Ice in Fuel

[Oldtechie]

Hullo,

It has been a long time since I was on these forums so please forgive me if this subject has already been covered or I am posting in the wrong place.

It is also a long time since I was associated with aircraft.

However I saw today a news story about the report on the Boeing 777 accident. I read that it was caused by ice in the fuel causing reduction of fuel to the engines and that this was not known as a problem at the time. That comment prompts this message.

In the mid 1960s I was a junior technician [radar] on Valiant bombers. As part of a starter crew one of the things I had to do was check that air was issuing from vents on the side of the aircraft and give a thumbs up to the crew chief when it did. Being curious, I eventually asked the reason for this, to me, meaningless ritual. The chief explained that fuel contained small amounts of water which might freeze and the crystals of ice would clog the fuel filters and the engines would be starved of fuel. The airflow I was checking was the heating for the filters.

Thing is if this problem was known in the 1960s and solved then why is it a problem in a modern airliner?


Just a puzzled X radar guy.

[Rigga]

Hi OT,

This particular airrfame and engine combination had not had this issue prior to this event - the two had made it through icing tests some years ago and got type certification from FAA and EASA (et al). No doubt testing for this condition will be (should be) added to new Type certification tests.

This icing apparently occurred due to a lack of normal fuel-flow variations and a stagnation of unheated fuel in cold pipes, causing fuel waxing. When the waxed fuel did move, it then clogged the heater matrix and caused fuel starvation.

Strange how more, similar, events were only reported afterwards?

But your right, it is the same fuel waxing issue as you were told about in the 60's - That's progress and the modern digital age for you!

Rigga

[Oldtechie]

Gosh! and there's me just a retired radar fairey.

[Alber Ratman]

The FOHE fitted to the Trent is a typical RR design, that as mentioned in RRs own book "The Jet Engine" can be compromised by icing to the entry of its capillary tubes, if the engines fuel flow is not regulary modulated. In the case of the LHR 777 accident, this was the case. An ex flight engineer I work with mentioned that he regulary had to adjust throttle settings to achieve such action during 747 sectors flown, due to the inablity of the Nigels driving to remember to.:eek:

[DERG]

Yes Oldtechie my thoughts were the same as yours. How could fuel icing cause such a complex modern aircraft like the 777 loose the engines..both of them..on the approach.

It seems that the aircraft had been cruising all the way from China at steady throttle settings which promoted the water in the fuel to congeal into ice.

This problem is as old as powered aviation and I still cannot grasp why the fuel is allowed to cool this far. My guess is that so much effort is put into efficiency gains...lean fuel and low noise...that the basics suffer.

Now I know the flight crew monitored the fuel temp guages because it says so in the report. This account shows the basics of the system and what failed.
Report: British Airways B772 at London on Jan 17th 2008, both engines rolled back on final approach (http://www.avherald.com/h?article=4270d893&opt=0)

If after reading this you are still asking..why the hell don't they heat the system...you would not be alone. This is incompetent design in my view.

[TopBunk]

RatmanAn ex flight engineer I work with mentioned that he regulary had to adjust throttle settings to achieve such action during 747 sectors flown, due to the inablity of the Nigels driving to remember to.
As a B747 skipper/nigel until recently, albeit on the -400, I can categorically state that there is no FCOM requirement to adjust throttle settings nor is there any autothrust subroutine that does it, other than a need to vary speed due changing winds/temps/weights whilst in level flight.

Maybe your ex FE colleague flew the clasic version without autothrust and occasionally had to act as an autothrottle for speed control rather than for any icing consideration?

[Alber Ratman]

Yeph, he flew Classics. He did however mention that he did it in consideration of the FOHEs, not for speed. He also mentioned that the nigels in question would berate him by asking "why did you do that for??"

His words, not mine.:eek: