AC 27-1B, Chg 1 2/12/03
(i) Cracks emanating from the edge of structural openings or cutouts which can be readily detected by visual inspection of the area.
(ii) A circumferential or longitudinal skin crack in the basic fuselage structure of such a length that it can be readily detected by a visual inspection of the surface area.
(iii) Complete severance of interior frame elements or stiffeners in addition to a visually detectable crack in the adjacent skin.
(iv) Failure of one element in a multiple load path design.
(v) Failure of primary attachments, including control hinges and fittings.
(3) Determination of Probable Crack Locations. The probable crack locations are to be determined by tests, analysis, or both. In cases of unusually critical or complex components or when initial fatigue loading may affect the rate or mode of cracking, the probable crack locations should be determined by fatigue test. When determination is made by analysis, sound engineering judgement should be used and a variety of factors such as the following should be taken into account:
(i) Conducting an analysis to locate areas of maximum stress and low margin of safety.
(ii) Conducting strain surveys on undamaged structure to establish points of high stress concentration as well as the magnitude of such concentration.
(iii) Examining static test results to determine locations where excessive deformation occurred.
(iv) Determining from fatigue analysis where cracks may initiate.
(iv) Selecting locations in an element where the stresses in adjacent elements would be the maximum with that element failed.
(v) Selecting partial fracture locations in an element wherein high stress concentrations are present in the residual structure.
(vi) Assessing design detail areas which are prone to fatigue damage such as joints, holes, and other features as based on service and test experience of similarly designed components.
(4) Fail-Safe Demonstration. It is to be demonstrated by analysis, tests, or both, that the structure with the partial failures as defined in paragraphs f(2) and f(3) can withstand the maximum load and the repeated loads expected in service during the period prior to detection. The repeated loads should be as defined in the loading Page MG 11 - 14
spectrum of paragraph d(2) and the structure should be capable of supporting this loading after a partial failure for a sufficient time with respect to the inspection interval to assure that catastrophic failure is extremely remote. In test demonstrations, the damage may be initiated or simulated by cuts made with a fine saw, sharp blade, or guillotine in those cases where it is not necessary and not practical to produce fatigue cracks by tests. In those cases where damage is simulated at joints or fittings, bolts may be removed to simulate failure if this condition would be representative of an actual failure. In some instances, the fail-safe characteristics may be shown analytically. The analytical approach may be used when the structural configuration involved is essentially similar to one already verified by fail-safe tests, whether on a previously approved type design, or on other similar areas of the design currently being evaluated.
The analytical approach may also be used when:
(i) It can be shown that the failure would be detected considerably before the critical crack length is reached;
(ii) The margins of safety resulting from the analysis are well in excess of the fail-safe residual static strength level; and,
(iii) The stress levels in the partially failed structure and the design are such as to assure adequate crack propagation time relative to the inspection interval.
(5) Inspection. Detection of fatigue cracks before they become dangerous is the ultimate control in ensuring the fail-safe characteristics of the structure. Therefore, the manufacturer should provide sufficient guidance information to assist operators in establishing the frequency and extent of the repeated inspections of the critical structure.
g. Further Considerations.
(1) Control of Fatigue Sensitive Parts. Control of the part in manufacture, operational service, and maintenance is vital to ensure the full benefits of the fatigue life substantiation process. Any part that has been selected for fatigue assessment should be considered using the following guidance. This is particularly important for safe life parts with no damage tolerance capability. The details of the manufacturing procedures and processes for fatigue sensitive parts, including material manufacture and source, forging procedures, machining operations and sequence, and inspection techniques and acceptance and rejection criteria should be established. Sensitivity should be established on the basis of identifying the processes, which if incorrectly completed could significantly affect the fatigue life. The tested components should be produced in accordance with the above manufacturing procedures. For life-limited and fatigue sensitive parts, the design and manufacturing standards should be frozen subject to further evaluation by the design authority. Parts produced in whole or in part under subcontracting or partnership arrangements should be subject to the same procedures.
Life-limited and fatigue sensitive parts should be marked with a serial number and records relating to the marking maintained, such that it is possible to establish the AC Page MG 11 - 15
AC 27-1B, Chg 1 2/12/03 relevant manufacturing modification and service history of the individual parts. Special instructions should be provided, as necessary, to ensure the part is handled in an appropriate manner, particularly during maintenance. Processes for determining the disposition of parts having manufacturing errors or material flaws should be established.
In a similar manner, processes controlling changes to the design or manufacture of the component or to its operating environment or loading spectrum are required. For any such changes, their effects on the fatigue evaluation of the part should be established.
This evaluation should involve further fatigue testing, unless it can be shown that testing is not necessary.
Page MG 11 - 16
MIN. DESIGN OR DEMONSTRATED RPM 27.33, 27.309 MAX. DESIGN OR DEMONSTRATED RPM 27.33, 27.309 VNE 27.1505 111% VNE
27.309
NORMAL OPERATING RPM
ROTOR RPM
AIRSPEED FIGURE AC 27.MG 11-2: Flight Regime to be Investigated Power-On Operation.
(Note: Dashed lines in these figures indicate test boundaries. Shaded areas indicate operating regimes.)
ROTOR RPM MIN. DESIGN OR DEMONSTRATED RPM 27.33, 27.309 MAX. DESIGN OR DEMONSTRATED RPM 27.33, 27.309 VNE 27.1505 111% VNE
27.309 MAX. PLACARD RPM 27.33, 27.1509 (95% MAX.DESIGN) MIN. PLACARD RPM 27.33, 27.1509 (105% MIN.DESIGN)
AIRSPEED FIGURE AC 27.MG 11-3: Flight Regime to be Investigated for Power-Off Operation AC Page MG 11 - 17
AC 27-1B, Chg 1 2/12/03 A B C Vibratory Stress D Estimated Mean Endurance Limit Adjusted Specimen Endurance Limit Operating Boundary (C=1/3B) Yield Stress Steady Stress FIGURE AC 27.MG 11-4: Simplified analytical method for safe life evaluation Vibratory Load or Stress Test Data Curve Statistically Reduced Strength Reduction Reduction Life Working Curve Cycles, N FIGURE AC 27.MG 11-5: Typical S/N curve for safe life evaluation.
Page MG 11 - 18
2/12/03 AC 27-1B, Chg 1 Measured Loads (Flight Load Survey) Component Strength (S/N Curve) Usage
(Flight Spectrum)
Miner’s
Rule
Life Determination FIGURE AC 27.MG 11-6: Elements of Safe-Life Determination AC Page MG 11 - 19
AC 27-1B, Chg 1 2/12/03 FIGURE AC 27.MG 11-7
FLIGHT LOAD MEASUREMENT PROGRAM CONDITIONS TO BE INVESTIGATED
1. GROUND CONDITIONS.
a. Normal start.
b. Rapid increases of RPM on ground to maximum power-on RPM of main rotor.
c. Taxiing with max allowable full cyclic control.
d. Landing run (if applicable).
e. Braking (if applicable).
f. Normal shutdown.
g. Special ground checks (if applicable).
2. HOVERING IN AND OUT
OF GROUND-EFFECT
(1) Steady with rotor at maximum side of RPM tolerance.
(2) Steady with rotor at minimum side of RPM tolerance.
(3) 90-degree right turn
(4) 90-degree left turn
(5) Control reversals
(6) Sideward flight
(7) Rearward flight
i) Longitudinal.
ii) Lateral.
iii) Rudder.
i) Left.
ii) Right.
3. MANEUVERING IN
GROUND EFFECT
(1) Jump takeoff.
(2) Normal takeoff (*) and accelerate to climb airspeed.
(3) Normal Landing (*)
(4) Full autorotational landing.
i) Multiengine.
ii) One-engine-inoperative.
Page MG 11 - 20
4. FORWARD FLIGHT-POWER ON.
a. Level flight. (**)
b. Maneuvers.
FIGURE AC 27.MG 11-7 (continued)
(1) 40 percent VH
(2) 60 percent VH
(3) 80 percent VH
(4) VH
(5) VNE
(1) Full power climbs. (**)
(2) Cyclic pull-ups.
(3) Normal acceleration from climb airspeed to 90 percent VH.
(4) Turns, right and left over a range of bank angles and speeds up to the lesser of VH or VNE and including:
(5) Control reversals at 90 percent VH.
(6) Deceleration from 90 percent VH to descent airspeed.
(1) Partial power descent. (*)
(2) Normal approach
(3) Steep Approaches (to landing) or Flare FIGURE AC 27.MG 11-7
i) Minimum side of main rotor RPM tolerance (RPM -) ii) Maximum side of main rotor RPM tolerance (RPM +)
i) (RPM +) ii) (RPM -)
i) (RPM +) ii) (RPM -)
i) (RPM +) ii) (RPM -)
i) (RPM +) ii) (RPM -)
i) All engines operative.
ii) One-engine-inoperative.
i) 60 percent VH.
ii) 90 percent VH.
i) Right at 60 percent VH and 90 percent VH.
ii) Left at 60 percent VH and 90 percent VH.
i) Longitudinal.
ii) Lateral.
iii) Rudder.
i) All engines.
ii) One engine out.
AC Page MG 11 - 21
AC 27-1B, Chg 1 2/12/03 (continued)
5. POWER TRANSITIONS.
a. All engines operating to one engine out.
(1) In full power climb.
(2) At 90 percent VH.
b. One engine out to all engines operating in powered descent.
c. All engines operating to autorotation.
