Hot Fuel Testing
Have you ever wondered how aircrafts are design to operate in hot environments, like desert regions, where they experience elevated fuel temperatures? Some of the challenges when operating in extreme conditions is the engine efficiency degradation, vapor lock caused by high temperatures, where fuel can vaporize, leading to vapor lock, which can disrupt fuel flow and cause engine failure, and fuel leaks due to fuel expansion since high temperatures cause fuel to expand, which can increase pressure in the fuel tanks and lines.
Our engineering team has been part of hot fuel testing in multiple occasions supporting the top aircraft manufactures in Europe, Canada and the USA.
Hot fuel testing is considered one of the most critical procedure in aircraft development and certification and serves to ensure the safe performance of the aircraft under extreme fuel temperature conditions. Hot fuel test is required for the following reasons:
· To verify engine and auxiliary power unit can be supplied fuel in worst case temperature conditions.
· Verify suction fuel feeding to target engine during worst case conditions, attitude and temperature.
· Determine the suction feed ceiling, Above Ground Level.
· Investigate the impact of maximum altitude for complete pump failure and the engine response during emergency power up at maximum altitude.
· Support demonstration of compliance with CFR 25.961(5)(b)and 25.991(a)(b)
Fuel System
The fuel system (ATA 28) is broken into three subchapters: Storage, distribution and indication.
● Storage includes: fuel tanks, access panels, and the structure of the tanks
● Distribution includes: refuel/defuel lines, inner tank movement, engine and Auxiliary Power Unit (APU) supply lines, booster pumps, shut-off valves (SOV), and ventilation lines.
● Indication includes: Fuel Quantity Gauging Computer (FQGC) communication, refuel control panel (RCP) communication, cockpit display, and logic for distributing warnings/information.
Certification Requirements
Aircraft level requirements are included in Part 25 [1]. The applicable requirements are:
· 25.951(a) General
o The fuel system must be constructed and arranged to ensure a flow of fuel at a rate and pressure for proper engine and auxiliary power unit function under each likely operating condition.
· 25.952(a)Fuel system analysis and test
o The fuel system should properly function under all probable operating conditions. These conditions must be shown by analysis. If testing is required, testing should be conducted using the airplane fuel system or test article that reproduces the operating characteristics of the fuel system to be tested.
· 25.955(a)(1)(3) Fuel flow
o Fuel system must provide at least 100 percent of the fuel flow required under each intended operating condition and maneuver. Compliance must be shown per:
§ (a)(1) Fuel must be delivered to each engine at a pressure within the limits specified in the engine type certification (TC)
§ (a)(3) Each main pump must be used that is necessary for each operating condition and attitude. Additionally, the appropriate emergency pump must be substituted for each main pump used.
· 25.961 Fuel system hot weather test
o The fuel system must perform satisfactorily in hot weather operations. This must be shown by the fuel system being pressurized under all intended envelop (to prevent vapor formation) and by climbing from takeoff to maximum altitude. For climbing condition there must be no evidence of vapor lock or any other malfunction during climb under the following conditions:
§ Engines must operate at maximum continuous power
§ The aircraft weight must be the weight with full fuel tanks
§ Fuel temperature my be at least 110 degF at takeoff.
· 25.903(e) Restart capability
o Requires a means to restart any engine in flight, establishment of an in-flight restart altitude and airspeed envelop, and provisions for adequate electrical energy for engine ignition following engine shutdown during windmill conditions.
· 25.991 Fuel system component fuel pump
o There must be emergency pumps or another main pump to feed each engine immediately after failure of any main pump.
· 25.1351(d) Operation without normal electrical power
o An airplane must be able to operate safely for not less than five minutes following loss of all normal electrical supply, including engine restart capabilities with loss of fuel tank boost pump power, if a boost pump in each fuel tank is not powered by the emergency electrical system.
Combining of tests
It should be noted, both 25.903 and 25.1351(d) are not directly related to hot fuel testing. But, previous experience has shown that these regulations are combined into hot fuel testing.
Regulation 25.903(e) is added to the aircraft takeoff and climb test case. After the engine is shutoff, the pilots will command the aircraft to descend and windmill start the engine before a given altitude, i.e 20,000 ft.
Regulation 25.1351(d) is tested via shutting off pumps at the altitude envelop ceiling for 5 minutes to observe engine behaviour.
And this is just the starting point! We have much more to share. From test equipment design, to definition of instrumentation requirements and test content, and of course best practices. Would you like to know how we heat up your fuel to 50 degrees C for testing?
Reach out if you are interested to know more!!