acceptable procedure to request information from Boeing for
additional structural repair instructions. The Service Letter describes what information
must be provided to Boeing before a structural repair can be provided.
11. The instructions in Paragraph 3.B., Work Instructions and the figures can include
operation of tools or test equipment. Boeing Engineering Tool Drawings, the Illustrated
Tool and Equipment Manual, and the Special Tool and Ground Handling Drawing
Index contain data on versions of the tools or test equipment that you can use. It is
permitted to use replaced tools. It is not permitted to use superseded tools.
12. If it is necessary to remove more parts for access, you can remove those parts. If
you can get access without removing identified parts, it is not necessary to remove
all of the identified parts. Jacking and shoring limitations must be observed.
13. Where the work instructions include installation of a kept part, a new or serviceable
part with the same part number can be installed as an alternative to the kept part.
14. Use of colors in Figures is based on guidance from the ATA e-Business Program
(ATA) iSpec 2200.
15. A General Visual Inspection (GVI) is defined as: A visual examination of an interior
or exterior area, installation or assembly to detect obvious damage, failure or
irregularity. This level of inspection is made from within touching distance, unless
otherwise specified. A mirror may be necessary to enhance visual access to all
exposed surfaces in the inspection area. This level of inspection is made under
normally available lighting conditions such as daylight, hangar lighting, flashlight or
drop-light and may require removal or opening of access panels or doors. Stands,
ladders or platforms may be required to gain proximity to the area being checked.
Basic cleaning may be required to ensure appropriate visibility.
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
22 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 23

16. When more than one OPTION is given for a CONDITION, do only one of the OPTION
numbers. When more than one ACTION is given for a CONDITION or an OPTION,
do all of the ACTION numbers for that CONDITION or OPTION.
17. The compliance times for the actions in Paragraph 3.B., WORK INSTRUCTIONS
are in Paragraph 1.E., Compliance.
B. WORK INSTRUCTIONS
Actions Required for Compliance
1. Requirements:
Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Refer to the listed
procedures in SB
737-53A1421
Original Issue or
later approved re-
visions as an ac-
cepted procedure
Method of ComplianceActionCondition
DO PART 1:
OPEN ACCESS.
-Do an external general
visual inspection of the aft
drain mast area and sur-
rounding fuselage skin for
any repair.
All Airplanes.
-FIGURE 1
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
23 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 24

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Refer to the listed
procedures in SB
737-53A1421
Original Issue or
later approved re-
visions as an ac-
cepted procedure
Method of ComplianceActionCondition
--CONDITION 1 (ACTION
1): Contact the Boeing
Company for alternative
inspection(s) and correc-
tive action instruction(s),
and do the alternative in-
spection(s) and applica-
ble on condition correc-
tive action(s).
CONDITION 1: ANY RE-
PAIR FOUND
-FIGURE 2CONDITION 1 (ACTION
2): Do an internal detailed
inspection of the aft drain
mast fitting, surrounding
fuselage skin, and struc-
ture for any crack and
any corrosion at the unre-
paired areas.
-FIGURE 3CONDITION 1 (ACTION
3): Do an external de-
tailed inspection of the
fuselage skin common to
the area where the aft
drain mast was removed
for any crack and any
corrosion, and external
surface High Frequency
Eddy Current (HFEC) in-
spection on the fuselage
skin around the fastener
holes and cutout for any
crack at the unrepaired
areas.
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
24 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 25

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Refer to the listed
procedures in SB
737-53A1421
Original Issue or
later approved re-
visions as an ac-
cepted procedure
Method of ComplianceActionCondition
DO PART 2:
CLOSE ACCESS.
-CONDITION 1.1 (AC-
TION 1): Repeat the inter-
nal detailed inspection of
the aft drain mast fitting,
surrounding fuselage
skin, and structure for any
crack and any corrosion.
(a)
CONDITION 1.1: NO
CRACK AND NO CORRO-
SION FOUND -FIGURE 2
-FIGURE 3CONDITION 1.1 (AC-
TION 2): Repeat the exter-
nal detailed inspection of
the fuselage skin com-
mon to the area where
the aft drain mast was re-
moved for any crack and
any corrosion, and exter-
nal surface HFEC inspec-
tion on the fuselage skin
around the fastener holes
and cutout for any crack.
(a)
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
25 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 26

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Refer to the listed
procedures in SB
737-53A1421
Original Issue or
later approved re-
visions as an ac-
cepted procedure
Method of ComplianceActionCondition
--CONDITION 1.2 (AC-
TION 1): Contact The
Boeing Company for re-
pair instructions and do
the repair.
CONDITION 1.2: ANY
CRACK OR ANY CORRO-
SION FOUND DO PART 2:CLOSE ACCESS.-
-FIGURE 2CONDITION 1.2 (AC-
TION 2): Repeat the inter-
nal detailed inspection of
the aft drain mast fitting,
surrounding fuselage
skin, and structure for any
crack and any corrosion.
(a)
-FIGURE 3CONDITION 1.2 (AC-
TION 3): Repeat the exter-
nal detailed inspection of
the fuselage skin com-
mon to the area where
the aft drain mast was re-
moved for any crack and
any corrosion, and exter-
nal surface HFEC inspec-
tion on the fuselage skin
around the fastener holes
and cutout for any crack.
(a)
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
26 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 27

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Refer to the listed
procedures in SB
737-53A1421
Original Issue or
later approved re-
visions as an ac-
cepted procedure
Method of ComplianceActionCondition
-FIGURE 2CONDITION 2 (ACTION
1): Do an internal detailed
inspection of the aft drain
mast fitting, surrounding
fuselage skin, and struc-
ture for any crack and
any corrosion.
CONDITION 2: NO RE-
PAIR FOUND
-FIGURE 3CONDITION 2 (ACTION
2): Do an external de-
tailed inspection of the
fuselage skin common to
the area where the aft
drain mast was removed
for any crack and any
corrosion, and external
surface HFEC inspection
on the fuselage skin
around the fastener holes
and cutout for any crack.
DO PART 2:
CLOSE ACCESS.
-CONDITION 2.1 (AC-
TION 1): Repeat the inter-
nal detailed inspection of
the aft drain mast fitting,
surrounding fuselage
skin, and structure for any
crack and any corrosion.
(a)
CONDITION 2.1: NO
CRACK AND NO CORRO-
SION FOUND -FIGURE 2
-FIGURE 3CONDITION 2.1 (AC-
TION 2): Repeat the exter-
nal detailed inspection of
the fuselage skin com-
mon to the area where
the aft drain mast was re-
moved for any crack and
any corrosion, and exter-
nal surface HFEC inspec-
tion on the fuselage skin
around the fastener holes
and cutout for any crack.
(a)
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
27 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 28

Table 1: Skin and Structure Common to the Aft Drain Mast - Inspection
Refer to the listed
procedures in SB
737-53A1421
Original Issue or
later approved re-
visions as an ac-
cepted procedure
Method of ComplianceActionCondition
--CONDITION 2.2 (AC-
TION 1): Contact The
Boeing Company for re-
pair instructions and do
the repair.
CONDITION 2.2: ANY
CRACK OR ANY CORRO-
SION FOUND DO PART 2:CLOSE ACCESS.-
-FIGURE 2CONDITION 2.2 (AC-
TION 2): Repeat the inter-
nal detailed inspection of
the aft drain mast fitting,
surrounding fuselage
skin, and structure for any
crack and any corrosion.
(a)
-FIGURE 3CONDITION 2.2 (AC-
TION 3): Repeat the exter-
nal detailed inspection of
the fuselage skin com-
mon to the area where
the aft drain mast was re-
moved for any crack and
any corrosion, and exter-
nal surface HFEC inspec-
tion on the fuselage skin
around the fastener holes
and cutout for any crack.
(a)
(a) It is not required to do the internal and external detailed inspection, and external surface HFEC inspection
in areas where a repair covers the affected inspection zone if the repair meets the conditions that follow:
1. The repair is the corrective action to the aft drain mast crack or corrosion condition identified in
Requirements Bulletin 737-53A1421 RB and installed after the Original Issue date of Require-
ments Bulletin 737-53A1421 RB AND
2. The installed repair was approved by the Boeing Organization Designation Authorization (ODA)
via a FAA Form 8100-9.
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
28 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 29 (OCR)

REQUIREMENTS BULLETIN 737-53A1421
ALERT  ALERT
AFT DRAIN MAST
STA  STA
986.50  1006
S-24L
S-25L-
FWD
VIEW LOOKING UP OF SKIN WITH
THE AFT DRAIN MAST REMOVED
A-A
GENERAL VISUAL INSPECTION  3079986
FIGURE 1: FUSELAGE SKIN COMMON TO THE AFT DRAIN MAST - EXTERNAL GENERAL VISUAL
INSPECTION
(SHEET 1 OF 2)
Original Issue: March 20, 2025
Export Controlled ECCN: 9E991  737-53A1421 RB
See page 1 for details  29 of 35

### Page 30

More DataQtyIdentificationNameTaskStep
(a)--SKINInspect1
(a) Do an external general visual inspection of the aft drain mast area and surrounding fuselage skin for any
repair.
FIGURE 1: FUSELAGE SKIN COMMON TO THE AFT DRAIN MAST - EXTERNAL GENERAL VISUAL
INSPECTION
(SHEET 2 OF 2)
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
30 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 31 (OCR)

