gradually exceed the defined temperature limits. To address this concern, some manufacturers have used engine operating limitations for ground operations that limit engine operation for discrete time periods. For example, engine operation at takeoff power or thrust may be limited to 2 minutes at ambient temperatures above 110 °F (43 °C), or engine operation at idle power or thrust is limited to 2 hours at ambient temperatures above 110 °F.
27.1.3.4 Temperature Correction.
Recorded ground temperatures should be corrected to the maximum ambient temperature selected, without consideration of the altitude temperature lapse rate. For example, if an auxiliary power unit is being tested for ground cooling margins, the cooling margin should be determined from the recorded ground temperature without regard to the test site altitude.
27.1.4 Data Acquisition.
All of the following data should be recorded at the time intervals specified in the particular test program. The data may be manually recorded unless the quantity and frequency necessitate automatic or semi-automatic means:
Outside air temperature (OAT).
Altitude.
Airspeed (knots).
Gas generator RPM.
Engine torque.
Time.
Fan RPM.
Engine oil temperature.
Pertinent engine temperature.
Pertinent nacelle and component temperatures.
27.1.5 Data Reduction.
27.1.5.1 Purpose.
Seldom is testing actually accomplished at the maximum required ambient temperature of at least 100 °F at sea level lapsed 3.6 °F (2 °C) per 1,000 feet pressure altitude. Component and fluid temperatures must therefore be corrected to derive the item temperature that would have been reached if the test day had matched exactly the maximum ambient temperature day. The applicant may select a higher maximum ambient temperature for cooling certification than the 100 °F (38 °C) sea level hot 27-6
day prescribed. Provisions are also made for selecting a maximum ambient temperature less than the 100 °F sea level hot day for winterization installations not intended to function at the hot day conditions.
27.1.5.2 Correction Factors for Ambient Conditions.
Unless a more rational method applies, a correction factor of 1.0 is applied to the temperature data as follows: co rrect ed temperature = true temperature + 1.0 [100 - 0.0036 (Hp) - true OAT]. A correction factor other than “degree-for-degree” should be based on engineering test data.
The corrected temperature is then compared with the maximum permissible tempe rature to determine compliance with the cooling requirements. No corrected temperatures may exceed establis hed limits.
Sample Calculation:
27.1.5.3 Correction Factor for Minimum Engine.
An important correction factor that is not discussed in the regulations but is frequently necessary to show the cooling adequacy required by § 25.1041, is the minimum engine (i.e., the thermal limit) correction factor. This factor is sometimes required if, at test day conditions, the engine measured temperature does not correspond to the engine temperature that would have occurred on a minimum specification engine in hot day conditions. The correction factor would not apply to those components not affected by changes in exhaust gas temperature (EGT) at a constant power or thrust. Typical items expected to be affected by changes in the EGT at constant power or thrust would be engine oil temperature, thermocouple harnesses, or other fluid, component, or ambient temperatures in the vicinity of the engine hot-section or exhaust gases. Other items remote from the hot section, like the starter-generator or fuel control, would not be expected to be influenced by EGT variations;
however, the items affected and the magnitude of the factor to be applied should be established by testing. There are several acceptable methods for establishing the appropriate correction factor during development testing.
The general idea is to establish a stabilized flight condition, typically during ground runs, and vary the measured EGT at approximately fixed power or thrust and OAT conditions. This may be accomplished by using engine anti-ice bleed air, customer bleed air, or by ingesting warmer than ambient air (either an external source or the engine bleed air) into the engine inlet. Care should be used when ingesting warmer than ambient air to assure that the warm air is diffused in order to avoid possible engine 27-7
surge. If it is not possible to attain an adequate variation in EGT by these methods, an acceptable, but conservative correction may be obtained by allowing both power or thrust and EGT to vary at stabilized engine operating conditions and OAT. The component temperature is plotted as a function of EGT, and the correction from test EGT for any flight condition, to the EGT that would have existed with minimum specification engines on a hot day, is then applied to derive the corrected component temperature. Both of these methods assume the inlet air and cooling air sources are essentially independent. Where they are not independent, such as those designs that take cooling air from the fan exit stream (e.g., via a fan box in bifurcation) rather than via a free stream scoop, care needs to be taken in selecting a technique to assure the results are neither overly conservative as may result from use of the hot air technique nor overly optimistic as may result from use of the higher power or thrust technique.