(1) At 60 percent VH.
(2) At maximum forward transition speed.
d. Stabilized autorotation to all engines operating at normal autorotation airspeed.
6. AUTOROTATION.
a. Stabilized. (1) At 70 percent VNE.
(2) At VNE.
b. Turns at 70 percent and 100 percent VNE.
(1) Right.
(2) Left.
c. Cyclic pull-up.
d. Control reversals. (1) Longitudinal.
(2) Lateral.
(3) Rudder.
(**) side slip conditions should be considered (*) max slope angle and aircraft headings should be considered Page MG 11 - 22
FIGURE AC 27.MG 11-8
LIFTING HELICOPTER LOAD MEASUREMENT in addition to normal conditions CONDITION %RATED LOAD % VNE
ROTOR START VERTICAL LIFT 100 87 HOVER including spot turns sideward and rearward flight and control reversal.
100 87 CRUISE with load 100 100 87 100 100 90 87 90 CRUISE no load 100 90 CLIMB max. rate DECELERATIONS max. rate
GROUND IDLE
SHUTDOWN AC Page MG 11 - 23
AC 27-1B, Chg 1 2/12/03 FIGURE AC 27.MG 11-9. EXAMPLE OF TWIN TURBINE USAGE SPECTRUM Notes:
1. Altitude, temperature, center of gravity, and weight variations may be appropriate where justified by service data; otherwise the worst case must be assumed throughout.
2. This spectrum is an example only; all probable types of operation must be accounted for unless prohibited--see Section 3.
CONDITION % of time Nb per hour ON GROUND inc START UP 3.5 5 TAXIING 3.5 APPROACH 5 2 NORMAL 3.5 UNDER HEAVY SLOPE 0.75 FLARE 0.375 1 QUICK-STOP 0.375 1 Dur. 14 (s) SPOT-TURN 1 5 30°/S/NORM (STOP)/RIGHT 0.32 1.6 Yaw speed rep.
80.00% 30°
15.00% 45°
5.00% max Stop rep.
80.00% normal
20.00% max 30°/S/NORM (STOP)/LEFT 0.32 1.6 30°/S/MAX (STOP)/RIGHT 0.08 0.4 30°/S/MAX (STOP)/LEFT 0.08 0.4 45°/S/NORM (STOP)/RIGHT 0.06 0.3 45°/S/NORM (STOP)/LEFT 0.06 0.3 45°/S/MAX (STOP)/RIGHT 0.015 0.075 45°/S/MAX (STOP)/LEFT 0.015 0.075 MAX/NORM (STOP)/RIGHT 0.02 0.1 MAX/NORM(STOP)/LEFT 0.02 0.1 MAX/MAX (STOP)/RIGHT 0.005 0.025 MAX/MAX (STOP)/LEFT 0.005 0.025 Dur. 7.2 (s) LANDING 1.5 5 NORMAL 1.482 5.94 1/20 hours slope 8° 1/100 hours slope 10° ON SLOPE/8DEG/RIGHT 0.0043 0.0145 ON SLOPE/8DEG/LEFT 0.0043 0.0145 8DEG/NOSE-UP 0.0038 0.0125 8DEG/NOVE-DOWN 0.0043 0.0145 ON SLOPE/10DEG/NOSE-UP 0.0006 0.002 COMB.SLOPE 0.0006 0.002 Dur. 11 (s) AUTOROTATION 1 1 Speed rep.
5.00% 70Kts
20.00% 70Kts
30.00% 70Kts
25.00% 120Kts
15.00% 135Kts
5.00% VNE RPM rep.
10.00% Mini
50.00% Nomi
36.00% Maxi
4.00% Maxi.trans VERTICAL 0.05 0.05 40 KTS 0.2 0.2 70 KTS 0.3 0.3 120 KTS 0.25 0.25 135 KTS 0.15 0.15 VNE 0.05 0.05 Dur. 36 (s) TAKE OFF 1.5 5 Dur. 11 (s) NORMAL 0.8892 2.964 60.00% Normal
40.00% Quick 1/20 hours slope 8° 1/100 hours slope 10° QUICK 0.5928 1.976 ON SLOPE/8DEG/RIGHT 0.0043 0.0145 ON SLOPE/8DEG/LEFT 0.0043 0.0145 ON SLOPE/8DEG/NOSE-UP 0.0038 0.0125 ON SLOPE/8DEG/NOVE-DOWN 0.0043 0.0145 ON SLOPE/10DEG/NOSE-UP 0.0006 0.002 COMB.SLOPE 0.0006 0.002 DESCENT 1.5 2 PARTIAL POWER/STD/STD 0.96 1.28 Speed rep.
80.00% Partial power
20.00% VNE Side slip PP values rep.
