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Aviation Forum / Country Specific / Australian Group / September 2008BA LHR 777 Crash - AD pending
FWIW - I understand that the long overdue, and anticipated (by me at least), AD addressing the possible role of ice in this crash will be issued in coming days. Ned > FWIW - I understand that the long overdue, and anticipated (by me at > least), AD addressing the possible role of ice in this crash will be > issued in coming days. > > Ned BA 777 crash inquiry believes icing behind fuel-flow restriction By David Kaminski-Morrow UK investigators believe that ice was the source of the fuel-system restriction which led a British Airways Boeing 777-200ER to experience a loss of engine power on approach to London Heathrow in January, and crash just short of the runway. In an interim report on the accident today, the Air Accidents Investigation Branch has issued three safety recommendations, including one directed specifically at Rolls-Royce Trent 800-powered 777s. The AAIB is recommending that US and European regulators - in conjunction with Rolls-Royce and Boeing - introduce "interim measures" to reduce the risk of restriction in Trent-equipped 777 fuel-feed systems caused by icing of water in fuel. Flight BA038 from Beijing crashed short of Heathrow's runway 27L on 17 January. All 152 passengers and crew survived the accident but the aircraft suffered severe damage. AAIB attention has focused on the external conditions during the flight, particularly given that the Trent-powered aircraft passed through areas of extremely cold air while in cruise. "The investigation has shown that the fuel flow to both engines was restricted, most probably due to ice within the fuel-feed system," says the AAIB. "The ice is likely to have formed from water that occurred naturally in the fuel while the aircraft operated for a long period, with low fuel flows, in an unusually cold environment." It stresses, however, that the 777 was being operated within its certified operational environment the whole time. US FAA and European Aviation Safety Agency regulators should take "immediate action to consider the implications" of the inquiry's findings on other aircraft, says the AAIB. It also recommends that both authorities review certification requirements to ensure aircraft and engines are "tolerant" to the build-up and possible sudden release of ice in their fuel-feed systems. >"The investigation has shown that the fuel flow to both engines was >restricted, most probably due to ice within the fuel-feed system," says >the AAIB. "The ice is likely to have formed from water that occurred >naturally in the fuel while the aircraft operated for a long period, >with low fuel flows, in an unusually cold environment." Interesting - the following was posted on the 23rd Jan and was ultimately somewhat sternly criticised : "Logic suggests that in this case, none of the reasons advanced for excluding fuel contamination (including your note on the CTR tank pumps, if that is where it was directed), really stack up to excluding that cause. For instance, switching to cold soaked wing tanks without crossfeed at start of descent could be the start of the chain of events, if the fuel had a wax (or even ice) problem that manifested itself on the unusually cold conditions (-70° C) that the aircraft encountered on this flight. An increasing reduction in the fuel flow available (through filter clogging) could easily be hidden when the power demand did not require full flow, but would manifest rather suddenly when increased power was required." >>"The investigation has shown that the fuel flow to both engines was >>restricted, most probably due to ice within the fuel-feed system," says >>the AAIB. "The ice is likely to have formed from water that occurred >>naturally in the fuel while the aircraft operated for a long period, >>with low fuel flows, in an unusually cold environment." On the top of page 7, Col 2, the report talks about the water scavanging system. It says it injects the "fluid" into each boost pump. Hello? Is this so low flow it can't alter engine performance, or they just hope it does not alter the thrust...?  SignatureA host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 >>>"The investigation has shown that the fuel flow to both engines was >>>restricted, most probably due to ice within the fuel-feed system," says [quoted text clipped - 6 lines] >this so low flow it can't alter engine performance, or they just hope it >does not alter the thrust...? 40 PPM ? I suppose it shouldn't have a great effect. >>On the top of page 7, Col 2, the report talks about the water scavanging >>system. It says it injects the "fluid" into each boost pump. Hello? Is >>this so low flow it can't alter engine performance, or they just hope it >>does not alter the thrust...? >40 PPM ? I suppose it shouldn't have a great effect. Where did that flow rate come from? SignatureA host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 >>>On the top of page 7, Col 2, the report talks about the water scavanging >>>system. It says it injects the "fluid" into each boost pump. Hello? Is [quoted text clipped - 4 lines] > >Where did that flow rate come from? Sorry, it wasn't a flow rate. It was an estimate of the average additional portion of water contributed by the scavenge pumps at engine flow rates in the order of 1,400 litres per hour. The scavenge pumps are operated by fuel flow - hence their output reduces at low flow rates and increases at high flow rate. This ratio could, I suppose, be substantially altered by operation of the fuel jettison system, but that seems irrelevant to this case. The scavenge pump has two inlets, by the way, one optimised for water scavenge and one for fuel scavenge. As the report is worded, it may give the impression that the scavenge delivered near the boost pump inlets is only from the water scavenge point. This isn't so. I also think you may have misinterpreted the report slightly. It doesn't inject scavenge into the boost pumps, but rather, near their inlets. This entrains fuel into that flow as well, so you shouldn't have solid slugs of heavily water contaminated fuel entering the boost pump inlets. Additionally, the tendency is for gross amounts of water to cause icing in the scavenge pumps which stops their operation (this usually doesn't damage the scavenge pumps). >I also think you may have misinterpreted the report slightly. It >doesn't inject scavenge into the boost pumps, but rather, near their [quoted text clipped - 3 lines] >to