REQUIREMENTS BULLETIN 737-53A1421
ALERT  ALERT
AFT DRAIN MAST
S-24L
S-25L
VIEW LOOKING DOWN
A-A
STA  STA
986.50  AFT DRAIN MAST  1006
SUPPORT FITTING
S-25L
S-24L
FWD
SKIN
AFT DRAIN MAST STRUCTURE  VIEW LOOKING DOWN  3079576
INTERNAL DETAILED INSPECTION
FIGURE 2: STRUCTURE COMMON TO THE AFT DRAIN MAST - INTERNAL DETAILED INSPECTION
(SHEET 1 OF 2)
Original Issue: March 20, 2025
Export Controlled ECCN: 9E991  737-53A1421 RB
See page 1 for details  31 of 35

### Page 32

The step numbers shown below agree with the numbers shown in the circle symbols in the figure. The QTY
numbers shown below are the number of parts necessary for this figure.
More DataQtyIdentificationNameTaskStep
(a)(b)1-AFT DRAIN MAST
SUPPORT FITTING
Clean1
(a)(b)--SKINClean
(a)(b)--STRUCTUREClean
(c)1-AFT DRAIN MAST
SUPPORT FITTING
Inspect2
(c)--SKINInspect
(c)(d)--STRUCTUREInspect
(a) Remove any dirt and grease from the inspection area. Refer to SOPM 20-30-03 as an accepted procedure.
(b) Make sure you only use approved compound removal tools. Refer to 737-600/700/800/900 AMM 51-31-
00 as an accepted procedure.
(c) Do an internal detailed inspection of the aft drain mast fitting, surrounding fuselage skin, and structure
for any crack and any corrosion.
(d) If any cracking, separation or corrosion is found on the surrounding sealant, remove the sealant, and inspect
the area for any cracking or corrosion.
FIGURE 2: STRUCTURE COMMON TO THE AFT DRAIN MAST - INTERNAL DETAILED INSPECTION
(SHEET 2 OF 2)
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
32 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

### Page 33 (OCR)

REQUIREMENTS BULLETIN 737-53A1421
ALERT  ALERT
AFT DRAIN MAST
09986
-79Z-S
FWD
VIEW LOOKING UP OF SKIN WITH
THE AFT DRAIN MAST REMOVED
EXTERNAL DETAILED INSPECTION  A-A  3082702
FIGURE 3: FUSELAGE SKIN COMMON TO THE AFT DRAIN MAST - EXTERNAL DETAILED INSPECTION
AND EXTERNAL SURFACE HIGH FREQUENCY EDDY CURRENT INSPECTION
(SHEET 1 OF 3)
Original Issue: March 20, 2025
Export Controlled ECCN: 9E991  See page 1 for details  737-53A1421 RB  33 of 35

### Page 34 (OCR)

REQUIREMENTS BULLETIN 737-53A1421
ALERT  ALERT
STA  (2 3) 5 LOCATIONS
986.50
S-24L
S-25L-  O
FWD
VIEW LOOKING UP OF SKIN WITH
THE AFT DRAIN MAST REMOVED
EXTERNAL SURFACE HFEC INSPECTION  A-A  3082772
FIGURE 3: FUSELAGE SKIN COMMON TO THE AFT DRAIN MAST - EXTERNAL DETAILED INSPECTION
AND EXTERNAL SURFACE HIGH FREQUENCY EDDY CURRENT INSPECTION
(SHEET 2 OF 3)
Original Issue: March 20, 2025
Export Controlled ECCN: 9E991  737-53A1421 RB
See page 1 for details  34 of 35

### Page 35

The step numbers shown below agree with the numbers shown in the circle symbols in the figure. The QTY
numbers shown below are the number of parts necessary for this figure.
More DataQtyIdentificationNameTaskStep
(a)--SEALANTRemove1
(b)--SKINInspect2
(c)--SKINInspect3
(a) Remove sealant from the skin and backup structure around the aft drain mast cutout. Also, remove sealant
from the skin exterior. Use a non-metallic tool. Refer to SOPM 20-50-19 as an accepted procedure.
(b) Do an external detailed inspection of the fuselage skin common to the area where the aft drain mast was
removed for any crack and any corrosion.
(c) Do an external surface High Frequency Eddy Current (HFEC) inspection on the fuselage skin around the
fastener holes and cutout for any crack in accordance with 737 NDT Manual Part 6, 51-00-00 Procedure
4 or 737 NDT Manual Part 6, 51-00-00 Procedure 23.
FIGURE 3: FUSELAGE SKIN COMMON TO THE AFT DRAIN MAST - EXTERNAL DETAILED INSPECTION
AND EXTERNAL SURFACE HIGH FREQUENCY EDDY CURRENT INSPECTION
(SHEET 3 OF 3)
Export Controlled ECCN: 9E991
See page 1 for details
Original Issue: March 20, 2025
737-53A1421 RB
35 of 35
REQUIREMENTS BULLETIN 737-53A1421
ALERTALERT

---
OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain, except the copies of manufacturers’ service documents, which belong to their rights holders.