AC 25-21

Historical

Advisory circular · Certification of Transport Airplane Structure

Approved
September 1, 1999
Last reviewed
March 31, 2017
Family
AC 25-21 · 1 version
14 CFR parts
25

Guidance, not mandatory unless a rule or AD requires it. DRS record last changed November 21, 2025.

Show pages 1–12

c. Background456

d. Acceptable compliance methods456

e. References456

[542-548] [RESERVED]456

549. Section 25.525 Application of loads456

a. Rule text456

b. Intent of rule456

c. Background456

d. Acceptable compliance methods456

e. References456

[550-559] [RESERVED]456

560. Section 25.527 Hull and main float load factors457

a. Rule text457

b. Intent of rule458

c. Background458

d. Acceptable compliance methods458

e. References458

[561-568] [RESERVED]458

569. Section 25.529 Hull and main float landing conditions458

a. Rule text458

b. Intent of rule459

c. Background459

d. Acceptable compliance methods459

e. References459

[570-575] [RESERVED]459

576. Section 25.531 Hull and main float takeoff condition459

a. Rule text459

b. Intent of rule459

c. Background459

d. Acceptable compliance methods460

e. References460

[577-583] [RESERVED]460

584. Section 25.533 Hull and main float bottom pressures460

a. Rule text460

b. Intent of rule462

c. Background462

d. Acceptable compliance methods462

e. References462

[585-589] [RESERVED]462

xvi

590. Section 25.535 Auxiliary float loads462

a. Rule text462

b. Intent of rule463

c. Background464

d. Acceptable compliance methods464

e. References464

[591-597] [RESERVED]464

598. Section 25.537 Seawing loads464

a. Rule text464

b. Intent of rule464

c. Background464

d. Acceptable compliance methods464

e. References464

[599-602] [RESERVED]464

Section 8. EMERGENCY LANDING CONDITIONS

603. Section 25.561 General539

a. Rule text539

b. Intent of rule540

c. Background540

d. Acceptable compliance methods540

e. References540

[604-610] [RESERVED]540

611. Section 25.562 Emergency landing dynamic conditions540

a. Rule text540

b. Intent of rule541

c. Background542

d. Acceptable compliance methods542

e. References542

[612-620] [RESERVED]542

621. Section 25.563 Structural ditching provisions542

a. Rule text542

b. Intent of rule542

c. Background542

xvii

d. Acceptable compliance methods542

e. References542

[622-628] [RESERVED]542

Section 9. FATIGUE EVALUATION

629. Section 25.571 Damage-tolerance and fatigue evaluation of structure

.593

a. Rule text593

b. Intent of rule595

c. Background595

d. Acceptable compliance methods596

e. References596

[630-638] [RESERVED]596

Section 10. LIGHTNING PROTECTION

639. Section 25.581 Lightning protection657

a. Rule text657

b. Intent of rule657

c. Background657

d. Acceptable compliance methods657

e. References657

640-650 [RESERVED]657

CHAPTER 2. DESIGN AND CONSTRUCTION Section 1. GENERAL

651. Section 25.601 General733

a. Rule text733

b. Intent of rule733

c. Background733

d. Acceptable compliance methods733

xviii

e. References733

[652-658] [RESERVED]733

659. Section 25.603 Materials733

a. Rule text733

b. Intent of rule734

c. Background734

d. Acceptable compliance methods734

e. References734

[660-668] [RESERVED]734

669. Section 25.605 Fabrication methods734

a. Rule text734

b. Intent of rule734

c. Background734

d. Acceptable compliance methods735

e. References735

[670-675] [RESERVED]735

676. Section 25.607 Fasteners735

a. Rule text735

b. Intent of rule735

c. Background735

d. Acceptable compliance methods735

e. References735

[677-683] [RESERVED]735

684. Section 25.609 Protection of structure736

a. Rule text736

b. Intent of rule736

c. Background736

d. Acceptable compliance methods736

e. References736

[685-692] [RESERVED]736

693. Section 25.611 Accessibility provisions736

a. Rule text736

b. Intent of rule736

c. Background737

d. Acceptable compliance methods737

e. References737

[694-698] [RESERVED]737

