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737NG engine questions

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737NG engine questions

Old 22nd Jan 2017, 06:27
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737NG engine questions

Dear all,

I would like to clear up some questions I have regarding these topics;

1. Reverser unlocked in flight

As per the video below;

https://www.youtube.com/watch?v=MEGX6h49ICY

The affected reverser system forward thrust lever goes straight to close (idle) with inadvertent actuation of reverse thrust (caused by a defect). This is the working on the B737-300/500 series.

What happens on the 737NG, same result?

I am curious since the B737NG has EEC?

For your reference I have looked at FCOM and QRH

2. Hard alternate EEC mode

"If the EECs are in the alternate mode, advancing the
thrust levers full forward provides some overboost and should be considered only during emergency situations when all other available actions have been taken and terrain contact is imminent."

"Hard alternate mode thrust is always equal to or greater than
normal mode thrust for the same lever position"

Is this hard alternate thrust significantly greater than EEC normal mode full foward stop?

Thank you
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Old 22nd Jan 2017, 08:34
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Short answers (it's after my bedtime) - I can provide further details if needed:
Reverser - on the 737-300/400/500, the thrust control is hydromechanical. If the T/R deploys uncommanded there is a mechanical linkage that will retard the throttle to idle. On the NG, the thrust control is FADEC (EEC) - if the T/R deploys uncommanded, the EEC will electronically retard the thrust to idle, but the thrust levers won't move.
Hard Alternate:
Hard alternate is an unrated mode, roughly equivalent to the old hydromechanical fuel control. Hard Alternate will always provide thrust equal or greater than the same thrust lever position in Normal mode. At corner point temperature, thrust will be basically equal between Normal and Alternate mode. Below corner point, Alternate mode will be greater - quite a bit greater if the temperature is much lower than corner point.
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Old 23rd Jan 2017, 21:55
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Could you explain what is corner point? Thanks
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Old 24th Jan 2017, 19:36
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Corner point is the 'flat rate' temperature limit - below that temperature the max rated thrust is constant, above that temperature the max rated thrust drops as the temperature increases (at what basically works out to a constant EGT). On EPR engines it's real obvious - if you look at max EPR vs. OAT, max EPR is basically constant up to corner point, then drops off roughly linearly as the temperature increases. N1 engines are not so obvious since N1 increases with temp (that old square root of theta term), but there is a pretty obvious change in slope of N1 at corner point.
Corner point temperature is typically IAS plus 15 deg. C, although there are exceptions and special hot weather thrust rating options.
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