7 seconds between 0 and -1.0 g, with a total accumulation of 20 seconds of negative acceleration operation.
23.13.2.4 Test data should be analyzed with regard to maintaining adequate fuel flow to the engines and APU and maintaining lubrication of critical components.
23.13.2.5 Test planning should consider that sufficient altitude be available to conduct a suction feed relight in the unlikely event of an all-engine flameout, in which case the tank pump feed lines will become uncovered and air will enter each feed line.
23.14 Thrust or Power Augmentation System—§ 25.945. [Reserved] 23-16
CHAPTER 24. POWERPLANT: FUEL SYSTEM
24.1 Unusable Fuel Supply—§ 25.959.
24.1.1 Explanation.
24.1.1.1 The purpose of this test is to determine, for each fuel tank, the quantity of fuel that is not available to the engines, as specified in § 25.959. The unusable fuel quantity is the quantity of fuel that can be drained from the fuel tank sump with the airplane in its normal level ground attitude after a fuel tank unusable fuel test has been performed, plus the quantity remaining in the fuel tank (undrainable fuel).
24.1.1.2 A fuel tank that is not designed to feed the engines under all flight conditions need be tested only for the flight regime for which it is designed to do so (e.g., cruise conditions). Tanks that are not subject to aeroelastic effects of flight, such as wing bending or tank flexing, may have their unusable fuel quantity determined during a ground test. Suitable instructions on the conditions under which the tank may be used should be provided in the AFM. Other part 25 requirements, such as fuel flow (§ 25.955(a)(2)) and fuel quantity gauge calibration (§ 25.1553), are also related to the unusable fuel quantity. These other requirements must also be considered when the unusable fuel quantity is being determined for each fuel tank.
24.1.2 Procedures.
24.1.2.1 The fuel system and tank geometry should be analyzed to determine the critical conditions for the specific tanks being considered (i.e., main and auxiliary or cruise tanks). The analysis should determine the amount of unusable fuel as a function of airplane pitch and roll attitudes, including those encountered when executing sideslips and dynamic maneuvers such as go-around pitch-up and acceleration. Particular attention should be directed toward the fuel tank or cell geometry and orientation with respect to the longitudinal axis of the airplane and location of the fuel tank outlets
(i.e., fuel pump inlets or pickups). Care should be taken in planning how the critical attitude conditions are tested, so that the test procedure does not result in a non-conservative unusable fuel quantity.
24.1.2.2 The term “most adverse fuel feed condition” is not intended to include radical or extreme conditions not likely to be encountered in operation.
Judgment should be used in determining what maneuvers are appropriate to the type of airplane being tested. The test conditions should be selected using good judgment with regard to the kind of conditions the airplane under test will be subjected to in operation.
24-1
24.1.2.3 Airplane attitude limitations may be used as a means of reducing the unusable fuel quantity, provided it is demonstrated that likely operational flight maneuvers can be accomplished with those attitude limits. Nose down pitch attitude should not be less than that for normal descent, approach, and landing maneuvers. Nose up pitch attitude consistent with a normal go-around condition, or a minimum of 10° nose up, whichever is less, should be considered. Roll attitude limitations should not be less than that required to enter a normal traffic pattern, intercept the final approach course, and land with a 10-knot crosswind.
24.1.2.4 After the most adverse fuel feed condition and the critical flight attitude have been determined for the specific fuel tanks being considered, the appropriate flight tests should be conducted. The flight testing should investigate the effects of the following:
24.1.2.4.1 Steady state sideslips anticipated during operation with the airplane in both the approach and landing configurations.
24.1.2.4.2 For those airplanes capable of high roll and pitch rates, abrupt maneuvers should be considered.
24.1.2.4.3 A go-around condition at maximum acceleration and maximum rotation rate to the maximum pitch attitude should be considered.
24.1.2.4.4 Effects of turbulence on unusable fuel quantity should be considered.
24.1.2.4.5 If the airplane includes a low fuel quantity warning system, it should be demonstrated that the airplane can complete a go-around, approach, and return to landing, without fuel feed interruption, using the normal goaround pitch attitude; this should include go-arounds accomplished with the aid of automated flight guidance systems.
24.1.2.5 If airplane attitude limitations are employed to reduce the unusable fuel quantity, those attitude limitations must be published in the AFM (per § 25.1581(a)(2)) as limits for flight maneuvers after the low fuel warning light/message illuminates. This will provide assurance that the fuel remaining that is above the unusable quantity can be used without risk of fuel feed interruption to the engines. Flight tests should be conducted to confirm that the proposed pitch attitude limit:
Is practical in terms of airplane flight characteristics for accomplishing a go-around; and Will not result in lift and drag characteristics that will increase the time and/or fuel quantity necessary to complete the go-around to a point where the fuel remaining is less than the unusable fuel quantity.
24.1.2.6 If fuel pump failure en route would result in a significant reduction in usable fuel, the unusable fuel supply test should include a determination of 24-2
this quantity. The effects of pump failure on the unusable fuel quantity should be presented in the AFM so that the flightcrew can take the reduction in usable fuel into account for flight planning purposes.
24.1.2.7 Auxiliary fuel tanks and fuel transfer tanks designed or restricted for use during cruise flight only (i.e., not suitable for takeoff and landing) should be tested for unusable fuel quantity by appropriate investigation of the cruise environment. This should include reasonable turbulence levels, asymmetrical power or thrust, adverse fuel feed/transfer configuration, etc.
However, the unusable fuel quantity should not be less than fuel tank sump quantity.
24.2 Fuel System Hot Weather Operation—§ 25.961.
24.2.1 Explanation.
24.2.1.1 A flight test is normally necessary to complete the hot weather operation tests required by § 25.961(a). If a ground test is performed, § 25.961(b) requires that the ground test closely simulate flight conditions. If a flight test is performed, the test should be conducted with hot fuel in the tanks normally used for takeoff and landing, and with the maximum number of engines drawing fuel from each tank, in accordance with the operating procedures provided in the AFM, to obtain the maximum anticipated fuel flow through the lines. In the case of symmetrical fuel tank systems, the tests may be confined to one of each such system. Critical fuel that is unweathered or has not been exposed to long storage periods should be used during the tests. This ensures that the fuel has the maximum Reid vapor pressure (RVP) for which approval is requested. Fuel samples should be taken from the fuel tank prior to the test; for typical JET B (JP-4) type fuels, a minimum RVP of 3.0 has been required. The fuel temperature, just prior to takeoff, should be as close as practical to the maximum value for which operational approval is sought, but not less than 110 °F as required by § 25.961(a)(5). If the fuel needs to be heated to this temperature, caution should be taken to prevent overheating during the process.
24.2.1.2 The desirable outside ambient air temperature at the airport from which the tests are being conducted should be at least 85 °F (29 °C). If tests are performed in weather cold enough to interfere with test results, § 25.961(b) requires insulating fuel tank surfaces, fuel lines, and other fuel system components from cold air to simulate hot-day conditions. It should not be necessary to provide additional heat to the fuel after the original fuel sample is heated to temperature during the hot weather tests.
However, if the fuel is used as the cooling medium for any heat exchangers, the maximum heat available to the fuel should be considered.
24-3
24.2.1.3 If auxiliary pumps are being considered for use as emergency pumps, they should be inoperative. This test may be used to establish the maximum pressure altitude for operation with these pumps off. A fuel pressure failure is considered to occur when the fuel pressure decreases below the minimum prescribed by the engine manufacturer, or the engine does not operate satisfactorily.
24.2.2 Test Procedures and Required Data.
24.2.2.1 The fuel temperature, just prior to takeoff, should be as close as practical to the maximum value for which operational approval is sought. If heating of the fuel is required, a takeoff and climb should be made as soon as possible after the fuel in the tank has been heated to avoid cooling of the fuel.
24.2.2.2 Power or thrust settings should be maintained at the maximum approved levels for takeoff and climb in accordance with § 25.961(a)(2).
