Preventing In Flight Deployment of Thrust Reverse 737NG
Hi all,
In the event of erroneous rad alt readings on the 737NG is there anything in place which would physically prevent the crew from being able to deploy the TRs in flight? This is based on the understanding they become deployable once <10' RA even with out air/ground sensing. Thank you in advance, CP |
Don't they need spin up of at least two wheels too?
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Originally Posted by TURIN
(Post 10917761)
Don't they need spin up of at least two wheels too?
If RA reads below 10ft (correct or false), you can deploy the reversers. |
Thrust Reverser
Each engine is equipped with a hydraulically operated thrust reverser, consisting of left and right translating sleeves. Aft movement of the reverser sleeves causes blocker doors to deflect fan discharge air forward, through fixed cascade vanes, producing reverse thrust. The thrust reverser is for ground operations only and is used after touchdown to slow the airplane, reducing stopping distance and brake wear. Hydraulic pressure for the operation of engine No. 1 and engine No. 2 thrust reversers comes from hydraulic systems A and B, respectively. If hydraulic system A and/or B fails, alternate operation for the affected thrust reverser is available through the standby hydraulic system. When the standby system is used, the affected thrust reverser deploys and retracts at a slower rate and some thrust asymmetry can be anticipated. The thrust reverser can be deployed when either radio altimeter senses less than 10ft altitude, or when the air/ground safety sensor is in the ground mode. Movement of the reverse thrust levers is mechanically restricted until the forward thrust levers are in the idle position. When reverse thrust is selected, an electro–mechanical lock releases, the isolation valve opens and the thrust reverser control valve moves to the deploy position, allowing hydraulic pressure to unlock and deploy the reverser system. An interlock mechanism restricts movement of the reverse thrust lever until the reverser sleeves have approached the deployed position. When either reverser sleeve moves from the stowed position, the amber REV indication, located on the upper display unit, illuminates. As the thrust reverser reaches the deployed position, the REV indication illuminates green and the reverse thrust lever can be raised to detent No. 2. This position provides adequate reverse thrust for normal operations. When necessary, the reverse thrust lever can be pulled beyond detent No. 2, providing maximum reverse thrust. Downward motion of the reverse thrust lever past detent No. 1 (reverse idle thrust) initiates the command to stow the reverser. When the lever reaches the full down position, the control valve moves to the stow position allowing hydraulic pressure to stow and lock the reverser sleeves. After the thrust reverser is stowed, the isolation valve closes and the electro–mechanical lock engages. I have a vague memory that the Classic B737 required wheel spin up. Not so on the NG |
Originally Posted by George Glass
(Post 10917815)
I have a vague memory that the Classic B737 required wheel spin up. Not so on the NG
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I've only managed one of those. Speed brake lever couldn't make it's mind up to come up or not... On a night flight early morning arrival on a LONG runway. AB off, idle reverse. slow roll out into a very slow taxi and smooth onto the stand. I swear all the passengers were asleep. Forgot to video it when I swung the flight deck door open to check. I'll never do that again so it's downhill as far as my flying goes.....
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Thanks for all of the replies guys - so it seems it would be possible to deploy a TR above 10' with a faulty Rad Alt.
Does any one know which RA the TRs work from, or where I can find out? Google hasn't come up with the goods yet! CP |
What's with the stupid questions.
It's like asking, is there anything to prevent the rudders being fully deflected by the pilot at 100 feet. Pointless. |
No first hand knowledge, but an old coworker once told me using the Radio Altimeter (aka LRRA) dates back to the original 737-100/200. Apparently the 737 can go into a ground effect 'float' such that even though the main gear are on the runway, there is insufficient weight on the gear to trigger the 'Weight on Wheels' sensor, or to reliably spin up the tires. Hence the change to allow it based solely on the LRRA. But even with an erroneous radio altimeter indicating less than 10 ft., it would still be necessary to lift the piggyback lever before you could get reverser deployment. I sincerely hope there are no pilots out there that are dumb enough to try it...
BTW, IIRC, the same is true of the 777 - the T/R inhibit is based on LRRA, not Weight on Wheels. |
Originally Posted by reamer
(Post 10918260)
What's with the stupid questions.
It's like asking, is there anything to prevent the rudders being fully deflected by the pilot at 100 feet. Pointless. Lord knows there’s been enough examples of jackass engineering. Like ADF antennae in square windows, rudder hard overs (73) and a little sumthing sumthing called MCAS. Lets not get started on Scarebus. |
Great input 👍
Originally Posted by reamer
(Post 10918260)
What's with the stupid questions.
