Hot Fuel Testing - Test Content
Have you ever wondered how aircrafts are tested to ensure they can 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)
Below is a breakdown of how we see Test Content based on experience.
General
Testing for Hot Fuel testing will involve testing an aircraft to simulate hot weather testing of the fuel system. The general idea for testing is to investigate how heat soaking fuel to worst case scenario will affect the performance of the aircraft. Examples of in-service fuel soaking include:
· External fuel tanks stored outside
· Fuel truck sitting in the sun
· Post refueling with aircraft parked in the sun
To simulate this, and to meet requirements, the fuel is first heated, then the aircraft is flown through an envelope of test points.
It is worth noting that the fuel temperature is targeted at aircraft takeoff. The fuel will need to be heated past 48 degC number to allow pilots time to complete required checklists.
Based on previous experience, fuel was heated within the fuel truck to 50-51 degC. Once loaded on the aircraft the fuel would drop 1 degC every 20 minutes.
Hot fuel testing must meet both CFR 25.961 and 25.991. Both requirements have to be established and agreed upon by the authorities. Such agreements need to include:
· Takeoff requirement and when to switch to a higher engine throttle
· Fuel temperature at takeoff
· Pilot action after engine shutdown
· When to turn pumps off/on
For worst case scenario tests would normally take place on the hottest practical test day (OAT of 50 degC) to allow extrapolation to the specified MHD. The temperature works in aircraft performance and demand on fuel system to accommodate. The minimum ambient temperature for testing is 85 degF. From previous experience, ambient temperature can have a decreases importance if the fuel is heated separately and as long as the fuel in the tanks at takeoff is within tolerance, the testing can be considered valid.
A pre-test with slightly heated fuel and earlier knock-it-off calls should be used to decrease potential dangers and increase the safety margin.
Test summary
From previous flight test campaigns, the test scope involves a flight profile whereby on engine is deemed the test engine. This engine is isolated by the fuel system via closing all crossfeed capabilities and only one tank is fueled with hot fuel. Additionally, the aircraft is fueled for minimum weight needed to complete the profile with a margin of safety to create the worst case head pressure. The flight profile includes a takeoff at TOGA then test side pumps are turned off and thrust moved to maximum continuous thrust for the remained of the test case. This is required to fulfil the worst-case condition of suction climb at maximum climb rate. This test case is deemed complete when the engine shuts off (or burner pressure modulates). Once this occurs the pilots will command test engine pumps on then restart the engine via windmill restart. Some OEMs will then add a second test point where the pumps are shutoff at maximum altitude. The time requirement for pumps off is directly related to the emergency procedure completion time.
Flight Crew and Dispatch should request prior clearance with ATC to achieve an uninterrupted climb with minimal turns or changes in direction. The ceiling of the test case is where the engine starves and shuts down. Prior test experience shows that the altitude of fuel starvation is 33,000 ft.
Prior to fueling the aircraft, fuel samples should be taken along with temperature of fuel. The fuel samples will be used to calculate reid vapor pressure (RVP) per ASTM D323 and flash point per ASTM D56.
Fuel Quality
Fuel quality is an important in data reduction post testing. For hot fuel testing, un-weathered and aerated fuel is required.
Un-weathered
Un-weathered fuel is defined as fresh fuel that has not been stored for long periods of time and has not experienced diminished light refraction component (reduction is vapor pressure, etc.)
Aerated fuel
Aerated fuel is defined as fuel stored on the ground that has been enriched by ambient gases. At sea level, aerated fuel can contain up to 14% of air by volume.
Test requirements
As regulation 25.961(5)(b) states 110 degF temperature is required, this temperature is targeted for takeoff (weight off wheels, WOW, is set to 0). To achieve this the fuel must be heated. Heating fuel can be done in 1 of three ways:
· Soaking the airplane in direct sunlight on a hot day.
o Additional black cladding should be used.
· Using a heated fuel truck.
o Such as the Oilmens AL44D
· Using a separate heater unit
o Such as the boiler/spring module.
