Re: ETOPS Question

Date:         09 Dec 97 03:54:25 
From:         Iain Stuart <big-iain@big-iain.demon.co.uk>
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Using his favourites wax crayons, <airliners.1997.2861@ohare.Chicago.COM
>, Matthew Willshee <96mjw@eng.cam.ac.uk> scrawled
>On 29 Nov 1997, John van Veen wrote:
>> When the combustion temperature, maybe 1100 degrees, of the fuel burning
>> in the APU is considered, there isn't much difference between -45
>> degrees and +90 degrees.

>I should think that the limiting factor on cold starts is oil temperature.
>If the oil is very cold then it will be too viscous to pump around the oil
>system.  This can lead to problems with bearings and local overheating of
>the oil (because the cold oil is too thick to pump into the hot bearing
>chambers to replace the heated oil there).

He's right you know, but it's very tricky to get down that low. When
Boeing did a trial cold start on a B757, they had to fly to Yakutsk
(sp?), Siberia in winter and leave the engine overnight + with no
special protection to get the engine to the required temp (-40C ?). No
commercial operator would be that cavalier about their property, or
would they ? Anyway, the engines started first time, and soon got up to
a sensible operating temperature.

>As to the temperature scales mentioned in John's post don't really apply.
>The oil will cold soak down to ambient temperature, but when the APU is
>running, the oil is cooled to stop it decomposing.  I think about 100
>degrees celsius is a reasonable maximum temperature for an oil system, but
>I may be wrong here.

A bit low.  IIRC the temperature limit on a B747 is 171C, so running at
100C should present no problems.

-----------
Iain Stuart