Engine icing problems on 747-8 and 787.
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Engine Icing...new style
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magic icing or something else !
So only engines are affected, right ?
No problem with any other part of aircraft, right?
If so, it has nothing to do with ICING, but with ice crystals filling some sophisticated engine electronic inputs i.e. "Bill Gates' Windows" probes...
All other aircraft including these will not lose power due to ice crystals but because PT2 or whatever is called tube if not properly heated and filled with ice crystals will give false indication to engine control or EPR etc... but will not affect engine power, unless Auto Throttle is on.
PS
Flying 50 or even 555 NM clear of CB will not solve the problem. Any stratus cloud or any cloud at all, may contain those crystals on high altitudes.
No problem with any other part of aircraft, right?
If so, it has nothing to do with ICING, but with ice crystals filling some sophisticated engine electronic inputs i.e. "Bill Gates' Windows" probes...
All other aircraft including these will not lose power due to ice crystals but because PT2 or whatever is called tube if not properly heated and filled with ice crystals will give false indication to engine control or EPR etc... but will not affect engine power, unless Auto Throttle is on.
PS
Flying 50 or even 555 NM clear of CB will not solve the problem. Any stratus cloud or any cloud at all, may contain those crystals on high altitudes.
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Do you mean cirrostratus clouds? Plain old stratus clouds are low altitude and composed of water - they are not an ice crystal hazard. Cirrostratus do contain ice crystals but these are a) Much bigger (by about one order of magnitude) and b) Much less dense (by about 2 orders of magnitude). This means they are nowhere near as big a hazard in terms of ICI than the very dense ice clouds we see being produced near Cbs.
Same with normal cirrus clouds, by the way: They're about an order of magnitude less dense than the Cb associated clouds and (as far as I know) have not been implicated in any rollback events.
Same with normal cirrus clouds, by the way: They're about an order of magnitude less dense than the Cb associated clouds and (as far as I know) have not been implicated in any rollback events.
The effect of ice crystals varies with engine type. The hazard involves large quantities of micro ice particles at high altitude. The previously quoted water/ice densities are like a heavy rain storm at ground level (a strong umbrella required), but with ice crystals there are additional criteria such as density, heat-flow, and ‘stickiness’.
In some engines, the heat flow of the compressor intake-anti icing (inside the engine) might be overwhelmed by the ice mass flow and heat loss. Or more likely result in partial melting enabling the ‘cold’ water to act as a glue to stick ice crystals together (run back ice), resulting in a rapid build-up which chokes the first stages of the compressor (cf pitot problems). The associated changes in air flow may be sensed by the fuel control system which reduces the fuel flow, which results in a reduction in engine speed, which affects the rate of ice build-up – and thence a continuing cycle resulting in ‘rollback’ to idle/sub idle. There may be a point at low rpm where fuel is added (to maintain idle) by the FCU or manual, but the engine is unable to respond and thus the additional fuel increases the engine temp resulting in a further control cycle with a risk of over-temp / stall / shutdown.
Engines may also suffer from ice lumps breaking off with potential damage to later stages of the compressor; and which can affect engine performance after an encounter – even if not detected at the time.
Different engine designs with slightly higher anti-ice heat-flow may protect them in most conditions; similarly an extended compressor intake lip may centrifuge the ‘relatively’ denser crystals (or a significant proportions of them) away from the compressor and exit harmlessly through the fan duct due to fan-whirl – spinner shape, fan design, by-pass duct shape, size, speed, etc.
All of these variables also depend on ice density, duration of exposure, ambient and compressor temperatures according to power setting / aircraft speed.
The safety problem is how to anticipate the combinations in a test program and ensure that you flight test in the appropriate conditions, except you don’t know what appropriate means.
Original designs should cope with a wide range of conditions (cert regs), but then there is longer term exposure to the real atmosphere and real flight operations; – in safety you never fight the war that you train for.
Thus modifications might consider changes to hardware, anti-ice heat flow, or fuel control system – air/fuel flow. And temporarily avoiding the most likely conditions, but with associated risk of human judgement of what is most likely (regulation advisory warning), and adherence to procedures (operator).
