Appendix C
APPENDIX C. GUIDELINES FOR SUPPLEMENTAL TYPE CERTIFICATES (STC)
AND AMENDED TYPE CERTIFICATES ON AIRPLANES
1. APPLICAT ION.
a. As stated in the “APPLICABILITY” section, the guidance in this AC applies to any STC or amended TC on an airplane for which the applicant wants approval under the provisions of § 23.1419. Increase in gross weight, changes in engine power, and propeller changes could affect approval in icing and these areas would have to be evaluated using AC
23.1419-2C as the method of compliance. An applicant wishing to use an alternate MOC needs to consult the Standards Office. Whether the certification basis for the STC or amended TC includes 14 CFR, § 23.1419 at amendment 23-14 or 23-43 is irrelevant as far as the tests that should be accomplished. The difference in the certification basis does not change the tests that must be accomplished, only their pass/fail criteria.
b. Compliance to icing regulations has been either an afterthought or totally disregarded on many modification programs on aircraft certified for flight in icing conditions. In some cases, the rationale used was that since the ice protection systems were not modified, icing regulations do not need to be addressed. This may be an incorrect assumption. Icing regulations may need to be addressed for any modification that could affect the following in icing conditions:
(1) Aircraft performance
(2) Flying qualities,
(3) Engine operation
(4) Essential system operation.
c. If it is desired to retain flight in icing approval of the modified airplane, the following examples are modifications in which compliance to icing regulations need to be revisited:
(1) Engine changes
(2) APU
(3) Propeller changes
(4) Engine inlet or accessory inlet changes
(5) Antennae installations or other external modifications
(6) Gross weight increases C-1
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(7) CG envelope increase
(8) Flight envelope increase
(9) Turboprop conversion
(10) Modifications to lifting surfaces
(11) Installation of vortex generators
(12) Modifications to ice protection systems
(13) Addition of/or re-location of fuel vents
(14) Addition of/or autopilot replacement.
d. The icing regulations that are addressed in this appendix are:
(1) § 23.929
(2) § 23.1093
(3) § 23.1301
(4) § 23.1309
(5) § 23.1416
(6) § 23.1419
e. The following guidance address some specific, common modifications:
(1) Engine Changes.
(a) Effects of increased engine power or thrust:
1. On airplanes with a certification basis at amendment 23-14 or higher, any degradation in stall characteristics, stability or control, or marginal characteristics, due to increased engine power or thrust, will require re-evaluation with ice accretions. Since the pass/fail criteria are qualitative, testing (original airplane and modified airplane) should be accomplished back to back by the same test pilot. Stall warning should also be evaluated.
Although the margins are not a concern at high power, they need to evaluate if higher power masks any stall warning cues. The following tests should be accomplished:
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• Stall characteristics and stability at minimum weight and maximum weight, aft CG limit
• Controllability at forward CG limit and critical weight
2. Susceptibility to ice contaminated tailplane stall should be addressed for airplanes in which the engine thrust increase is greater than 10 percent or the airplane has a service history of ICTS susceptibility. Maximum power is usually more critical than idle. This cannot be done with analysis on a propeller airplane with reversible controls due to second order effects. Flight testing should be accomplished with 40-grit sandpaper and intercycle/residual ice on horizontal tailplane, intercycle/residual ice on vertical stabilizer, and 45-minute ice accretions on unprotected leading edge tail surfaces.
• Engine Induction Icing. Refer to AC 23-16A for guidance.
(c) Ice Shedding. Engine compliance data to § 33.77 should be compared between the currently installed engine and the proposed engine. If the ice slab used to show compliance is smaller for the proposed engine, ice shedding from the airframe should be readdressed. Data on ice shedding may be available from the airplane TC holder. Engine inlet lip ice shedding should be addressed. The amount of ice mass that could be shed should be compared to a similar, approved engine installation or to part 33 engine compliance data.
(d) Effects of Decreased Engine Power, Thrust or Bleed Air.
1. Ice Protection System Operation. Bleed air mass flows, pressures and temperatures of the proposed engine and of the existing, certified engine should be compared. If there is a reduction, the effectiveness of the ice protection systems must be substantiated.
2. Airplane Performance. Airplane performance in icing conditions should be re-evaluated.
(2) Essential APU. When an essential APU is modified or added, operation in icing conditions should be addressed similarly to engines since essential APUs are covered by § 23.1093.
(3) Propeller Changes.
(a) Section 23.929 states that propellers (except wooden propellers) and other engine installations must be protected against the accumulation of ice as necessary to enable satisfactory functioning without appreciable loss of thrust when operated in the icing conditions for which certification is requested.
