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HAWK21M
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T/R Overide Switch On RB211-535E4

Wed Jul 04, 2012 6:49 pm

Anyone aware of the history leading to the Installation of the T/R overide switch on the RB211-535E4.
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Tristarsteve
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RE: T/R Overide Switch On RB211-535E4

Wed Jul 04, 2012 9:38 pm

On our engines, the override switch was to bypass the eng oil low px switch so that you could test the thrust reverser with the engine stopped. Sometime in 2000 to 2005 the low oil px sw was removed, the thrust reverser was always live, so the override switch was removed as it was now redundant. ( I think, we haven't got any B757 anymore)
 
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HAWK21M
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RE: T/R Overide Switch On RB211-535E4

Thu Jul 05, 2012 8:46 am

Quoting Tristarsteve (Reply 1):
the override switch was removed as it was now redundant.

I think would have still been effective considering that the T/R can be deployed with Hydraulics pressurised on ground with Engines shut.
The safety is not when the TR gets erronously deployed,but it should never be restored when the error is realised without ground clearence.
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767eng
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RE: T/R Overide Switch On RB211-535E4

Fri Jul 06, 2012 10:43 am

It is exactly as Steve says, originally you needed Oil Pressure (ie engine running) to deploy the reverser. To allow maintenance to deploy without the engine running the override switch was fitted.

We did the mod to remove the oil pressure signal as it was another failure path for the reverser in normal use. At the same time the switch was deactivated as no longer required.

It was originally a sound idea but as the aircraft aged all it took was a poor connection somewhere in the oil pressure circuit and your reverser would fail to deploy - not good.
 
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HAWK21M
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RE: T/R Overide Switch On RB211-535E4

Fri Jul 06, 2012 11:28 am

Quoting 767eng (Reply 3):
It was originally a sound idea but as the aircraft aged all it took was a poor connection somewhere in the oil pressure circuit and your reverser would fail to deploy - not good

Makes sense....suprisingly RR never thought about a safety switch on the flight deck aka B737.
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boeingfixer
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RE: T/R Overide Switch On RB211-535E4

Sat Jul 07, 2012 7:11 pm

Quoting HAWK21M (Reply 4):
Makes sense....suprisingly RR never thought about a safety switch on the flight deck aka B737.

Actually I find the current RR system on the B757 better than that of the B737. The B737 Override switches are there for maintenance purposes and serve no function to the aircrew as far as the QRH is concerned.

The B757 T/R Override switch on the RB211 ended up being a maintenance issue and was therefore removed. Our company policy is to place a ground observer with interphone contact to the flight deck when we are operating the reverser for maintenance work. All other hangar operations dictate the deactivation of the T/R's to prevent accidental operation of them during maintenance on the aircraft.

As a side note, on Pratt powered B757's all you need to do is power up the EEC's on the P-61(or overhead on some aircraft) panel and you can operate the T/R's with the engines not running.

Cheers,

John
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HAWK21M
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RE: T/R Overide Switch On RB211-535E4

Mon Jul 09, 2012 10:19 am

Quoting boeingfixer (Reply 5):
The B737 Override switches are there for maintenance purposes and serve no function to the aircrew as far as the QRH is concerned.

True but you would def not make an error on the B737 unless it was deliberate....as deployment in air would mean:-
1.Fwd Thrust levers in Idle
2.Landing Gear down
3.Overide selected.
4.Hydraulics present
5.Air-Ground relay on Ground mode.
So you would need to be knowing what you were doing.
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boeingfixer
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RE: T/R Overide Switch On RB211-535E4

Wed Jul 11, 2012 4:25 pm

Quoting HAWK21M (Reply 6):
True but you would def not make an error on the B737 unless it was deliberate....as deployment in air would mean:-
1.Fwd Thrust levers in Idle
2.Landing Gear down
3.Overide selected.
4.Hydraulics present
5.Air-Ground relay on Ground mode.
So you would need to be knowing what you were doing.

Actually you can take #5 out of the equation if the override switch is placed to override in flight. This directly powers the T/R isolation valve from the Batt Bus as long as the respective fire handle is IN. This also bypasses the air/ground relay system.

The B757 has more safety features than the B737-100/200 when it comes to the T/R. The auto re-stow and sync lock actuator are very good at what they do. Unless there are multiple concurrent systems failures you won't get a B757 RB211 T/R to deploy in flight. Unlike The B737 with the override switches, there is nothing the aircrew can do to make the T/R's deploy in flight on the B757.

A change to the B737-100/200 T/R system was brought about by a Pacific Western Airlines crash not too far from where I live back on 11 Feb. 1978. Long story short, during an aborted landing, after the T/R's were deployed, the crew elected to go-around due to snow clearing equipment on the runway. One of the T/R's bucket doors didn't reach the stowed and locked position before the Air/Ground Relay took power away from the isolation valve. The buckets on one engine then fully deployed after the aircraft was in the air causing it to crash. This is why there is a holding relay in the Engine Accessory Unit to keep the Isolation Valve energized until both reverser door locks indicate Stowed and Locked.

Cheers,

John
Cheers, John YYC

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