The FAA has issued an airworthiness directive affecting Rolls-Royce RB211 Trent 800-powered Boeing 777-200/300 jetliners that are operated in extremely cold temperatures.
The AD that is effective Sept. 29 requires revision of the aircraft flight manual to include in-flight procedures for pilots to follow in certain cold weather conditions and requires fuel circulation procedures on the ground when certain conditions exist.
The AD results from an incident of uncommanded reduction in thrust on both engines because of reduced fuel flow. “We are issuing this AD to prevent ice from accumulating in the main tank fuel feed system, which, when released, could result in a restriction in the engine fuel system. Such a restriction could result in failure to achieve a commanded thrust, and consequent forced landing of the airplane,” stated the FAA.
On Jan. 17, 2008, a British Airways Boeing 777-200 equipped with Rolls-Royce RB211 TRENT 895-17 turbofans crashed short of a runway at London Heathrow. During final approach, the autothrottles commanded an increase in thrust from both engines, and the engines initially responded. However, at a height of about 720 feet above the ground, the thrust of the right engine reduced, and seven seconds later, the thrust on the left engine also reduced.
Air Safety investigators say the uncommanded reduction in thrust on both engines was the result of reduced fuel flows. The engine control system detected the reduced fuel flows and commanded increased thrust. However, there was no appreciable change in the fuel flow to either engine.
The UK crash probe determined that over a long period of low power fuel flows and low fuel temperatures associated with cruise flight, ice can accumulate in the main tank fuel feed system and then release as a result of increased fuel flow when high thrust is commanded.
When released, the ice could create a restriction within the engine fuel system. A restriction in the engine fuel system, if not corrected, could result in failure to achieve a commanded thrust and dire consequences.
All of the testing and research was conducted on Boeing 777-200/300s equipped with Rolls-Royce RB211-TRENT 800 engines. “Initial review of 777 other airplane engine combinations has not revealed the same vulnerability to the identified unsafe condition,” the FAA noted.
This AD requires revising the Limitations section of the airplane flight manual to include procedures for pilots to follow in certain cold weather conditions and requires fuel circulation procedures on the ground when certain conditions exist.
The AD requires that the fuel circulation procedures be accomplished by a certified mechanic. The FAA said it included this requirement because of the complexity of the procedure. “We recognize that persons other than mechanics who are properly trained might also be capable of accomplishing this procedure. Therefore, we would be receptive to requests for approval of alternative methods of compliance to allow others to accomplish the procedure if the request includes training and oversight provisions to ensure that the procedure is accomplished properly,” said the FAA.
“We have determined that the loss of engine thrust was likely due to ice accumulating in the main tank fuel feed system during long exposure to cold fuel temperatures and low power fuel flows. It is necessary to issue interim mitigating actions in order to prevent an additional accident,” the FAA stated.
“Because an unsafe condition exists that requires the immediate adoption of this AD, we find that notice and opportunity for prior public comment hereon are impracticable and that good cause exists for making this amendment effective in less than 30 days.” It added.
As regards aircraft flight manual (AFM) revisions, the FAA said within 10 days after the effective date of this AD, affected operators must revise the Limitations section of the AFM to include the following statement.
”On ground, after refueling, check fuel temperature if fuel temperature indication is operative. If fuel temperature is colder than 0 degrees C or if fuel temperature indication is inoperative, verify that a record exists certifying that the approved fuel circulation procedure was performed.”
”Perform all step climbs using VNAV or maximum climb thrust.”
”In flight, within 3 hours of top of descent, but not less than 15 minutes before top of descent, check fuel temperature. If fuel temperature is colder than -10 degrees C, perform a step climb using maximum climb thrust. If a step climb using maximum climb thrust cannot be accomplished, verify cruise speed is set to 0.84 Mach or less, and manually advance thrust levers to maximum (autothrottles may be overridden). After reaching maximum climb thrust, hold for 10 seconds or until reaching 0.86 Mach, whichever occurs first. Check engines to ensure they have achieved maximum climb thrust and operate normally.”
The FAA AD also includes a new fuel circulation procedure. If the fuel temperature has not exceeded 0 degrees Celsius during the ground turn, before further flight, using the main tank fuel boost pumps, pump fuel through the fuel manifold using the high flow mode for a minimum of one minute.
“We consider this proposed AD interim action. If final action is later identified, we might consider further rulemaking then,” The FAA stated.