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(b) If the deicing system is listed on the propeller TCDS, it does not indicate that compliance to § 23.929 was shown. It means that the deicing system was shown to function properly, the deicing system complies with propeller structural and vibration regulations, and deicing system failure modes, as discussed in § 23.929 of AC 23-16, cannot cause an unairworthy condition.
(c) The typical analysis report from the deicing boot manufacturer is not sufficient by itself to show compliance to § 23.929. The typical report calculates intercycle ice thickness for various flight and icing conditions, but does not calculate the effect on propeller efficiency, which must be done to show no appreciable loss of thrust. For STCs, it would be acceptable to show that intercycle ice is equal to or less than the accretions obtained on the same propeller on an airplane that was flight tested in icing conditions and shown to have no appreciable loss in thrust.
(d) The typical deicing boot manufacturer report also contains a caveat that it does not address propeller runback ice. Similarity to another propeller that was flight tested in icing conditions is usually done to address runback. Similarity would include propeller and deicing boot aerodynamic and thermal similarity, deicing cycle time, propeller RPM, and flight conditions. Note that metal and composite propellers have different thermal masses.
(e) As a final qualitative check for both intercycle and runback ice on new airplane programs, airplane performance is checked during flight tests in icing conditions. A test point as close to minus 22 degrees F as possible should be included in the flight tests.
(f) The propeller installation, including spinner and cowl geometry, must be compared to previously tested installations in icing conditions. Changes that could allow moisture to reach the brush blocks must be avoided.
(g) If the proposed propeller is calculated to have higher efficiency than the existing, approved propeller, the guidance in paragraph 1e(1)(a) of this appendix should be followed.
(h) If the proposed propeller(s) and/or deicing system are predicted to increase the size of intercycle ice, the effects of propeller ice shed onto other parts of the airplane should be addressed.
(i) For New Propeller Deice Electrical Power Systems:
1. The surface temperature characteristics of the propeller boots should be shown to be the same as original certified system.
(aa) If the temperature characteristics and deice timing cycle are shown to be changed, flight testing in measured natural icing conditions are required to evaluate propeller deicing and airplane performance C-4
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(bb) If the temperature characteristics and deice timing cycle are shown to be unchanged, a demonstration of propeller deicing and airplane performance in natural icing conditions should be performed.
(cc) Flight testing should be accomplished as close to –22 degrees F as possible.
2. A rational analysis of the heat generated by the system should be made and compared to the existing system if the system is located in areas where ice accretion and runback could be affected, such as the spinner.
(4) Engine Inlet or Accessory Inlet Changes. Guidance is provided in the “engine changes” section above. It should be noted that § 23.1093 applies to engine oil and accessory cooling inlets as well as induction inlets.
(5) Antennas, Installations or Other External Modifications.
(a) When antennas, cameras, fairings for such installations, or other external installations such as drain masts are installed on aircraft, the installer should show the following:
1. The predicted ice accretion does not contribute significantly to drag;
2. There is no ice-shedding hazard due to impact or ingestion on downstream structure, engines or propellers. See paragraph 13 of the AC for guidance on ice shedding.
3. There is no ice related reduction performance of lifting surfaces;
4. There is no ice related effect on downstream air data sensors or ice detectors.
(b) A very conservative, simple analysis may be accomplished first to show the objectives (a)1 and (a)2. If the conservative analysis fails, the analysis can be refined to determine if the initial analysis was overly conservative. The conservative analysis can assume the following:
1. The water catch area is the full frontal area of the installation;
2. Collection efficiency is one.
3. No runback or evaporation of impinging water.
4. Assume the shape on blade antenna will be similar to airfoils and the shape on low profile antennae will be single horn shapes.
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(c) The installer should determine the critical icing condition, and the 45-minute hold in continuous maximum conditions needs to be included. If the analysis shows a problem, then one or more of the following can be accomplished:
1. Determine realistic collection efficiency either with an ice accretion code or with the “FAA Icing Handbook”;
2. Determine the real impingement limits by using an icing code, which may reduce the collection area;
3. Run the full configuration in an icing code to determine if the installation is in a shadow zone.
4. If drag is a problem, run an ice accretion code to determine a more realistic ice shape.
(d) Flight tests in measured natural icing or with simulated ice shapes should be accomplished to determine if there are any detrimental effects due to the ice accretions if:
1. The installation is upstream of air data sensors or an ice detector; or
2. The installation is on a lifting surface or
3. The installation could create a wake on a lifting surface. As an example, if an external modification is large enough (e.g. dish antenna), it may interfere with the flow field around the tail and the susceptibility to ICTS may need to be addressed.