60.00% 5°
30.00% 10°
8.00% 15°
2.00% 20° PARTIAL POWER/5 DEG/RIGHT 0.072 0.096 PARTIAL POWER/5 DEG/LEFT 0.072 0.096 PARTIAL POWER/10 DEG/RIGHT 0.036 0.048 PARTIAL POWER/10 DEG/LEFT 0.036 0.048 Side slip rep.
80.00% Without
20.00% with PARTIAL POWER/15 DEG/RIGHT 0.0096 0.0128 PARTIAL POWER/15 DEG/LEFT 0.0096 0.0128 Side slip VNE values rep.
90.00% 5°
10.00% 10° PARTIAL POWER/20 DEG/RIGHT 0.0024 0.0032 PARTIAL POWER/20 DEG/LEFT 0.0024 0.0032 VNE AT MCP/STD/STD 0.24 0.32 VNE AT MCP/5 DEG/RIGHT 0.027 0.036 VNE AT MCP/5 DEG/LEFT 0.027 0.036 VNE AT MCP/10 DEG/RIGHT 0.003 0.004 VNE AT MCP/10 DEG/LEFT 0.003 0.004 Page MG 11 - 24
FIGURE AC 27.MG 11-9. EXAMPLE OF TWIN TURBINE USAGE SPECTRUM (continued) CONDITION % of time Nb per hour CLIMB 3 4 MCP 75 KTS/STD/STD 1.8 2.4 Climb rep.
75.00% 75 Kts
25.00% Vertical Side slip values rep.
80.00% 05 10
18.00% 15 20
2.00% 25 30 MCP 75 KTS/05-10 DEG/RIGHT 0.18 0.24 MCP 75 KTS/05-10 DEG/LEFT 0.18 0.24 MCP 75 KTS/15-20 DEG/RIGHT 0.0405 0.054 MCP 75 KTS/15-20 DEG/LEFT 0.0405 0.054 MCP 75 KTS/25-30 DEG/RIGHT 0.0045 0.006 Side slip rep.
80.00% Without
20.00% with MCP 75 KTS/25-30 DEG/LEFT 0.0045 0.006 TOP/VERTICAL 0.75 1 Dur. 27 (s) LEVEL FLIGHT 56.5 80% MCP/STD/STD 12.4 Speed rep.
28.3% 80% MCP
36.3% 90% MCP
35.4% MCP Side slip rep.
80.00% Without
20.00% with 80% MCP/5 DEG/RIGHT 1.08 80% MCP/5 DEG/LEFT 1.08 80% MCP/10 DEG/RIGHT 0.54 80% MCP/10 DEG/LEFT 0.54 80% MCP/15 DEG/RIGHT 0.144 80% MCP/15 DEG/LEFT 0.144 Side slip typ rep.
60.00% 5°
30.00% 10°
8.00% 15°
2.00% 20° 80% MCP/20 DEG/RIGHT 0.036 80% MCP/20 DEG/LEFT 0.036 90% MCP/STD/STD 16.3 90% MCP/5 DEG/RIGHT 1.26 90% MCP/5 DEG/LEFT 1.26 90% MCP/10 DEG/RIGHT 0.63 90% MCP/10 DEG/LEFT 0.63 90% MCP/15 DEG/RIGHT 0.21 90% MCP/15 DEG/LEFT 0.21 MCP/STD/STD 17 MCP/5 DEG/RIGHT 0.9 MCP/5 DEG/LEFT 0.9 MCP/10 DEG/RIGHT 0.45 MCP/10 DEG/LEFT 0.45 MCP/15 DEG/RIGHT 0.15 MCP/15 DEG/LEFT 0.15 PULL-UP 0.22 1.1 PULL-UP/VNE 0.01 0.05 PULL-UP/LEVEL FLIGHT 80%MCP 0.1 0.5 PULL-UP/LEVEL FLIGHT 90%MCP 0.1 0.5 PULL-UP/AUTOROTATION 0.01 0.05 Dur. 7.2 (s) HOVERING 10 10 HOVERING/IGE 9 9 HOVERING/OGE 1 1 Dur. 36 (s) TRANSITION 4.52 19.6 TRANSITION/CLIMB TO VMAX 0.292 5 TRANSITION/90%MCP TO V.DESC(ISO-V) 0.292 5 TRANSITION/LEVEL.F TO AUTO.(65 KTS) 0.292 0.5 TRANSITION/FORW.F AFTER REAR.F 0.06 0.1 TRANSITION/HOVERING TO FWD FLIGHT 3 5 Dur. 22 (s) TRANSITION/PITCH INCREASE(65 KTS) 0.292 2 TRANSITION/PITCH DECREASE(65 KTS) 0.292 2 Dur. 3.6 (s) AC Page MG 11 - 25