cause icing in the scavenge pumps which stops their operation (this >usually doesn't damage the scavenge pumps). I found it interesting because usually the idea is to keep the water out of combustion engines, not send it there. I agree that in very low percentages, it won't disrupt combustion -- some engines actually intentionally inject H2O, the HP7 and early B52's among them.. BUT fuel system icing is the topic at hand, and water+cold often equals ice. SignatureA host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 >>I also think you may have misinterpreted the report slightly. It >>doesn't inject scavenge into the boost pumps, but rather, near their [quoted text clipped - 11 lines] >BUT fuel system icing is the topic at hand, and water+cold often equals >ice. Too true. But off course, the water scavenge is intended to prevent the build up of water to the point where it could be a large part of the flow. The maximum scavenge over a long flight would be in the order of litres, whilst the total fuel used would be in the order of tens of thousands of litres. David Lesher wrote... > I found it interesting because usually the idea is to keep the water > out of combustion engines, not send it there. I agree that in very > low percentages, it won't disrupt combustion -- some engines actually > intentionally inject H2O, the HP7 and early B52's among them.. Quite a few stationary jet engines (power generation, etc.) use water injection to control NOX emissions, and use fine sprays at the compressor inlets to increase the air mass and hence performance on warmer to hot days. But the introduction of water is at high loads, and given what I know about these engines, I think that this would be occuring for an aircraft equivalent load of "take off thrust", I think it's called. If the engine is at low loads and water is introduced into the mix then it'd probably do what our engines were doing when the WI system failed to shut off - flame out somewhat spectacularly.. David Lesher wrote... > Is this so low flow it can't alter engine performance, or they just hope it > does not alter the thrust...? Eh? >David Lesher wrote... >> Is this so low flow it can't alter engine performance, or they just hope it >> does not alter the thrust...? >Eh? Read the whole post. The relevant part is: On the top of page 7, Col 2, the report talks about the water scavenging system. It says it injects the "fluid" into each boost pump. Hello? Is this so low flow it can't alter engine performance, or they just hope it does not alter the thrust...? and is talking about deliberately injecting water into the fuel. SignatureA host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 > FWIW - I understand that the long overdue, and anticipated (by me at > least), AD addressing the possible role of ice in this crash will be > issued in coming days. > > Ned Interim report published: http://www.aaib.gov.uk/cms_resources/G-YMMM%20Interim%20Report.pdf Ned said the following on 4/09/2008 9:22 PM: > FWIW - I understand that the long overdue, and anticipated (by me at > least), AD addressing the possible role of ice in this crash will be > issued in coming days. Another article http://www.theregister.co.uk/2008/09/04/heathrow_777_verdict/ Ice in fuel caused Heathrow 777 crash Nasty chill provoked reduced fuel flow By Lester Haines Published Thursday 4th September 2008 16:37 GMT The Air Accidents Investigation Branch (AAIB) has concluded that the 17 January crash-landing of a Boeing 777 at Heathrow was probably caused by "ice within the fuel feed system" which restricted flow to the engines. BA038 (G-YMMM), after a routine flight from Beijing, suffered reduced thrust in both engines while coming into land and fell short of the runway. The AAIB explained earlier this year: The first officer took control for the landing at a height of approximately 780 ft, in accordance with the briefed procedure, and shortly afterwards the autothrottles commanded an increase in thrust from both engines. The engines initially responded but, at a height of about 720 ft, the thrust of the right engine reduced. Some seven seconds later, the thrust reduced on the left engine to a similar level. The engines did not shut down and both engines continued to produce thrust at an engine speed above flight idle, but less than the commanded thrust. The engines failed to respond to further demands for increased thrust from the autothrottles, and subsequent movement of the thrust levers fully forward by the flight crew. Following examination of the aircraft, the AAIB was able to report: "The high pressure (HP) fuel pumps from both engines have unusual and fresh cavitation damage to the outlet ports consistent with operation at low inlet pressure. "The evidence to date indicates that both engines had low fuel pressure at the inlet to the HP pump. Restrictions in the fuel system between the aircraft fuel tanks and each of the engine HP pumps, resulting in reduced fuel flows, is suspected." Quite what caused this restricted flow proved a bit of a poser, but the AAIB now confirms (pdf): The investigation has shown that the fuel flow to both engines was restricted; most probably due to ice within the fuel feed system. The ice is likely to have formed from water that occurred naturally in the fuel whilst the aircraft operated for a long period, with low fuel flows, in an unusually cold environment*; although, G-YMMM was operated within the certified operational envelope at all times. The AAIB, while describing the incident as "the first known occurrence of this nature in any large modern transport aircraft", stresses: "All aviation fuel contains water which cannot be completely removed, either by sumping or other means. Therefore, if the fuel temperature drops below the freezing point of the water, it will form ice. The majority of flights have bulk fuel temperatures below the freezing." Among its recommendations attached to the latest report, the AAIB urges "that the Federal Aviation Administration and the European Aviation Safety Agency, in conjunction with Boeing and Rolls-Royce, introduce interim measures for the Boeing 777, powered by Trent 800 engines, to reduce the risk of ice formed from water in aviation turbine fuel causing a restriction in the fuel feed system". Bootnote *During its investigation, the AAIB noted that during flight BA038 "there was a region of particularly cold air, with ambient temperatures as low as -76°C, in the area between the Urals and Eastern Scandinavia". |
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