(e) The one exception to (d) 2 is fairings. An analysis to show the impact on maximum lift coefficient, in combination with flight tests with no ice accretions, may be acceptable.
(6) Gross Weight Increases.
(a) At the increased angle of attack for a given airspeed, the impingement limits will change. An impingement analysis needs to be accomplished to show the ice protection coverage remains adequate.
(b) The impingement analysis should also evaluate unprotected areas such as fuel vents.
(c) If the following flight testing with no ice show no degradation from the unmodified aircraft, and no marginal characteristics, flight testing with ice (or simulated ice accretions) are not required:
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1. Stall warning, stall characteristics, and stability at maximum weight, aft CG limit
2. Stall speeds and controllability at maximum weight, forward CG limit.
(d) Operational speeds and AFM/POH performance data in icing conditions need to reflect higher stall speeds.
(e) An analysis should show increased weight makes the airplane equal or less susceptible to tailplane stall. The analysis should evaluate tail trim requirements and tail ice accretion at the higher airplane angle of attack.
(7) CG Envelope Increase. Generally, the same guidance used for gross weight increases can be used for CG envelope increases. The one exception is when an increase of forward center of gravity limit on airplanes makes an airplane more susceptible to ice contaminated tailplane stall. This should be addressed by flight testing for airplanes with unpowered, reversible elevators or with propellers. An analysis may be acceptable for other configurations.
(8) Flight Envelope or Operating Procedure Changes.
(a) If an increase in maximum operating altitude is applied for, the applicant should demonstrate:
1. The ice protection system operating pressures (for pneumatic systems) or temperatures (for hot air systems) by dry air testing; and
2. The stall speeds and stall characteristics associated with ice accretions if these are shown to be influenced by Mach number.
(b) The effect of increased cruise airspeeds and increased altitudes that could affect windshield ice accretion, and adequacy of the windshield heat, should be addressed.
(c) The effect of different operating airspeeds and altitudes that could affect critical ice accretions.
(9) Turboprop Conversion.
(a) If the ice protection systems utilize engine bleed air for operation, the pneumatic lines may accumulate more water than the current unmodified type design. This water can subsequently refreeze and block the pneumatic lines, resulting in failure of some or all zones of the pneumatic system. The applicant needs to show that the pneumatic deicing system will continue to function in icing conditions.
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(b) The pneumatic deicer operating pressure may also decrease at lower engine RPMs. A minimum engine RPM for acceptable pneumatic operating pressure, which should allow for descent, should be established and published in the AFM/POH.
(10) Modifications to Lifting Surfaces.
(a) Critical ice accretions (including pre-activation, intercycle, residual, and runback) may have to be re-defined, especially if the changes affect wing angle of attack. Stall strips are good collectors of ice and are an example where leading edge ice accretions should be re-defined. If ice accretions are changed or the modifications could affect control power or hinge moments, flight testing with simulated ice accretions should be accomplished to evaluate one or more of the following:
1. Stall warning, stall characteristics, and stability at maximum weight, aft CG limit;
2. Stall speeds, stall warning, controllability and performance at maximum weight, forward CG limit;
3. ICTS susceptibility at light weight, forward CG limit if the aircraft has unpowered, reversible elevators or propellers.
4. For unprotected winglets, flutter margins needs to be addressed.
(b) Susceptibility to ICTS should be addressed for either horizontal or vertical tail modifications or wing modifications that are predicted to increase ICTS susceptibility. ICTS susceptibility may be addressed by analysis on jet-powered aircraft with irreversible elevator controls.
(11) Installation of Vortex Generators.
For vortex generators that are installed near the leading edge, the applicant should provide data on expected ice accretions. Flight conditions to consider are the 45-minute hold, descent, and approach. Substantiation of the effects on stall speeds, stall characteristics, and stability and control should be provided.
(12) Modifications to Ice Protection Systems.
(a) Critical ice accretions may have to be re-defined. If ice accretions are changed or the modifications could affect control power or hinge moments, flight testing with simulated ice accretions should be accomplished to evaluate one or more of the following:
1. Stall warning, stall characteristics, and stability at maximum weight, aft CG limit;
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2. Stall speeds, stall warning and controllability at maximum weight, forward CG limit;
3. ICTS susceptibility at light weight, forward CG limit if the aircraft has unpowered, reversible elevators or propellers.
(13) Addition of or Re-location of Fuel Vent.
As a minimum an impingement analysis and/or similarity should be used to show that ice does not obstruct the fuel vents.
(14) Addition or Replacement of Autopilot
(a) Guidance in this AC, paragraphs 11 and 12 for autopilots should be consulted.
