I had a chance to talk to Boeing about what they're now recommending
and why. Although Boeing are not saying so I understand that the
various manufacturers suspect that the risk only applies to
Trent-powered aircraft due to the design of the fuel/oil heating system
which Pratt and GE don't have. Boeing are investigating whether other Rolls-Royce-powered current and legacy models are at risk. Anecdotally I also understand that it's by no means certain that Trent-powered Airbus widebodies will be affected due to different fuel-tank designs. Anyway, below is what Boeing is telling operators.
There are two new in-flight procedures to be implemented whenever
conditions resemble those that affected BA038; a new ground procedure
likewise when conditions dictate; and a new non-normal checklist if it
all goes wrong (although it wouldn't have helped BA038).
The first procedure is intended for any flight when the fuel temperature falls below 0deg C. It's pretty simple: when you conduct a step-climb (ie climbing to the next level as the aircraft gets lighter), you do it using maximum thrust. That's in order to induce a high fuel-flow and help dislodge any ice while it's still of a size that the fuel/oil heat exchanger can handle that. Something that didn't happen on BA038.
The simple way to achieve that is to use the Vnav function in the FMS - which demands a new flight level - rather than the vertical speed mode (which is what the BA038 crew used.) That's a change from usual practice. Crews tend to use vertical speed precisely because it induces a gentle power change and doesn't disturb the passengers in the middle of what is often a night-time cruise. Apparently the fuel penalty is negligible.
The second procedure is basically for use when the occasion doesn't arise for a step climb during a flight. So if if you find yourself getting close to top-of-descent, the fuel temp is below -10deg C (which happens on about half of all flights), and you haven't done a high-thrust manoeuvre (like a step-climb) for more than three hours - now's the time to do one. What you do is go to full thrust for 10 seconds or until you reach M0.86 - whichever comes sooner.
There is another procedure for use on the ground. Anytime you're refuelling and it becomes apparent that the fuel will not reach 0deg C, you operate the fuel pumps at high power for one minute. That was the case for BA038 at Beijing.
Finally, if you find yourself in the unhappy situation that the BA guys did, but at a higher altitude, which is still pretty unhappy, then there's a new checklist to follow. You throttle back for 30 seconds on the affected engine (or engines - one elephant, two elephants, three elephants....keep smiling...30 elephants) while the ball of slush/ice melts on the heat-exchanger. The shortcomings of this procedure if you are trying to squeak it onto Heathrow 27L without spilling any pints at The Green Man will be immediately apparent.
All of this is pretty much predicated on the assumption that the slug of ice getting lodged in the heat-exchanger is probably right. Boeing was repeatedly able to replicate that pheonomenon in test circumstances, whereas they were unable to replicate the alternative scenario of ice accreting in the fuel line and gradually choking it off.
All of this is expected to end up in an airworthiness directive anytime now.
The first procedure is intended for any flight when the fuel temperature falls below 0deg C. It's pretty simple: when you conduct a step-climb (ie climbing to the next level as the aircraft gets lighter), you do it using maximum thrust. That's in order to induce a high fuel-flow and help dislodge any ice while it's still of a size that the fuel/oil heat exchanger can handle that. Something that didn't happen on BA038.
The simple way to achieve that is to use the Vnav function in the FMS - which demands a new flight level - rather than the vertical speed mode (which is what the BA038 crew used.) That's a change from usual practice. Crews tend to use vertical speed precisely because it induces a gentle power change and doesn't disturb the passengers in the middle of what is often a night-time cruise. Apparently the fuel penalty is negligible.
The second procedure is basically for use when the occasion doesn't arise for a step climb during a flight. So if if you find yourself getting close to top-of-descent, the fuel temp is below -10deg C (which happens on about half of all flights), and you haven't done a high-thrust manoeuvre (like a step-climb) for more than three hours - now's the time to do one. What you do is go to full thrust for 10 seconds or until you reach M0.86 - whichever comes sooner.
There is another procedure for use on the ground. Anytime you're refuelling and it becomes apparent that the fuel will not reach 0deg C, you operate the fuel pumps at high power for one minute. That was the case for BA038 at Beijing.
Finally, if you find yourself in the unhappy situation that the BA guys did, but at a higher altitude, which is still pretty unhappy, then there's a new checklist to follow. You throttle back for 30 seconds on the affected engine (or engines - one elephant, two elephants, three elephants....keep smiling...30 elephants) while the ball of slush/ice melts on the heat-exchanger. The shortcomings of this procedure if you are trying to squeak it onto Heathrow 27L without spilling any pints at The Green Man will be immediately apparent.
All of this is pretty much predicated on the assumption that the slug of ice getting lodged in the heat-exchanger is probably right. Boeing was repeatedly able to replicate that pheonomenon in test circumstances, whereas they were unable to replicate the alternative scenario of ice accreting in the fuel line and gradually choking it off.
All of this is expected to end up in an airworthiness directive anytime now.
on September 8, 2008 2:32 PM | Reply
Even if true, it was still quite a coincidence to have two slugs of ice block both engines' fuel flow simultaneously.
As I understand the 777 fuel system, each engine is fed by it's corresponding wing tank, with the center fuel tank keeping the wing tanks topped off until it is exhausted.
on September 11, 2008 4:12 AM | Reply
The information was useful to the pilot's community in addition to the Checklist.
on October 4, 2010 2:49 PM | Reply
This is a matter of people's lives so they have got to do something about it or else we will end up entering aircrafts with a scared feeling. These things should be secured.