It's like asking, is there anything to prevent the rudders being fully deflected by the pilot at 100 feet. Pointless. |
Thanks mate - this was my thought, you’d have to do so many dumb things to get there but theoretically it is possible.
Originally Posted by tdracer
(Post 10918286)
No first hand knowledge, but an old coworker once told me using the Radio Altimeter (aka LRRA) dates back to the original 737-100/200. Apparently the 737 can go into a ground effect 'float' such that even though the main gear are on the runway, there is insufficient weight on the gear to trigger the 'Weight on Wheels' sensor, or to reliably spin up the tires. Hence the change to allow it based solely on the LRRA. But even with an erroneous radio altimeter indicating less than 10 ft., it would still be necessary to lift the piggyback lever before you could get reverser deployment. I sincerely hope there are no pilots out there that are dumb enough to try it...
BTW, IIRC, the same is true of the 777 - the T/R inhibit is based on LRRA, not Weight on Wheels. |
Originally Posted by reamer
(Post 10918260)
What's with the stupid questions.
It's like asking, is there anything to prevent the rudders being fully deflected by the pilot at 100 feet. Pointless. |
reamer,
There is NO such thing as a stupid question in aviation!!! McHale. |
Originally Posted by excrab
(Post 10918514)
So why isn’t there something to prevent the rudders being fully deflected by the pilot at 100 feet ? It’s not necessarily a stupid question. This site is for professional pilots, and not all professional pilots fly jet transports or have any knowledge of them, so if there is something that they want to ask why shouldn’t they ask. The only stupid thing is not asking the question.
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If the pilot flying decides they want to crash the aircraft, it's pretty hard to design it so he/she can't.
And if you do, there's not much reason to have a human pilot... |
Originally Posted by Derfred
(Post 10918985)
Because you need full rudder deflection in the event of a single-engine go-around at 100 feet... ;)
At speeds above approximately 135 kts, both hydraulic system A and B pressure are each reduced within the main PCU by approximately 25% each. This function limits full rudder authority in flight after takeoff and before landing. But even with an erroneous radio altimeter indicating less than 10 ft., it would still be necessary to lift the piggyback lever before you could get reverser deployment. I sincerely hope there are no pilots out there that are dumb enough to try it... |
Re: Wheel spin up
Originally Posted by FlyingStone
(Post 10917767)
Not on the 737.
If RA reads below 10ft (correct or false), you can deploy the reversers. |
737 T/R Logic
Forgive me for a poor memory, but maybe 35 years ago, I recall an accident somewhere in the U.S. where a 737-200 ran off the end of the runway on landing. Can’t remember who/where. But, I do remember it was the typical unstable approach to a wet runway...can’t remember if there was a slight tailwind involved...not sure.
Anyway, typical deal...touched down fast, beyond the touchdown zone, very smooth touchdown. No wheel spin-up, not enough weight to compress the struts, therefore no auto speed brake deployment. Unable to deploy thrust reverse because of all this. Aircraft smoothly glided off the end. As a result, Boeing incorporated into the logic a Radio Altimeter switch function that would allow T/R deployment below a specified altitude...like maybe ten or fifteen feet RA. (Don’t remember details.) Logic opened isolation valve, or whatever it was called, so hydraulic pressure was available to the selector valves. Again, this accident occurred on the 737-200 (I’m guessing it was the advanced model...don’t know.), and I don’t know when, at what point, the 737 serial numbers had this feature. But, it was well before the NG. Maybe someone in this forum has a better memory than I, or can research this. Fly Safe, PantLoad |
Originally Posted by PantLoad
(Post 10919402)
Forgive me for a poor memory, but maybe 35 years ago, I recall an accident somewhere in the U.S. where a 737-200 ran off the end of the runway on landing. Can’t remember who/where. But, I do remember it was the typical unstable approach to a wet runway...can’t remember if there was a slight tailwind involved...not sure.
Anyway, typical deal...touched down fast, beyond the touchdown zone, very smooth touchdown. No wheel spin-up, not enough weight to compress the struts, therefore no auto speed brake deployment. Unable to deploy thrust reverse because of all this. Aircraft smoothly glided off the end. ... PantLoad Piedmont flight 467 at Charlotte, NC in 1986. Here's the report: http://libraryonline.erau.edu/online...s/AAR87-08.pdf At the time of the flight 467 accident the 737-200 required weight on main gear to allow reverser deployment. The conditions required for reverser deployment on that airplane were described in detail in the report. Originally the 737 required weight on the nose gear as well. The SB that eliminated the nose gear part of the logic was mentioned in the Cranbrook accident report as having been incorporated on that airplane. |
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