699. Section 25.613 Material strength properties and design values737

a. Rule text737

b. Intent of rule738

c. Background738

xix

d. Acceptable compliance methods738

e. References738

[700-707] [RESERVED]738

708. Section 25.615 [Removed]738

709. Section 25.619 Special factors738

a. Rule text738

b. Intent of rule739

c. Background739

d. Acceptable compliance methods739

e. References739

[710-718] [RESERVED]739

719. Section 25.621 Casting factors739

a. Rule text739

b. Intent of rule740

c. Background740

d. Acceptable compliance methods741

e. References741

[720-728] [RESERVED]741

729. Section 25.623 Bearing factors741

a. Rule text741

b. Intent of rule741

c. Background741

d. Acceptable compliance methods741

e. References741

[730-738] [RESERVED]741

739. Section 25.625 Fitting factors741

a. Rule text741

b. Intent of rule742

c. Background742

d. Acceptable compliance methods742

e. References742

[740-745] [RESERVED]742

746. Section 25.629 Aeroelastic stability requirements742

a. Rule text742

b. Intent of rule744

c. Background744

d. Acceptable compliance methods744

e. References744

[747-759] [RESERVED]745

xx

760. Section 25.631 Bird strike damage745

a. Rule text745

b. Intent of rule745

c. Background745

d. Acceptable compliance methods745

e. References745

[761-769] [RESERVED]745

Section 2. CONTROL SURFACES

770. Section 25.651 Proof of strength845

a. Rule text845

b. Intent of rule845

c. Background845

d. Acceptable compliance methods845

e. References845

[771-778] [RESERVED]845

779. Section 25.655 Installation845

a. Rule text845

b. Intent of rule845

c. Background846

d. Acceptable compliance methods846

e. References846

[780-787] [RESERVED]846

788. Section 25.657 Hinges846

a. Rule text846

b. Intent of rule846

c. Background846

d. Acceptable compliance methods846

e. References846

[789-793] [RESERVED]846

xxi

Section 3. CONTROL SYSTEMS

794. Section 25.671 General921

a. Rule text921

b. Intent of rule921

c. Background921

d. Acceptable compliance methods922

e. References922

[795-800] [RESERVED]922

801. Section 25.672 Stability augmentation and automatic and power-

operated systems922

a. Rule text922

b. Intent of rule923

c. Background923

d. Acceptable compliance methods923

e. References923

[802-810] [RESERVED]923

811. Section 25.673 Two-Control Airplanes - [Removed]923

812. Section 25.675 Stops923

a. Rule text923

b. Intent of rule924

c. Background924

d. Acceptable compliance methods924

e. References924

[813-818] [RESERVED]924

819. Section 25.677 Trim systems924

a. Rule text924

b. Intent of rule924

c. Background925

d. Acceptable compliance methods925

e. References925

[820-827] [RESERVED]925

828. Section 25.679 Control system gust locks925

a. Rule text925

b. Intent of rule925

c. Background925

d. Acceptable compliance methods925

e. References925

[829-835] [RESERVED]925

xxii

836. Section 25.681 Limit load static tests926

a. Rule text926

b. Intent of rule926

c. Background926

d. Acceptable compliance methods926

e. References926

[837-844] [RESERVED]926

845. Section 25.683 Operation tests926

a. Rule text926

b. Intent of rule926

c. Background927

d. Acceptable compliance methods927

e. References927

[846-851] [RESERVED]927

852. Section 25.685 Control system details927

a. Rule text927

b. Intent of rule927

c. Background927

d. Acceptable compliance methods927

e. References927

[853-860] [RESERVED]927

861. Section 25.689 Cable systems928

a. Rule text928

b. Intent of rule928

c. Background928

d. Acceptable compliance methods928

e. References928

[862-867] [RESERVED]928

868. Section 25.693 Joints929

a. Rule text929

b. Intent of rule929

c. Background929

d. Acceptable compliance methods929

e. References929

[869-875] [RESERVED]929

876. Section 25.695 Power-boost and power-operated control system ­

[Removed]929

a. Rule text929

877. Section 25.697 Lift and drag devices, controls930

a. Rule text930

b. Intent of rule930

xxiii