In some engines, the heat flow of the compressor intake-anti icing (inside the engine) might be overwhelmed by the ice mass flow and heat loss. Or more likely result in partial melting enabling the ‘cold’ water to act as a glue to stick ice crystals together (run back ice), resulting in a rapid build-up which chokes the first stages of the compressor (cf pitot problems). The associated changes in air flow may be sensed by the fuel control system which reduces the fuel flow, which results in a reduction in engine speed, which affects the rate of ice build-up – and thence a continuing cycle resulting in ‘rollback’ to idle/sub idle. There may be a point at low rpm where fuel is added (to maintain idle) by the FCU or manual, but the engine is unable to respond and thus the additional fuel increases the engine temp resulting in a further control cycle with a risk of over-temp / stall / shutdown.
Engines may also suffer from ice lumps breaking off with potential damage to later stages of the compressor; and which can affect engine performance after an encounter – even if not detected at the time.
Different engine designs with slightly higher anti-ice heat-flow may protect them in most conditions; similarly an extended compressor intake lip may centrifuge the ‘relatively’ denser crystals (or a significant proportions of them) away from the compressor and exit harmlessly through the fan duct due to fan-whirl – spinner shape, fan design, by-pass duct shape, size, speed, etc.
All of these variables also depend on ice density, duration of exposure, ambient and compressor temperatures according to power setting / aircraft speed.
The safety problem is how to anticipate the combinations in a test program and ensure that you flight test in the appropriate conditions, except you don’t know what appropriate means.
Original designs should cope with a wide range of conditions (cert regs), but then there is longer term exposure to the real atmosphere and real flight operations; – in safety you never fight the war that you train for.
Thus modifications might consider changes to hardware, anti-ice heat flow, or fuel control system – air/fuel flow. And temporarily avoiding the most likely conditions, but with associated risk of human judgement of what is most likely (regulation advisory warning), and adherence to procedures (operator).
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Why icing in 2 spool and not 3 spool?
I happen to have the temp versus station diagrams for the GEnx and the T1000 (I analyze engines over at Airliners.net) so I thought I post them here as well, here the link to the Airlines post:
How Does A Software Change Solve Icing Issues? — Tech Ops Forum | Airliners.net
The diagrams show very clearly why a 2 shaft booster engine can have the type of icing problems discussed and why a 3 shaft is less affected. First the GEnx-1B:
Then the T1000:
As said these are the design point diagrams,top of climb in these cases. The cruise temps will be somewhat lower, the ToC thrust is around 17klbf and the typical crusie thrust around 10-12klbf at FL350 and higher.
Now one can see that there is low temps before the HPC in the 2 spool as the booster works with very low pressure rise (less then 1.5 in cruise) and hence temps, the temps might just be enough for the thawing and refreezing cycle discussed. The 3 spool is cold until the IPC comes and then it raises the temp fast as it works with optimal RPMs (the booster spins with the fans low RPM), hence the ICI does not thaw until they thaw completely and don't reattach.
How Does A Software Change Solve Icing Issues? — Tech Ops Forum | Airliners.net
The diagrams show very clearly why a 2 shaft booster engine can have the type of icing problems discussed and why a 3 shaft is less affected. First the GEnx-1B:
Then the T1000:
As said these are the design point diagrams,top of climb in these cases. The cruise temps will be somewhat lower, the ToC thrust is around 17klbf and the typical crusie thrust around 10-12klbf at FL350 and higher.
Now one can see that there is low temps before the HPC in the 2 spool as the booster works with very low pressure rise (less then 1.5 in cruise) and hence temps, the temps might just be enough for the thawing and refreezing cycle discussed. The 3 spool is cold until the IPC comes and then it raises the temp fast as it works with optimal RPMs (the booster spins with the fans low RPM), hence the ICI does not thaw until they thaw completely and don't reattach.
Re AD 2013-24-01 (#49); another masterpiece from DOT / FAA, but at least they do try 
“…prohibit operation in moderate and severe ICI conditions” This is a very thin safety line particularly where the conditions cannot be detected directly, only by supposition based on WXR defined Cb activity.
And what are moderate to severe ICI conditions vice conventional icing conditions – don’t you have to be in them to know the difference.
The industry has a new hazard; MCS – “a large Mesoscale Convective System … where several thunderstorms have merged, with a continuous cloud larger than 100 kilometers (62 miles) across.”
Events with other aircraft types were not necessarily constrained to “warm geographic locations” or above 30000ft. Also, not all of the likely conditions would be identified by the crew, e.g. as being in cloud, some of the very thin wispy cirrus conditions may not constitute IMC.