27.2 [Reserved.] 27-8
AC 25-7D CHG 1 CHAPTER 28. POWERPLANT: INDUCTION SYSTEM
28.1 Air Induction—§ 25.1091.
28.1.1 Explanation—§ 25.1091(d)(2).
The turbine engines of transport category airplanes are susceptible to surge, stall, and flameout when excessive amounts of water are ingested. The certification requirements for turbine engines include the demonstration of a capability to operate in simulated rainfall with no adverse operating effects. The quantities of water spray that may be directed toward the engine inlets, resulting from the airplane passing through standing water on taxi and runways, may exceed that used in the simulated rainfall ingestion testing. This becomes particularly important during takeoff, where the engines are operated at high power or thrust settings and the airplane will experience a wide range of speeds. During takeoff and landing ground rolls, the airplane’s tires generate bow waves, side spray, and “rooster tails” (spray that is thrown off the tires as they rotate), which can collect into concentrated streams of water that, if ingested into the engines, APU, or air conditioning systems, could affect operation sufficiently to cause an unsafe condition. Similarly, water ingestion into the airspeed system during ground operations may cause errant cockpit indications of airspeed. It is often practical to investigate these effects concurrently with the engine water ingestion evaluation.
28.1.2 References.
AC 20-124.
AC 91-79B.
28.1.3 Procedures.
28.1.3.1 Method of Compliance.
The applicant may show compliance with the requirements of § 25.1091(d)(2) by test or by reasonable analysis. The analytical approach may be acceptable in cases where an airplane and/or its engines were modified, but the overall geometry and configuration remained unchanged.
28.1.3.2 Water Depth.
Takeoffs should not be attempted when the depth of standing water, slush, or wet snow is greater than one-half inch over an appreciable part of the runway. (See AC 91-79A). Therefore, one-half inch of standing water is the accepted criteria for conducting tests to demonstrate compliance with the water ingestion requirements of § 25.1091(d)(2). Testing may be conducted in specially constructed water test beds, where the selected water depth is maintained over not less than 90 percent of the test bed area. If the airplane successfully completes testing with a one-half inch water depth, no operating limitations will be imposed for wet runway operations.
28-1
28.1.3.3 Airplane Configuration.
Testing should be conducted to demonstrate all critical phases of taxi, takeoff, and landing operations. High lift devices and landing gear doors should be in the position appropriate to each phase of operation tested. All portions of the airplane that may affect water spray patterns should be in the production configuration desired for approval (e.g., chine tires, fenders). The most critical powerplant operating configuration (bleed air, electrical load, ignition system, etc.) that is consistent with the applicant’s recommended procedures for operation on wet runways should be used.
28.1.3.4 Test Facility.
The test facility needs to have adequate space for simulated takeoff and landing runs. Intermediate dams may be used in the water test bed to maintain the selected water depth over not less than 90 percent of the test bed area. The test bed length should not be less than that required to produce a spray pattern of one second duration at critical test speeds. The test bed width need only accommodate one landing gear, if acceptable data are presented to the FAA that show the combined nose and main gear spray is not more hazardous than the spray produced by either gear separately.
28.1.3.5 Test Procedures.
The tests should be conducted with the appropriate power or thrust setting.
To establish the critical speed ranges, test runs should be made through the water test bed in speed increments of not more than 20 knots. If the rotated airplane attitude is suspected to be critical, at least one run at VR should be conducted with the airplane rotated to the normal rotation pitch attitude. If thrust reversers or reversing propellers are provided, testing should also be conducted with those systems operating.
28.1.3.6 Test Data.
The applicant is expected to provide personnel and equipment to collect the following data:
28.1.3.6.1 Test Site Data.
The outside air temperature, wind velocity, and water depth should be recorded before each test run.
28.1.3.6.2 Airplane Performance Data.
The airplane velocity should be recorded when it enters and exits the water test bed. Engine rotor speeds and interstage turbine temperature (ITT) or EGT, as applicable) should be recorded and any engine abnormal engine sounds noted.
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28.1.3.6.3 Water Spray Pattern.
Appropriate areas of the airplane should be coated with agents that will permit identification of water spray impingement. Suitable high-speed photography or video equipment should be used to record the origin, trajectory, and configuration of the water spray.
28.1.3.6.4 Pilot Comments.
Pilot comments should be noted to determine if the takeoff would have been affected by abnormal audible engine sounds, airplane instrument anomalies, etc.