There are specific scenarios in which autopilots can get the pilot into trouble in an airplane approved for flight into known icing. Those scenarios resulted in accidents and are factual.
Based on our service experience, even though there are no regulatory requirements addressing autopilots in airplanes approved for known icing, applicants are strongly encouraged to include features that mitigate these autopilot induced accident scenarios. Where it would be impractical to add such a feature, the design should include adequate trim in motion cues. For replacement autopilots, the design of the original and replacement autopilots should be compared.
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Appendix D
APPENDIX D. GUIDELINES FOR CERTIFYING ICE PROTECTION SYSTEMS
ON AIRPLANES NOT CERTIFICATED FOR FLIGHT IN ICING
1. APPLICABILITY. There may be times when applicants may want to certificate an ice protection system installation on an airplane that is to remain not certificated for flight in icing.
This used to be called a “non-hazard” basis. This means that the aircraft is prohibited from flight in icing conditions but there is some ice protection to facilitate an exit from an inadvertent icing encounter. The following guidance provides a reference; novel systems may require additional considerations.
2. SUBPART B – FLIGHT.
a. The applicant must show that installation of the system (not operating) does not affect performance, stalls, controllability, maneuverability, stability, trim, ground/water handling, vibration and buffet, and, if applicable, high speed characteristics. If any of these are affected, it should be shown that applicable regulations are still complied with and place the appropriate information in the AFM. Compliance should be accomplished with dry air flight tests. If the system is being evaluated as an amended TC or STC, it is not necessary to investigate all weight and CG combinations and flight conditions when results from the airplane certification testing clearly indicate the most critical combination to be tested.
b. In some cases the effect of system operation may need to be evaluated. For pneumatic deicing boots, the operation of the boots (inflation) should have no hazardous affect on airplane performance and handling qualities. The effect of pneumatic boot operation on stall speed and stall warning should be evaluated and appropriate information placed in the AFM. Freezing point depressant systems when operating have been shown to increase drag.
3. SUBPART D – DESIGN AND CONSTRUCTION.
a. The ice protection systems should be evaluated to determine the impact to the airframe structure per the applicable regulations in subpart D. The load conditions determined from Subpart C should be used in this evaluation. The individual ice protection system components should also be evaluated to determine that they would withstand the load conditions from Subpart C if their failure can cause a hazard. Thermal effects on structure of thermal ice protection systems and fluid/structure compatibility of freezing point depressant systems should be evaluated.
b. An evaluation of flutter characteristics to account for the added mass of the ice protection systems should be made per § 23.629.
c. If a thermal windshield ice protection system is installed, an evaluation of the visibility due to distortion effects through the protected area should be made. In accordance with § 23.775(g), a probable single failure of a transparency heating system should not adversely affect the integrity of the airplane cabin or create a potential danger of fire.
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d. For wing and empennage electrical deicing systems, the indirect effects of lightning maybe an issue. The airplane must be shown to be protected against catastrophic effects from lightning in accordance with § 23.867. As a minimum the effect of the ice protection system installation should be addressed by analysis and design.
4. SUBPART E – POWERPLANT. For an airplane not certificated for flight in icing, compliance to § 23.929 is not required but compliance to § 23.1093 is required.
5. SUBPART F – EQUIPMENT.
a. On airplanes with a certification basis of amendments 23-20 and higher, compliance to § 23.1301 must be shown. Compliance to § 23.1301 would entail a functional flight test in dry air supplemented by previous certification data on other, similar installations that showed the system functioned in part 25, Appendix C icing. If this data were not available the applicant would have to perform some testing in icing conditions.
b. Compliance to the latest applicable amendment of § 23.1309 should be shown. See AC 23.1309-1C for additional guidance. To show compliance to § 23.1309 the following would apply:
(1) Show that installation of the system, and normal operation of the system, does not affect operation of essential equipment. This should include electromagnetic interference testing.
(2) Show that hazards are minimized on single engine airplanes and prevented for multi-engine airplanes in the event of a probable failure. Examples of failures that should be addressed:
(a) Auto inflation of deicing boots
(b) Failures that could cause an asymmetric wing condition
(c) Bleed air leaks of thermal systems
(d) Electrical shorts in electrothermal systems.
(3) Compliance can be by analysis or test or a combination. The loss of the ice protection system would not have to be considered since the airplane is not approved for flight in icing. For the purposes of the current regulation, the system would not be an essential load.
(4) Show that the system when operating normally does not create a greater hazard than operating with no ice protection system. For example, on systems where there is runback the applicant should show that the runback ice does not cause a greater hazard than the ice accretion with no ice protection. Hazards to address would be stalls, tailplane stalls, and engine operation if applicable.
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