Is the industry pushing the safety assumptions a bit far by allowing a dispatch after only three engines have been inspected. Is it more likely that a ‘damaged’ engine will fail at high thrust during takeoff .
What if ICI conditions are encountered and an engine suffers damage without any EICAS alerting?
“ … unrecoverable thrust loss on multiple engines can lead to a forced landing.” There must an award for such a statement.
“…prohibit operation in moderate and severe ICI conditions” This is a very thin safety line particularly where the conditions cannot be detected directly, only by supposition based on WXR defined Cb activity.
And what are moderate to severe ICI conditions vice conventional icing conditions – don’t you have to be in them to know the difference.
The industry has a new hazard; MCS – “a large Mesoscale Convective System … where several thunderstorms have merged, with a continuous cloud larger than 100 kilometers (62 miles) across.”
Events with other aircraft types were not necessarily constrained to “warm geographic locations” or above 30000ft. Also, not all of the likely conditions would be identified by the crew, e.g. as being in cloud, some of the very thin wispy cirrus conditions may not constitute IMC.
Is the industry pushing the safety assumptions a bit far by allowing a dispatch after only three engines have been inspected. Is it more likely that a ‘damaged’ engine will fail at high thrust during takeoff .
What if ICI conditions are encountered and an engine suffers damage without any EICAS alerting?
“ … unrecoverable thrust loss on multiple engines can lead to a forced landing.” There must an award for such a statement.
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Yeah, it's fun to ridicule government employees, isn't it?
The FAA's published requirements were developed jointly with Boeing and GE, and used terminology in the preamble that came from messages Boeing sent to their operators. This is done to be consistent with manufacturer publications whenever possible. The statement about the results of multiple engine power loss is required by policy and by FAA counsel - the AD must state the unsafe condition, even if it's obvious.
In this case there was a lot of discussion to reach agreement on an appropriate limitation because there is no direct way to detect the ice crystal conditions in advance of being in them, and wind is a factor. This was not an easy issue to positively address without potentially creating a large economic impact.
It would be a mistake to assume the FAA engineering staff is incompetent. Just ask Tim (tdracer).
The FAA's published requirements were developed jointly with Boeing and GE, and used terminology in the preamble that came from messages Boeing sent to their operators. This is done to be consistent with manufacturer publications whenever possible. The statement about the results of multiple engine power loss is required by policy and by FAA counsel - the AD must state the unsafe condition, even if it's obvious.
In this case there was a lot of discussion to reach agreement on an appropriate limitation because there is no direct way to detect the ice crystal conditions in advance of being in them, and wind is a factor. This was not an easy issue to positively address without potentially creating a large economic impact.
It would be a mistake to assume the FAA engineering staff is incompetent. Just ask Tim (tdracer).
Dave,
“… it's fun to ridicule government employees, isn't it?” yes, but … “It would be a mistake to assume the FAA engineering staff is incompetent …”. I totally agree, but there are occasions where mild ridicule which refocuses attention might improve the objectives of a safety message.
Undoubtedly this issue is being considered by many parties with great expertise and who have access to more information than is made public, but the quality of these activities is often judged by what is said / published.
DOT/FAA are the custodians of safety and publications – even though the text may be drafted elsewhere; they, at the end of the process, should be expected to ask if what is being conveyed makes sense and will it achieve the objective. Are the explanations and procedures written from their point of view, with their responsibilities for safety in mind, or from the views of operators and the pilots who have to judge and manage the risks in potentially hazardous situations.
Regulatory authorities provide guidance on risks - as to what might be acceptable or not; in simple terms they ‘take a risk’, based on well-judged and proportioned reasoning to maintain the required safety level. However, it is the pilots who ‘run the risks’; theirs’ is the final safety call which often depends on situation assessment, which in this instance requires clear and concise depiction of the conditions and how they can be identified.
This is an onerous task for everyone in the process and warrants careful thought, which from the current public view might have been better considered.
Yes the above is posted with hindsight, but better to consider this hindsight as foresight now, rather than start again after an incident; then the judgement of economic aspects, etc (a fine balance with safety) will be wrong, whereas today, ‘it is right’.