28.1.3.7 Test Results.
No hazardous quantities of water are considered to have been ingested when review of the collected test data shows that:
28.1.3.7.1 No engine flameout, performance degradation, distress, or airspeed fluctuations occurred that would create a safety hazard; and
28.1.3.7.2 No abnormal engine sounds occurred, such as pops or bangs, or cockpit instrument indications of incipient engine surge or stall, or they were not of sufficient magnitude to cause a pilot in service to abort the planned operation.
28.2 Induction System Icing Protection—§ 25.1093.
28.2.1 References.
AC 20-73A.
AC 20-147A.
28.2.2 Reciprocating Engines—§ 25.1093(a).
28.2.2.1 Explanation.
28.2.2.1.1 Conditions for Tests.
The carburetor air temperature measurement has been found to provide satisfactory average readings through the use of a minimum of three thermocouples so arranged as to give an average air temperature. This indicator should be calibrated prior to the test. Operationally, it has been determined that the tests should be conducted at an altitude where the free air temperature is 30 °F (-1 °C), or at two altitudes of different temperatures, one of which is near 30 °F (-1 °C).
28-3
28.2.2.1.2 Configuration.
The test should be conducted in the configuration that follows:
Weight—optional.
CG Position—optional.
Wing Flap Position—optional.
Landing Gear Position—optional.
Engines—60 percent maximum continuous power.
Cowl Flaps—appropriate for flight condition.
Mixture Setting—normal cruising position.
28.2.2.2 Procedures.
Test procedures and required data.
28.2.2.2.1 After all temperatures have stabilized (i.e., when the rate of temperature change is less than 2 °F (1 °C) per minute), and with the airplane in level flight and full cold carburetor at 60 percent maximum continuous power, the following data should be recorded:
Pressure altitude.
Ambient air temperature.
Indicated airspeed.
Carburetor air temperature.
Engines’ RPM and manifold pressure.
Torque pressure.
Mixture setting.
Cowl flap setting.
28.2.2.2.2 Preheat should then be applied slowly (power may be restored to 60 percent maximum continuous at the applicant’s option) and the above data recorded again after the temperatures have stabilized. The carburetor heat rise is determined by comparing the results of the data obtained with and without preheat.
28.2.3 Turbine Powered Airplanes—§ 25.1093(b).
28.2.3.1 Explanation.
28.2.3.1.1 Section 25.1093(b) requires that each turbine engine operate throughout its flight power or thrust range without adverse effect on engine operation or serious loss of power or thrust in the icing conditions specified, including 28-4
those related to falling and blowing snow. This requirement must be met for all operating conditions of the airplane and is not limited to operations where icing can be predicted or when icing penetrations are specifically intended. It is clear that the engines require protection against the possible hazardous effects of ice ingestion for all operating conditions, while the remaining airframe need be protected only if certification for flight in icing conditions is desired. This can also be verified by a review of § 25.1093(b) and 25.1419 and 14 CFR 33.68.
28.2.3.1.2 One purpose of flight in natural and/or tanker-provided icing conditions is to demonstrate that chunks of ice discharged from unprotected surfaces do not cause damage to the engine or other critical parts of the airplane, and to demonstrate that ice discharged from protected surfaces after an undetected icing encounter, during which the ice protection system was inoperative, does not cause engine damage or malfunction, or damage to other critical parts of the airplane. In order to accomplish these objectives, the airplane should be exposed to an icing condition of a magnitude and duration sufficient to accumulate enough ice to produce an acceptable demonstration. If, after installation on the airplane, it is possible for the engines to ingest ice shed from the airframe, which will cause an adverse effect or serious loss of power or thrust on the engine, compliance with § 25.1093(b) has not been achieved, and the required level of airworthiness for the engine installation has not been obtained.
28.2.3.1.3 It has been established from experience that turbine engines can be affected seriously and adversely by inadvertent in-flight ice encounters in which only a minor performance loss could be attributable to the amount of ice accumulated on the airframe. Based upon this experience, it was deemed necessary to require a higher level of ice protection for the engines than was found necessary for the airframe. The requirements of §§ 25.1093(b), 25.1419, and 33.68 reflect this concept. Satisfactory operation of protected surfaces at the design point condition, as described in AC 20-73A, should be demonstrated by icing tunnel tests or by analysis supported by tests. The ability to de-ice and anti-ice protected surfaces should be demonstrated during the natural and/or tanker-provided icing tests.
