Previously unknown issues in airliner operations in severe cold in the upper atmosphere are cited by the FAA in the US in serving notice of compulsory design changes to Rolls-Royce powered versions of the Boeing 777-200ER jet overnight.
While there are no RR engined 777s in service with Australian carriers the directive is interesting in that they raised discussion as to whether external air temperatures were the common environmental factor in the chain of events leading to the incidents.
The flip side of lower atmospheric warming because of the enhancing of the greenhouse gas effect by the large scale burning of fossil fuels is upper level cooling caused by reduced re-radiation of heat energy back into space.
However those who see themselves as defending the inconvenient truth of global warming adhere to models that actually predict higher air temperatures up to levels well above airline operations, so reports of aircraft encountering gradually lower external air temperature environments are somewhat ‘inconvenient’ as well.
The NPRM or notice of proposed rule making issued by the American regulator follows similar action by its European counterpart, and will require the design, certification and installation of a new fuel and engine oil heat exchange system on all of the affected airliners by early 2011.
In January 2008 a British Airways 777 crashed just short of the runway at London’s Heathrow Airport after a long sub polar trans Siberian flight from Beijing in what pilots in other jets in the same area were quoted as saying were unusually cold conditions.
This was later contradicted by the British inquiry. Apparently -75C is really quite normal, which is a notion that might get tested a bit more in coming years.
However both engines on the BA 777 failed to respond almost simultaneously to a command for more power on the final approach, and ‘rolled back’ to a low thrust setting independently of each other.
This contradicted some of the core design assumptions of large twin engined jets and led to a detailed search for a common cause for a fuel blockage in an airliner where each engine is not only separately served by the devices that control their power output but maintained by separate ground crews to prevent any error made in the care of one engine being replicated by the same team of mechanics on the other .
After fending off rat bag suggestions of mobile phone interference, or terrorists disguised as plane spotters beaming jamming devices at the flight as it approached the perimeter of the airport, the inquiry turned to ice, the evidence of which would have melted soon after the impact in which all on board survived.
Extensive tests showed that ice could clog the innards of the RR fuel distribution system in which hot engine oil is cooled and cold fuel is heated prior to entering the engines via a heat exchanger. That ice tended to form most aggressively at around -12C but also at somewhat colder levels inside the fuel lines. Those temperatures would also occur within the jet after very long flights in very cold outside air, such as the -75C experienced at cruising level by the BA jet.
In November last year a similar RR engined Delta 777 on a long range flight from Shanghai to Atlanta also experienced a simultaneous uncommanded engine thrust roll back, but which was overcome and did not cause a crash.
In its reasons for the rule the FAA says with two such incidents happening within a year and without precedent the probability of more occurring was sufficiently high for compulsory safety action to be taken.
Airlines have also been advised previous to these two incidents to alter flight paths or flight altitudes on all long range flights where practicable to avoid known extremely cold atmospheric conditions and to ensure that fuel reserves are circulated with sufficient frequency to prevent them turning to jelly. As the high latitude and polar routes came into increased use so did the need for carriers to be aware that in some locations additional factors needed to be considered in planning operations. Airbus and Boeing have both indicated that they are monitoring instances of abnormally low external temperatures with a view to modifying jets or the way they are flown as necessary in the future.
These two incidents may assume additional importance with time.
3 Comments
Forgive my ignorance, but am I correct in thinking that when an engine fails to respond properly to a “power up” command then (as a safety feature) they “rolled back” to idle? (just trying to get my head around this)
I can think of a number of reasons why this would be so, but it does, in a way, seem counterintuitive.
Like I said, I’m ignorant on such matters, just trying to understand a little better
BA038 had made a ’straight in’ approach toward Heathrow at an essentially smooth rate of descent and a low power setting consistent with that increasingly popular technique. Sometimes called ‘gliding’ approaches in the popular media, even though the engines are still generating minimal thrust until the last moment. As the jet dropped to the level where more power and a higher head up profile, or flare, was called for the engines didn’t respond and then ‘rolled back’, in what is found to have been a response to impeded fuel flow. The pilots only had seconds in which to respond to the initial failure, quickly commanded more thrust once more, and when that failed, optimised the attitude of the jet as best they could before ‘gliding’ across the perimeter road at very low altitude and hitting the grass short of the runway. Straight in approaches are increasingly used to save time and fuel.
The term roll back is occasionally used in other instances when engines fail to deliver on cue. There were two instances last year of sudden roll back on the different types of engine used on the Boeing 737, once in the crash of a Turkish Airlines flight at Amsterdam, which caused some fatalities on board, and once soon after in a Qantas flight where the situation was caught by the standard operating procedure in the airline in which one pilot has his hands on the engine throttles throughout the final approach to a landing. However those roll backs were not attributed to ice but a fault in one of the radar altimeters on the jets which fed incorrect height data to the flight control computers causing them in the case of the Turkish jet to shut down the engines because the altitude -8 feet that was suddenly indicated when the jet was still at 1000 feet, conformed to the below sea level altitude of the airport, as entered in the flight plan. So the system in the 737 assumed the jet had landed and adjusted the throttle setting accordingly. Not in hindsight a terrific endorsement of the logic behind that particular system. The pilots of the Turkish flight were unable to react in time to the situation.
Personally I think ‘roll back’ is something I only want to hear applied to groceries and bank charges. Throttle malfunctions would be a much better term to use to incidents like these on airliners.
Thanks, I get it now