“… it's fun to ridicule government employees, isn't it?” yes, but … “It would be a mistake to assume the FAA engineering staff is incompetent …”. I totally agree, but there are occasions where mild ridicule which refocuses attention might improve the objectives of a safety message.
Undoubtedly this issue is being considered by many parties with great expertise and who have access to more information than is made public, but the quality of these activities is often judged by what is said / published.
DOT/FAA are the custodians of safety and publications – even though the text may be drafted elsewhere; they, at the end of the process, should be expected to ask if what is being conveyed makes sense and will it achieve the objective. Are the explanations and procedures written from their point of view, with their responsibilities for safety in mind, or from the views of operators and the pilots who have to judge and manage the risks in potentially hazardous situations.
Regulatory authorities provide guidance on risks - as to what might be acceptable or not; in simple terms they ‘take a risk’, based on well-judged and proportioned reasoning to maintain the required safety level. However, it is the pilots who ‘run the risks’; theirs’ is the final safety call which often depends on situation assessment, which in this instance requires clear and concise depiction of the conditions and how they can be identified.
This is an onerous task for everyone in the process and warrants careful thought, which from the current public view might have been better considered.
Yes the above is posted with hindsight, but better to consider this hindsight as foresight now, rather than start again after an incident; then the judgement of economic aspects, etc (a fine balance with safety) will be wrong, whereas today, ‘it is right’.
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I understand that there is an FAA statement that if there is another occurrence of ICI in GEnx engines the TCs for the 747-8 and 787/GEnx will be withdrawn.
If that's already been posted, apologies; I didn't find it. Is there any corroboration of that information?
If that's already been posted, apologies; I didn't find it. Is there any corroboration of that information?
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another occurrence of ICI in GEnx engines the TCs for the 747-8 and 787/GEnx will be withdrawn
barit1, excepting that the narrowly-defined region is ill-defined and depends on the crew to identify it. In addition, the probability of total power loss in not the usual certification consideration where independent probabilities of engine failure might be used; with ICI it is one single event capable of affecting all engines simultaneously; the failures are not mutually exclusive.
The events so far have not involved all engines. It would be interesting to understand why, which might refute the probability theory. However, 2 engines out on a 747 should be manageable, but 2 out on a 787 … …
If there is another event then the situation and circumstances might be very revealing in many (unfortunate) ways.
It is possible that the justification for the current restriction has considered that most engines recovered power and were not damaged beyond immediate use; however the sample size is very small for an ‘unanticipated’ situation.
The AD is a very fine safety line which depends on human judgement, hence the need for quality advice.
The events so far have not involved all engines. It would be interesting to understand why, which might refute the probability theory. However, 2 engines out on a 747 should be manageable, but 2 out on a 787 … …
If there is another event then the situation and circumstances might be very revealing in many (unfortunate) ways.
It is possible that the justification for the current restriction has considered that most engines recovered power and were not damaged beyond immediate use; however the sample size is very small for an ‘unanticipated’ situation.
The AD is a very fine safety line which depends on human judgement, hence the need for quality advice.
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Boeing 787 engines' ice risk spurs FAA to warn airlines - Bloomberg
U.S. regulators are poised to order airlines to avoid flying Boeing Co. (BA) 787 Dreamliners and 747-8 jumbo jets with General Electric Co. (GE) engines near thunderstorms after some of the planes experienced ice buildup.
A directive due this week is an 'interim action' to ensure pilots fly clear of icing conditions that could reduce thrust from GEnx engines, the Federal Aviation Administration said yesterday. The U.S. move follows Japan Airlines Co. (9201)' s decision to shift to other jets from 787s on some Asia routes.
The icing risk adds urgency for pilots to steer clear of thunderstorms already shunned because of potentially deadly lightning and turbulence. Jets flying at high altitudes through tropical zones can be at risk from powerful storms that promote the formation of performance-sapping ice, according to GE, the world's largest maker of jet engines.
'It's a relatively rare phenomenon because it requires just the right meteorological conditions, ‘Hans Weber, president of San Diego-based aviation consultant Tecop International Inc., said by telephone. 'This isn't a problem that will be limited to GE engines. These crystals have been found in all engines at high altitudes near thunderstorms.'
JAL, United
The twin-engine Dreamliner, the first jet made chiefly of composite materials, entered service with ANA Holdings Inc. (9202)'s All Nippon Airways in October 2011. Tokyo-based ANA, the biggest Dreamliner operator, uses Rolls-Royce Holdings Plc (RR/) engines on its planes.
