AMC 25.929(a) Propeller de-icing

ED Decision 2016/010/R

1.       Analysis.

The applicant should perform an analysis that:

(1)     substantiates ice protection coverage in relation to chord length and span.

(2)     substantiates the ice protection system power density.

(3)     consider the effect of intercycle ice accretions and potential for propeller efficiency degradation for all flight phases.

(4)     assess the different propeller Ice Protection System failure modes which are not extremely improbable and leading to the:

(i)      highest propeller performance level degradation, and

(ii)     highest propeller vibration levels taking also into account possible ice shedding.

(5)     assess the impact of ice released by the propeller on the vibration levels, the adjacent components (if any) and the aircraft structure, both for normal operation and in the different propeller de-icing system failure modes.

Similarity to prior designs with successful service histories in icing may be used to show compliance. A demonstration of similarity requires an evaluation of both system and installation differences. The applicant should show specific similarities in the areas of physical, functional, thermodynamic, pneumatic, and aerodynamic characteristics as well as in environmental exposure. The analysis should show that propeller installation, operation, and effect on the aeroplane’s performance and handling are equivalent to that of the same or similar propeller in the previously approved configuration. Differences should be evaluated for their effect on IPS functionality and on safe flight in icing. If there is uncertainty about the effects of the differences, the applicant should conduct additional tests and/or analysis as necessary and appropriate to resolve the open issues.

For showing compliance with the CS-25 certification specifications relative to SLD icing conditions represented by Appendix O, the applicant may use a comparative analysis. AMC 25.1420(f) provides guidance for comparative analysis.

2.       Compliance Tests.

2.1     Surface temperature measurements should be made and monitored in dry air flight testing. These measurements are useful for correlating analytically predicted dry air temperatures with actual temperatures, and as a general indicator that the system is functioning and that each de-icer is heating. It is suggested that system current, brush block voltage (i.e., between each input brush and the ground brush) and system duty cycles be monitored to ensure that adequate power is applied to the de-icers.

2.2     System operation should be checked throughout the full rotation speed range. and propeller cyclic pitch range expected during flight in icing. Additionally, if the propeller Ice Protection System is regulated based on different outside parameters such as temperature, then system operation should also be checked against those parameters. All significant vibrations should be investigated.

2.3     The analysis assessing the effect of intercycle ice accretions and potential for propeller efficiency degradation should be adequately validated by tests.

2.4     The Ice Protection System failure modes determined in 1.4 above should be adequately validated by tests.

2.5     The applicant should consider the maximum temperatures a composite propeller blade may be subjected to when de-icers are energized. It may be useful to monitor de-icer bond-side temperatures. When performing this evaluation, the most critical conditions should be investigated (e.g., aeroplane on the ground; propellers not rotating) on a hot day with the system inadvertently energized.

2.6     Shedding procedures and post failure procedures mentioned in the AFM should be demonstrated by test.

3.       Runback Ice.

Water not evaporated by thermal ice protection systems and unfrozen water in near-freezing conditions (or in conditions when the freezing fraction is less than one) may run aft and form runback ice. This runback ice can then accumulate additional mass from direct impingement. Computer codes may be unable to estimate the characteristics of the runback water or resultant ice shapes (rivulets or thin layers), but some codes may be able to estimate the mass of the runback ice. Thus runback ice should be determined experimentally, or the mass determined by computer codes with assumptions about runback extent and thickness similar to those used successfully with prior models. The runback ice should be determined both for normal operation and for propeller Ice Protection System failure modes when not operating in the predefined cycles.

The applicant should consider potential hazards resulting from the loss of propeller performance, the increased vibration level and the runback ice shedding.

[Amdt 25/16]

[Amdt 25/18]