c. Background930

d. Acceptable compliance methods930

e. References930

[878-886] [RESERVED]931

887. Section 25.699 Lift and drag device indicator931

a. Rule text931

888. Section 25.701 Flap and slat interconnection931

a. Rule text931

b. Intent of rule932

c. Background932

d. Acceptable compliance methods932

e. References932

[889-896] [RESERVED]932

Section 4. LANDING GEAR

897. Section 25.721 General1007

a. Rule text1007

b. Intent of rule7007

c. Background1007

d. Acceptable compliance methods1007

e. References1007

[898-907] [RESERVED]1007

908. Section 25.723 Shock absorption tests1008

a. Rule text1008

b. Intent of rule1008

c. Background1008

d. Acceptable compliance methods1008

e. References1008

[909-913] [RESERVED]1008

914. Section 25.725 Limit drop tests1009

a. Rule text1009

b. Intent of rule1010

c. Background1010

d. Acceptable compliance methods1010

e. References1010

[915-921] [RESERVED]1010

xxiv

922. Section 25.727 Reserve energy absorption drop tests1010

a. Rule text1010

b. Intent of rule1011

c. Background1011

d. Acceptable compliance methods1011

e. References1011

[923-928] [RESERVED]1011

929. Section 25.729 Retracting mechanism1011

a. Rule text1011

b. Intent of rule1013

c. Background1013

d. Acceptable compliance methods1013

e. References1013

[930-938] [RESERVED]1013

939. Section 25.731 Wheels1013

a. Rule text1013

[940-946] [RESERVED]1014

947. Section 25.733 Tires1014

a. Rule text1014

[948-954] [RESERVED]1015

955. Section 25.735 Brakes1015

a. Rule text1015

[956-960] [RESERVED]1017

961. Section 25.737 Skis1018

a. Rule text1018

[962-966] [RESERVED]1018

Section 5. FLOATS AND HULLS

967. Section 25.751 Main float buoyancy1093

a. Rule text1093

[968-973] [RESERVED]1093

974. Section 25.753 Main float design1093

a. Rule text1093

[975-977] [RESERVED]1093

978. Section 25.755 Hulls1093

a. Rule text1093

[979-981] [RESERVED]1093

xxv

Section 6. PERSONNEL AND CARGO ACCOMMODATIONS

982. Section 25.771 Pilot compartment1143

a. Rule text1143

[983-988] [RESERVED]1143

989. Section 25.772 Pilot compartment doors1143

a. Rule text1143

[990-996] [RESERVED]1144

997. Section 25.773 Pilot compartment view1144

a. Rule text1144

[998-1003] [RESERVED]1145

1004. Section 25.775 Windshields and windows1145

a. Rule text1145

b. Intent of rule1146

c. Background1146

d. Acceptable compliance methods1146

e. References1146

[1005-1015] [RESERVED]1146

1016. Section 25.777 Cockpit controls1146

a. Rule text1146

[1017-1022] [RESERVED]1147

1023. Section 25.779 Motion and effect of cockpit controls1147

a. Rule text1147

[1024-1029] [RESERVED]1148

1030. Section 25.781 Cockpit control knob shape1149

a. Rule text1149

[1031-1037] [RESERVED]1149

1038. Section 25.783 Doors1149

a. Rule text1149

b. Intent of rule1150

c. Background1150

d. Acceptable compliance methods1150

e. References1151

[1039-1049] [RESERVED]1151

1050. Section 25.785 Seats, berths, safety belts, and harnesses1151

a. Rule text1151

[1051-1054] [RESERVED]1153

xxvi

1055. Section 25.787 Stowage compartments1153

a. Rule text1153

[1056-1059] [RESERVED]1154

1060. Section 25.789 Retention of items of mass in passenger and crew

compartments and galleys1154

a. Rule text1154

b. Intent of rule1154

c. Background1154

d. Acceptable compliance methods1154

e. References1154

[1061-1068] [RESERVED]1154

1069. Section 25.791 Passenger information signs and placards1155

a. Rule text1155

[1070-1072] [RESERVED]1155

1073. Section 25.793 Floor surfaces1156

a. Rule text1156

[1074-1075] [RESERVED]1156

Section 7. EMERGENCY PROVISIONS

1076. Section 25.801 Ditching1257

a. Rule text1257

b. Intent of rule1257

c. Background1257

d. Acceptable compliance methods1257

e. References1258

[1077-1085] [RESERVED]1258

Section 8. FIRE PROTECTION 1086. Section 25.865 Fire protection of flight controls, engine mounts, and

other flight structure1279

a. Rule text1279

b. Intent of rule1279

xxvii