JAL's 787s have GEnx engines, as do the Dreamliners flown by United Continental Holdings Inc. (UAL), the only U.S. airline flying 787s. Chicago-based United hasn't changed schedules or routes for its Dreamliners, said Christen David, a spokeswoman.
Atlas Air Worldwide Holdings Inc. (AAWW), the lone U.S. operator of 747-8s, adjusted operations after Boeing's Nov. 23 warning for GEnx-equipped jets to stay 50 nautical miles (93 kilometers) from storms, said Bonnie Rodney, a spokeswoman.
Any disruptions for the freighters from the Purchase, New York-based company 'will be minimal and can be managed with only minor reroutings,' Rodney said yesterday.
Replacement 747-400s
Cathay Pacific Airways Ltd. (293), the Hong Kong-based airline, said it has 10 Boeing 747-8 freighters in its fleet that are powered by GE's GEnx engines. As a precautionary measure, it's standard operating procedure for 747-8 freighters to avoid flying into thunderstorms, Cathay said in an e-mailed response.
The European Aviation Safety Agency has adopted the FAA safety directive, spokesman Dominique Fouda said today.
Deutsche Lufthansa AG (LHA), the largest customer for the 747-8, said that from December the nine jets already delivered will not operate above 30,000 feet or avoid storms. Weather conditions en-route will be closely reviewed and, in some cases, other aircraft such as 747-400s equipped with other engines will be used, Lufthansa said by e-mail.
'This looks a lot like a classic teething issue,' Richard Aboulafia, an aerospace analyst at Fairfax, Virginia-based consultant Teal Group, said by e-mail. 'It's probably isolated to just the engine, and even then just one of the two engines available as options. It's also probably easily fixed with a software tweak, rather than any kind of hardware modification.'
Software Modifications
GE said it's making software modifications to eliminate the ice-buildup risk and expects them to be available in the first quarter. Marc Birtel, a spokesman for Chicago-based Boeing, said the engines' design and maintenance practices, together with the new instructions, allow for the jets' 'continued safe operation.'
'The FAA has been working closely with Boeing and GE to monitor and understand these events as the companies develop a permanent solution,' the FAA said in a statement. It didn't give a specific day for issuing the airworthiness directive on the planes, which only covers U.S. carriers.
Both the 787 and 747-8 have had bumpy debuts. The 747-8 was two years late in starting service in 2011, and slack demand forced Boeing to cut output. The Dreamliner, whose 2011 entry was 3 1/2 years late, was grounded for three months in January after meltdowns in the lithium-ion battery packs on two planes.
Six Cases
Boeing fell 2.2 percent to $133 at the close in New York, while GE declined 1.3 percent to $26.73.
There have been six cases since April of planes with GEnx engines temporarily losing thrust in high-altitude icing conditions, Fairfield, Connecticut-based GE said Nov. 23. Five were with 747-8s and one was with a 787, according to the e-mailed statement.
Japan Air will replace 787 Dreamliners on flights between Tokyo and Delhi with Boeing 777s until Nov. 30, and will switch to 767s on its Tokyo-Singapore route, according to a Nov. 23 statement. The Tokyo-based carrier will make a decision this week on flights past Dec. 1, said Jian Yang, a spokesman.
Boeing surpassed 1,000 orders for the Dreamliner with its haul at the Dubai Air Show this month. The company handed over 57 of the four-engine 747-8s, through the end of last month, most of which are freighters.
'Airlines wouldn't be too concerned about engines in terms of costs,' K. Ajith, a Singapore-based analyst at UOB Kay Hian (UOBK) Pte. 'There will be some of sort compensation for airlines. Despite the problems, the aircraft is quite popular.'
A directive due this week is an 'interim action' to ensure pilots fly clear of icing conditions that could reduce thrust from GEnx engines, the Federal Aviation Administration said yesterday. The U.S. move follows Japan Airlines Co. (9201)' s decision to shift to other jets from 787s on some Asia routes.
The icing risk adds urgency for pilots to steer clear of thunderstorms already shunned because of potentially deadly lightning and turbulence. Jets flying at high altitudes through tropical zones can be at risk from powerful storms that promote the formation of performance-sapping ice, according to GE, the world's largest maker of jet engines.