28.2.3.1.4 The secondary effects of falling and blowing snow should also be considered in the evaluation of susceptibility to, and protection from, ice accumulations. The critical ambient temperatures should be defined; this evaluation should take into consideration not only temperatures in the immediate vicinity of the freezing point, where “wet, sticky snow” is likely, but also colder temperatures where snow may adhere to partially heated interior inlet surfaces, melt, and refreeze in a colder location. Any hardware or ancillary systems (e.g., screens, particle separators, oil coolers) installed in turbine engine inlets may facilitate the accumulation of snow and potential for generation of ice. The effects of falling and 28-5
blowing snow should be evaluated for both ground and flight conditions.
The number of actual airplane tests within the critical snow and temperature environment should be maximized. In accordance with § 21.35(a)(3), the test article must be in production configuration with respect to surface finish, texture, and material type, to assure accurate representation of the in-service configuration. Recognizing that the desired atmospheric conditions may be difficult to find, some conditions may be substantiated by analysis, provided that analysis is supported by some form of test data (e.g., temperature survey data).
28.2.3.1.5 It would be inconsistent to consider that § 25.1093(b) pertains only to the engine or engine inlet lip. The wording is broad and objective. It should be noted that the icing requirements pertaining specifically to the engine are included in § 33.68. Section 25.1093(b) was included to assure that the engine, as installed in the airplane, did not suffer adverse effects when the airplane was subjected to the icing envelope requirements covered by appendix C to part 25. It is not logical to exclude any part of the airplane that might shed ice so as to produce an adverse effect on the engine.
28.2.3.1.6 The preceding paragraphs are intended to describe the minimum program to achieve certification for flight in icing conditions.
28.2.3.2 Procedures.
None.
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CHAPTER 29. POWERPLANT: EXHAUST SYSTEM
29.1 General—§ 25.1121.
29.1.1 Explanation: Section 25.1121(a)—Carbon Monoxide Contamination.
Carbon monoxide detection tests are conducted in accordance with this requirement to determine that the disposal of exhaust gases from each exhaust system does not cause carbon monoxide contamination of any personnel compartment.
29.1.2 References.
Also see information related to the evacuation of other personnel compartment atmosphere contaminants contained in paragraphs 19.1 and 31.2 of this AC addressing the requirements of §§ 25.831 and 25.1197, respectively.
29.2 [Reserved.] 29-1
CHAPTER 30. POWERPLANT: CONTROLS AND ACCESSORIES [RESERVED] 30-1
CHAPTER 31. POWERPLANT: FIRE PROTECTION
31.1 Drainage and Ventilation of Fire Zones—§ 25.1187.
31.1.1 Explanation.
Section 25.1187 requires that each part of each designated fire zone be drained completely of flammable fluids to minimize hazards resulting from malfunctioning or failing components that contain flammable fluids. The drainage means must be:
31.1.1.1 Effective under conditions expected to prevail when drainage is needed;
31.1.1.2 Arranged so that no discharged fluid will cause an additional fire hazard;
and
31.1.1.3 Arranged so that no discharged fluid will enter any other fire zone.
31.1.2 Procedures.
None.
31.2 Fire Extinguishing Systems—§ 25.1197.
31.2.1 Explanation: Carbon Dioxide in Flightcrew Compartments.
Carbon dioxide has been found to adversely affect flightcrew personnel in the performance of their duties. Therefore, in airplanes equipped with built-in carbon dioxide fuselage compartment fire extinguisher systems, the carbon dioxide concentration occurring at the flightcrew stations as a result of discharging the fire extinguishers should be determined in accordance with the procedures of this section (also see paragraph 19.1 of this AC), except that such determination is not considered necessary if:
31.2.1.1 Five pounds or less of carbon dioxide will be discharged into any one such fuselage compartment in accordance with established fire control procedures; or
31.2.1.2 Protective breathing equipment is provided for each flight crewmember on flight deck duty.
31.2.2 Procedures: Flight Test Investigation.
31.2.2.1 The carbon dioxide concentrations at breathing level at the flightcrew stations should be determined in flight tests during which fuselage compartment fire extinguishers are discharged in accordance with established fire control procedures. Since carbon dioxide is heavier than air, a nose-down attitude is likely to produce the critical concentrations in the crew compartment. Perform the tests described in paragraphs 31.2.2.1.1 and 31.2.2.1.2 below.
31-1