ED Decision 2003/15/RM
(a) The air induction system for each engine must supply the air required by that engine under the operating conditions and manoeuvres for which certification is requested.
(b) Each cold air induction system opening must be outside the cowling if backfire flames can emerge.
(c) If fuel can accumulate in any air induction system, that system must have drains that discharge fuel –
(1) Clear of the rotorcraft; and
(2) Out of the path of exhaust flames.
(d) For turbine engine-powered rotorcraft:
(1) There must be means to prevent hazardous quantities of fuel leakage or overflow from drains, vents, or other components of flammable fluid systems from entering the engine intake system; and
(2) The air inlet ducts must be located or protected so as to minimise the ingestion of foreign matter during take-off, landing, and taxying.
CS 27.1093 Induction system icing protection
ED Decision 2003/15/RM
(a) Reciprocating engines. Each reciprocating engine air induction system must have means to prevent and eliminate icing. Unless this is done by other means, it must be shown that, in air free of visible moisture at a temperature of –1°C (30°F) and with the engines at 75% of maximum continuous power:
(1) Each rotorcraft with sea-level engines using conventional venturi carburettors has a preheater that can provide a heat rise of 50°C (90°F);
(2) Each rotorcraft with sea-level engines using carburettors tending to prevent icing has a sheltered alternate source of air, and that the preheat supplied to the alternate air intake is not less than that provided by the engine cooling air downstream of the cylinders;
(3) Each rotorcraft with altitude engines using conventional venturi carburettors has a preheater capable of providing a heat rise of 67°C (120°F); and
(4) Each rotorcraft with altitude engines using carburettors tending to prevent icing has a preheater that can provide a heat rise of:
(i) 56°C (100°F); or
(ii) If a fluid de-icing system is used, at least 22°C (40°F).
(b) Turbine engines
(1) It must be shown that each turbine engine and its air inlet system can operate throughout the flight power range of the engine (including idling):
(i) Without accumulating ice on engine or inlet system components that would adversely affect engine operation or cause a serious loss of power under the icing conditions specified in appendix C of CS-29; and
(ii) In snow, both falling and blowing, without adverse effect on engine operation, within the limitations established for the rotorcraft.
(2) Each turbine engine must idle for 30 minutes on the ground, with the air bleed available for engine icing protection at its critical condition, without adverse effect, in an atmosphere that is at a temperature between –9°C and –1°C (15° and 30°F) and has a liquid water content not less than 0.3 grams per cubic metre in the form of drops having a mean effective diameter of not less than 20 microns, followed by momentary operation at take-off power or thrust. During the 30 minutes of idle operation, the engine may be run up periodically to a moderate power or thrust setting in a manner acceptable to the Agency.
(c) Supercharged reciprocating engines. For each engine having superchargers to pressurise the air before it enters the carburettor, the heat rise in the air caused by that supercharging at any altitude may be utilised in determining compliance with sub-paragraph (a) if the heat rise utilised is that which will be available, automatically, for the applicable altitude and operating condition because of supercharging.
AMC1 27.1093(b)(1)(i) Induction system icing protection
ED Decision 2023/001/R
This AMC is primarily applicable to rotorcraft equipped with air intake external screens (or any other air intake prone to the same kind of icing which may exist downstream), and has been developed based on in-service experience.
In icing conditions, as defined in CS-27 Appendix C, when the outside air temperature (OAT) is quite cold, typically below -5°C, the water droplets freeze at the helicopter air intake external screen that, once clogged, acts as passive protection by preventing subsequent super-cooled droplets to enter the engine duct and plenum. The air, then, enters the engine intake through screen areas where water droplets do not accrete, or through an air intake by-pass, if necessary.
For warmer temperatures, typically between -5°C and 0°C, a critical temperature can exist at which the water droplets do not freeze completely and immediately on the external screen and therefore icing conditions may exist downstream in the engine air intake ducts or engine internal screen.