'It's a relatively rare phenomenon because it requires just the right meteorological conditions, ‘Hans Weber, president of San Diego-based aviation consultant Tecop International Inc., said by telephone. 'This isn't a problem that will be limited to GE engines. These crystals have been found in all engines at high altitudes near thunderstorms.'
JAL, United
The twin-engine Dreamliner, the first jet made chiefly of composite materials, entered service with ANA Holdings Inc. (9202)'s All Nippon Airways in October 2011. Tokyo-based ANA, the biggest Dreamliner operator, uses Rolls-Royce Holdings Plc (RR/) engines on its planes.
JAL's 787s have GEnx engines, as do the Dreamliners flown by United Continental Holdings Inc. (UAL), the only U.S. airline flying 787s. Chicago-based United hasn't changed schedules or routes for its Dreamliners, said Christen David, a spokeswoman.
Atlas Air Worldwide Holdings Inc. (AAWW), the lone U.S. operator of 747-8s, adjusted operations after Boeing's Nov. 23 warning for GEnx-equipped jets to stay 50 nautical miles (93 kilometers) from storms, said Bonnie Rodney, a spokeswoman.
Any disruptions for the freighters from the Purchase, New York-based company 'will be minimal and can be managed with only minor reroutings,' Rodney said yesterday.
Replacement 747-400s
Cathay Pacific Airways Ltd. (293), the Hong Kong-based airline, said it has 10 Boeing 747-8 freighters in its fleet that are powered by GE's GEnx engines. As a precautionary measure, it's standard operating procedure for 747-8 freighters to avoid flying into thunderstorms, Cathay said in an e-mailed response.
The European Aviation Safety Agency has adopted the FAA safety directive, spokesman Dominique Fouda said today.
Deutsche Lufthansa AG (LHA), the largest customer for the 747-8, said that from December the nine jets already delivered will not operate above 30,000 feet or avoid storms. Weather conditions en-route will be closely reviewed and, in some cases, other aircraft such as 747-400s equipped with other engines will be used, Lufthansa said by e-mail.
'This looks a lot like a classic teething issue,' Richard Aboulafia, an aerospace analyst at Fairfax, Virginia-based consultant Teal Group, said by e-mail. 'It's probably isolated to just the engine, and even then just one of the two engines available as options. It's also probably easily fixed with a software tweak, rather than any kind of hardware modification.'
Software Modifications
GE said it's making software modifications to eliminate the ice-buildup risk and expects them to be available in the first quarter. Marc Birtel, a spokesman for Chicago-based Boeing, said the engines' design and maintenance practices, together with the new instructions, allow for the jets' 'continued safe operation.'
'The FAA has been working closely with Boeing and GE to monitor and understand these events as the companies develop a permanent solution,' the FAA said in a statement. It didn't give a specific day for issuing the airworthiness directive on the planes, which only covers U.S. carriers.
Both the 787 and 747-8 have had bumpy debuts. The 747-8 was two years late in starting service in 2011, and slack demand forced Boeing to cut output. The Dreamliner, whose 2011 entry was 3 1/2 years late, was grounded for three months in January after meltdowns in the lithium-ion battery packs on two planes.
Six Cases
Boeing fell 2.2 percent to $133 at the close in New York, while GE declined 1.3 percent to $26.73.
There have been six cases since April of planes with GEnx engines temporarily losing thrust in high-altitude icing conditions, Fairfield, Connecticut-based GE said Nov. 23. Five were with 747-8s and one was with a 787, according to the e-mailed statement.
Japan Air will replace 787 Dreamliners on flights between Tokyo and Delhi with Boeing 777s until Nov. 30, and will switch to 767s on its Tokyo-Singapore route, according to a Nov. 23 statement. The Tokyo-based carrier will make a decision this week on flights past Dec. 1, said Jian Yang, a spokesman.
Boeing surpassed 1,000 orders for the Dreamliner with its haul at the Dubai Air Show this month. The company handed over 57 of the four-engine 747-8s, through the end of last month, most of which are freighters.
'Airlines wouldn't be too concerned about engines in terms of costs,' K. Ajith, a Singapore-based analyst at UOB Kay Hian (UOBK) Pte. 'There will be some of sort compensation for airlines. Despite the problems, the aircraft is quite popular.'