Hot Soak
Depending on the climate of the test, hot soaking the aircraft is a viable option. Mesa, Arizona, is a previous test location where hot soaking could have been used. To assist in trapping heat, black tarps/cladding was used. This method is time consuming and requires the flight crew to be briefed, prepped and ready as soon as the temperature is reached. Prior airport coordination should be used here to aid in a smooth takeoff within the temperature tolerance band.
Heated fuel truck
Some fuel trucks have the ability to heat fuel. The Oilmen AL44D is such a truck. It uses internal tank heating with pumps to circulate the hot fuel. The fuel should be heated to slightly above required temperature as fuel loading and taxi take time to complete where fuel temperature can drop.
Separate heater unit
Prior experience has shown using a separate heater unit works well when the above options are not feasible. The idea of the heater is to use a large water boiler to heat copper coils. These copper coils are placed inside the fuel truck from the access man holes on the ceiling of the fuel truck.
Prior communication, setup and testing is required with the local fueling OEM. In addition to the copper coils, an air compressor is used to spin a circulation fan that is lowered inside the copper springs to move the fuel within the tank. Before fueling is ready, the fuel truck should start circulating fuel in the fueling hoses to prevent any trapped cold fuel from being loaded onto the aircraft.
Takeoff climb profile
Worst case thermal conditions for the fuel system is high power thrust with low head pressure. To achieve this the test engine side is loaded with minimal hot fuel. The pilots are to command maximum thrust at takeoff, then move to maximum continuous thrust, CON, above 1,000 ft (per regulation 25.961) on the test engine. At the time of switching to CON, pilots should command test side fuel pumps off. The test engine is now operating at CON with suction fuel. This is to simulate worst case scenario of low fuel and pumps unavailable with a hot soaked airplane.
The non-test engine should be set to IDLE. The aircraft is to continuously climb with minimal turns until the engine flames out. The flame out will follow the following progression:
1. Burner pressure fluctuation
a. Burner pressure (PB) will drop throughout the climb but plateau towards the end of the climb. At this moment, a small oscillation will be observed. Flame out will occur very shortly after this.
2. N1, N2 and fuel flow
a. These parameters will fluctuate very quickly, and flameout will occur at this point.
3. Engine status flags
a. The FADEC will post flameout and engine fail.
A buildup test can be used to increase the margin of safety. To do this, as soon as PB oscillates, pilots are to command fuel pumps on. The uptake in fuel pressure will be quick enough to prevent an engine flameout. This test also achieves an expected altitude for when the engine will flameout. This can be used to create better testcard planning and fuel loading.
It is acceptable to expand the scope of this testing to include the restart capability regulation, 25.903(e). Once the engine has flamed out, the pilots will move the4 engine run switch to cutoff. Then, proceed to the windmill restart checklist. The aircraft will have a predetermined and accepted altitude the restart must be complete by. Once the restart is complete, the engine will be moved back to CON thrust for a small duration. Test is deemed complete after this.
High altitude suction fuel
The operation without normal electrical power regulation, 25.1351(d), was demonstrated during hot fuel testing. To validate this test, the aircraft conducted a continuous climb to the altitude envelop ceiling. From here the test engine side fuel pumps were commanded off. This demonstrates a loss of electrical power to all pumps. Normal pilot operation would be to descend in altitude and deploy the ram air turbine, RAT. The RAT then will provide power to the electrical emergency bus, which normally includes 1 or more fuel pumps. The estimated time to complete this is no less than 5 minutes.
As a result, the testing simulates this by flying straight and level flight for 5 minutes with test engine side fuel pumps off. The fuel level should be low to reduce the head pressure creating the worst-case scenario. After the 5 minutes have elapsed, pumps are commanded on and test is complete. If the engine starves for fuel and flames out, then a windmill restart is to be conducted.
Instrumentation
To support data collection in hot fuel testing, temperature probes are required. The following are locations of temperature probes that should be recorded as well as monitored in real time:
· Inside fuel tank
o Preferably in collator tank near fuel line pickup point
· APU inlet
o Preferably at the inlet of fuel manifold. If the APU has a fuel/oil heat exchanger (FOHE) it is recommended that temperature is measured at the FOHE inlet
· Engine inlet
o Preferable at the inlet of the fuel manifold.