Furthermore, ice accretions behind the air intake screen can then be released during an engine acceleration or a rotorcraft descent in a warmer atmosphere and thus may lead to engine damage, surge or in-flight shutdown.
In the case where the engine is also protected by its own screen, then the engine screen can then become clogged by ice. This may also lead to high pressure drop or distortion across the engine screen, resulting into engine surge, engine damage or engine shutdown.
The purpose of this AMC is to provide specific and complementary guidance for demonstrating compliance with CS 27.1093(b)(1)(i) in the determination of this critical temperature, but does not provide any other guidance to demonstrate full compliance with CS 27.1093(b)(1)(i) to cope with icing conditions as detailed in Appendix C to CS-27.
Analysis only should not be considered in the determination of the critical temperature due to the level of accuracy required for such an assessment. Its determination should be validated during combined rotorcraft (air intake / engine) icing tests in a wind tunnel or a similar test facility where the temperature can be controlled accurately showing whether icing conditions downstream the air intake screen are an issue or not. Typically, an accuracy of 0.5°C could be envisaged.
If the above-mentioned testing is done without the engine, it should be first demonstrated that the engine flow is correctly simulated, and the engine thermal impact adequately considered and validated on air intake. In a second step, the repercussion of any ice accretion should be assessed at engine level both in terms of airflow distortion and engine ingestion and duly validated by appropriate means. It has to be noted that this alternative approach without the engine may lead to difficulties in interpreting the results at engine level.
During these tests, the engine should be run at critical power in the icing conditions defined in CS-29 Appendix C depending on the claimed certification (inadvertent icing encounter or full icing certification). The critical power could be determined following a critical point analysis (other methodologies might be acceptable) to assess the engine operability with regard to the feared events such as airflow distortion or engine ice ingestion.
To determine the temperature at which the water does not freeze on the external screen, the test temperature may be decreased by accurate steps (typically a value of 0.5°C is suggested) from 0°C until accretion downstream the external air intake screen, if any, is maximised. If no ice is observed after 15 minutes of water injection, the test point is believed to be performed at a too warm temperature and can be stopped.
When decreasing the temperature step by step, if no ice accretion is observed downstream the helicopter external screen — typically for temperatures below -5°C the external screen catches the majority of the super-cooled droplets — it means that the above-described phenomenon does not occur.
Some other method can be proposed to reduce the test point number.
The test should demonstrate that, at the determined critical temperature, the maximum potential ice accretions downstream the rotorcraft screen do not have an adverse effect on the engine both in the full range of claimed operation and when the rotorcraft then descends in an atmosphere with a positive OAT.
As an example, the following test procedure may be considered:
— A 1st run: at the end of the test (in fact, when reaching the highest measured pressure drop in the air intake), perform three consecutive engine quick decelerations (from maximum power to idle) / accelerations (from idle to maximum power).
— A 2nd run: at the end of the test (in fact, when reaching the highest measured pressure drop in the air intake), simulate a quick descent in atmosphere with a positive OAT considering a tunnel warm-up procedure.
Quick accelerations / decelerations are to be understood as the maximum acceleration / deceleration rates that can be performed by a pilot during flight operation. The intent is to simulate a real-life engine behaviour which affects the flow/ice ingestion accordingly. For example, values close to one second from minimum to maximum power have been considered in the past for such testing.
As specified in CS 27.1093(b)(1)(i), these tests shall demonstrate that the engine operation is not adversely affected by icing conditions.
Whenever an applicant is willing to use previous icing wind tunnel tests, an analysis might be an acceptable means of compliance provided that this analysis is adequately validated and covers as a minimum the changes in configurations (air intakes, engines, engine installations, etc.), engine operability (airflow, ingestion capabilities, surge margins, etc.) and thermal environment of the air intake.
For rotorcraft certified in full icing conditions, in order to determine the rotorcraft performance in icing conditions, this test point should be used to identify the engine installation losses for flight into known icing conditions, in particular if the engine is also equipped with its own screen.
[Amdt 27/10]