Section 25.961(a)(3) requires the weight of the airplane to be the weight with full fuel tanks, minimum crew, and the ballast necessary to maintain the CG within allowable limits. The airspeed during the climb should not exceed that speed used in demonstrating the requirements specified in § 25.961(a)(4). The combination of power or thrust, airplane weight and climb airspeed establishes the climb rates that must be considered when determining the critical conditions for testing. The airplane fuel load and rate of climb are the critical parameters for this test. Although the weight of the airplane must be based on full fuel tanks, experience has shown that full fuel tanks may not always be the most critical fuel load. Generally, full tanks result in the maximum head pressure over the tank pumps. A lesser tank quantity results in a lesser head pressure that may be more critical to the formation of fuel vapor in the pumps and fuel system lines.
A greater rate of climb results in more air per unit time being released in the feed lines during the climb. An analysis may be needed to determine the critical combination of fuel quantity state and airplane climb rate that is operationally likely to occur. The test should be conducted at this critical combination of fuel quantity state and airplane climb rate with the airplane loaded to the weight and flown within the range of climb airspeeds at the power or thrust required by the regulation.
Note: The fuel systems of some airplanes are designed to feed fuel to the engines from auxiliary and main fuel tanks. This results in variations in fuel feed configurations as part of the approved fuel management.
Variations in fuel feed configurations, including changes in fuel feed configuration such as transition from feeding fuel to the engine(s) from one tank to feeding from another, should be considered in assessing fuel feed performance in demonstrating compliance to §§ 25.951 or 25.955 and considered in establishing the critical conditions for the hot fuel evaluation.
24-4
24.2.2.3 If the engines are normally operated with the auxiliary/emergency pumps “off,” they should remain “off” until fuel pressure failure occurs.
Restoration of fuel pressure should be noted and the climb continued to the maximum operating altitude selected by the applicant for certification.
If a lower altitude is substantiated, appropriate operating limitations should be established and furnished in the AFM.
24.2.2.4 The tests should be conducted with the fuel system operating and configured normally, in accordance with the normal procedures outlined in the AFM. The following data should be recorded at reasonable time intervals:
1. Fuel temperature in the tank.
2. Engine fuel pressure, measured at the engine/airplane interface, at the start of the test and during climb (note any pressure failure, fluctuation, or variation).
3. Main and auxiliary/emergency fuel pump operation, as applicable.
4. Pressure altitude.
5. Ambient air temperature.
6. Airspeed.
7. Engine power or thrust setting and operating parameters (i.e., engine pressure ratio, gas generator speed, fan speed, exhaust gas temperature, fuel flow, etc.).
8. Comments on engine operation.
9. Fuel quantities in fuel tanks.
10. Fuel grade or designation determined prior to test.
11. Airplane pitch and roll attitudes.
24.2.2.5 If significant fuel pressure fluctuations occur during testing of the critical flight conditions, but pressure failure does not occur, additional testing should be considered to determine that pressure failure may not occur during any expected operating mode. Also, the fuel system should be evaluated for vapor formation when switching from different fuel feed configurations, or at low fuel flow and idling approach and landing.
24.3 Fuel Tank Vents and Carburetor Vapor Vents—§ 25.975.
24.3.1 Explanation: Approval of Fuel Tank Vents (§ 25.975(a)).
24.3.1.1 The tank venting arrangement must prevent siphoning of fuel during normal operation. Also, the venting capacity and vent pressure levels must 24-5
maintain acceptable differences of pressure between the interior and exterior of the tank during:
Normal flight operation, Maximum rate of ascent and descent, and Refueling and defueling.
24.3.1.2 No vent or drainage provision may end at any point where the discharge of fuel from the vent outlet would constitute a fire hazard, or where the discharge of fumes could enter personnel compartments.
24.3.1.3 Each carburetor vent system must have means to avoid stoppage by ice.
24.3.2 Procedures.
24.3.2.1 Tests should be conducted to ensure that no hazardous quantities of fuel will be siphoned overboard during any likely maneuvers encountered during normal operations. Maneuvers that may require evaluation include, but are not necessarily limited to, the following:
24.3.2.1.1 Taxi turns and turning takeoff maneuvers with fuel tanks filled to the maximum volume (below the required 2 percent expansion space) allowed by the type design, including consideration of tolerances in the volumetric shutoff level. Typically, left and right-hand turns are conducted in a “figure eight” maneuver, followed by a maximum pitch and ascent rate takeoff.
24.3.2.1.2 Maximum climb power or thrust ascent at high climb angles.
24.3.2.1.3 Simulated turbulent air oscillations at or near the natural yawing and pitching frequency of the airplane.
24.3.2.1.4 Rapid descent with high initial pitch-down rate.
24.3.2.1.5 In-flight power-up turns.
24.3.2.1.6 Sideslip maneuvers on approach.
24.3.2.2 The changes in tank secondary barrier cavity pressure during all airplane maneuvers, including emergency descent, should be accounted for in the design of the fuel tank. Bladder type tanks may be critical under emergency descent conditions, depending on the cavity vent line sizing.
According to § 25.975(a)(3), the vent/drain configuration must provide the required positive and negative pressure relief between the outer shell and the bladder or inner wall, to prevent collapse or over-expansion of the inner tank. Depending on the location of the overboard vent/drain exit and the airflow characteristics around the exit or exit mast, a flight test may be 24-6
required to evaluate the ascent and/or emergency descent characteristics of the cavity vent system with the airplane in both the “clean” and “wheels and flaps down” configuration.
24.3.2.3 Verification that liquid discharge from the vent mast will flow clear of the airplane, not attaching itself to any airplane surface or re-entering any compartment of the airplane, may need to be accomplished by impingement tests conducted inflight. Small discharges of fuel from the fuel tank vent outlet have been acceptable, provided the fuel discharges clear of the airplane and does not result in siphoning of fuel from the tank.
This can be accomplished using dyed fluid and/or coating the surfaces required to be free of impingement with powder compounds that will be washed away if contacted by liquid. If dyed water or other liquid is used, it may be necessary to add chemicals to prevent freezing during the test.
Sufficient test maneuvers should be accomplished to ensure that impingement will not occur during any inadvertent discharge from the venting system.
24.3.2.4 Carburetor vent systems may require flight testing to ensure against stoppage by freezing. Such tests can be conducted in conjunction with the tests required by § 25.1093 and/or § 25.1101 (see the “Procedures” for those sections in this AC).
24-7
CHAPTER 25. POWERPLANT: FUEL SYSTEM COMPONENTS
25.1 Fuel Jettisoning System—§ 25.1001.
25.1.1 Explanation.
25.1.1.1 Section 25.1001(a) prescribes the conditions governing the need for installation of fuel jettisoning systems; if an airplane can meet the climb requirements of §§ 25.119 and 25.121(d), at the weight existing after a 15-minute flight consisting of a maximum weight takeoff and immediate return landing, a fuel jettisoning system is not required. Credit is given for the actual or computed weight of fuel consumed in the 15-minute flight using the airplane configurations, power or thrust settings, and speeds appropriate to each flight segment.
25.1.1.2 If a fuel jettisoning system is required, § 25.1001(b) prescribes the conditions that will determine the minimum flow rate of the system.
Section 25.1001(b) requires the fuel jettisoning system to be capable of reducing the weight of the airplane, within 15 minutes of operation, from that specified in § 25.1001(a) to a weight at which the airplane will meet the climb requirements of §§ 25.119 and 25.121(d). Since the weight defined in § 25.1001(a) allows credit for a 15-minute fuel burn, a literal interpretation of this rule would result in a 15-minute jettisoning period beginning after a 15-minute takeoff, go-around, and approach flight. In application, the 15-minute jettisoning period will occur during a 30-minute flight in which weight reduction credit will be given for the fuel consumed and jettisoned. The airplane must be able to meet the specified climb requirements at the weight existing at the end of this 30-minute flight.
25.1.1.3 Airplanes should also be investigated for other elements that may limit their ability to safely accomplish an immediate return landing without a fuel jettisoning system. Advances in wing design and propulsion technology have resulted in transport category airplane designs that can take off at weights considerably above their maximum landing weights.
Many of these airplanes are capable of meeting the climb requirements of §§ 25.119 and 25.121(d), following a 15-minute flight, without a fuel jettisoning system. Some of these airplanes, however, may not be capable of landing without exceeding other certification limits such as maximum brake energy, landing distance, and tire speed. This is particularly true when non-normal procedures, implemented as a result of failures that have been shown to be foreseeable events, call for reduced flap settings and increases of as much as 30 knots, for a given weight, over speeds associated with the normal landing flap setting. Margins to flap placard limit speeds and flap load-relief activation speeds should be established and maintained for non-normal configurations that may be used in immediate return landings.
25-1
25.1.1.4 An additional consideration that is representative of actual operating conditions is the ability to perform a go-around from field elevation with the flaps in the approach position and the landing gear down. Through compliance with § 25.1001(b), assurance will be obtained that the airplane can accomplish an all-engines-operating balked landing go-around, with normal landing flaps, followed by a one-engine-inoperative climb-out with approach flaps and landing gear up. However, non-normal procedures generally call for one-engine-inoperative landings to be made with the flaps in the position used to show compliance with the approach climb requirements of § 25.121(d). It should therefore be determined under what combinations of weight, altitude, and temperature the airplane can establish a positive rate-of-climb with one-engine-inoperative and the other operating at go-around power or thrust, with the flaps in the appropriate go-around position and the landing gear down.
25.1.2 Procedures.
The basic purpose of these tests is to verify that the minimum jettisoning rate will allow the airplane to safely execute an immediate return landing, and to determine that the required amount of fuel may be safely jettisoned under reasonably anticipated operating conditions within the prescribed time limit, without danger from fire, explosion, or adverse effects on the flying qualities.
25.1.2.1 Jettisoning Rate.
25.1.2.1.1 In determining the minimum jettisoning rate, the tanks, tank combinations, or fuel feed configurations that are critical should be selected for demonstrating the flow rate.
25.1.2.1.2 It should be determined if airplane attitude or configuration has an effect on the jettisoning rate.
25.1.2.1.3 It should be demonstrated that the means to prevent jettisoning of the fuel in the tanks used for takeoff and landing, below the level to meet the requirements of § 25.1001(e) and (f), are effective.
25.1.2.1.4 It should be demonstrated that operation of the jettisoning system does not have a detrimental effect on operation of the engines (and the APU if installed and approved for inflight operation).
25.1.2.2 Fire Hazard.
25.1.2.2.1 The fuel jettisoning flow pattern should be demonstrated from all normally used tank or tank combinations on both sides of the airplane, whether or not both sides are symmetrical.
25-2
25.1.2.2.2 The fuel jettisoning flow pattern should be demonstrated for the flight conditions specified in § 25.1001(d)(1), (2), and (3). Steady-state sideslips anticipated during operation should be conducted during flight conditions.
25.1.2.2.3 Fuel in liquid or vapor form should not impinge upon any external surface of the airplane during or after jettisoning. Colored fuel, or surface treatment that liquid or vaporous fuel changes the appearance of, may be used on airplane surfaces for detection purposes. Other equivalent methods for detection may be acceptable.
25.1.2.2.4 Fuel in liquid or vapor form should not enter any portion of the airplane during or after jettisoning. The fuel may be detected by its scent, a combustible mixture detector, or by visual inspection. In pressurized airplanes, the check for the presence of liquid or vaporous fuel should be accomplished with the airplane unpressurized.
25.1.2.2.5 There should be no evidence of leakage after the fuel jettisoning valve is closed.
25.1.2.2.6 Testing should be conducted with the wing flaps in all available positions and during transition from each position to the next. If there is any evidence that wing control surface (flaps, slats, etc.) positions may adversely affect the flow pattern and allow fuel to impinge on the airplane, the airplane should be placarded and a limitation noted in the AFM.
25.1.2.3 Control.
25.1.2.3.1 Changes in the airplane control qualities during the fuel jettisoning tests should be investigated, including asymmetrical jettisoning.
25.1.2.3.2 Discontinuance of fuel jettisoning should be demonstrated in flight.
25.1.2.4 Residual Fuel.
The quantity of usable fuel that cannot be jettisoned should be determined to meet the requirements of § 25.1001(e) or (f), as applicable. One acceptable means to show compliance with the requirement for sufficient fuel remaining to permit continued flight has been to drain the remaining fuel from the test tank(s) after landing. Applicants may propose other means of compliance.
25.2 [Reserved.] 25-3
CHAPTER 26. POWERPLANT: OIL SYSTEM [RESERVED] 26-1