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Preventing In Flight Deployment of Thrust Reverse 737NG

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Preventing In Flight Deployment of Thrust Reverse 737NG

Old 3rd Nov 2020, 09:20
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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
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Old 3rd Nov 2020, 10:24
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Don't they need spin up of at least two wheels too?
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Old 3rd Nov 2020, 10:33
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Originally Posted by TURIN View Post
Don't they need spin up of at least two wheels too?
Not on the 737.

If RA reads below 10ft (correct or false), you can deploy the reversers.
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Old 3rd Nov 2020, 11:42
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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 electroĖmechanical 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 electroĖmechanical lock engages.

I have a vague memory that the Classic B737 required wheel spin up. Not so on the NG
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Old 3rd Nov 2020, 16:51
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Originally Posted by George Glass View Post
I have a vague memory that the Classic B737 required wheel spin up. Not so on the NG
No, same logic on the Classic. Have my classic FCOM only in paper, but even the wording is exactly the same as your quoted NG excerpt. Happened to me once in my early days as a commercial pilot on a -300, the smoothest touch down i ever had, and a slightly puzzled captain asking me during rollout "did you mean to open the reversers in the air?". I explained that i honestly thought we were on the ground already. Well, that was the debriefing done, and nothing else happened. Today it would probably mean at least a trip to the head office for a very uncomfortable talk, or, considering the surplus in personnel, a very short trip out the door.
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Old 3rd Nov 2020, 17:05
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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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Old 3rd Nov 2020, 19:14
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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
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Old 4th Nov 2020, 00:03
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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.
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Old 4th Nov 2020, 01:29
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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.
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Old 4th Nov 2020, 06:37
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Originally Posted by reamer View Post
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.
Systems questions are rarely stupid
Lord knows thereís 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.
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Old 4th Nov 2020, 10:25
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Great input 👍

Originally Posted by reamer View Post
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.
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Old 4th Nov 2020, 10:27
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Thanks mate - this was my thought, youíd have to do so many dumb things to get there but theoretically it is possible.

Originally Posted by tdracer View Post
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.
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Old 4th Nov 2020, 10:43
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Originally Posted by reamer View Post
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.
So why isnít there something to prevent the rudders being fully deflected by the pilot at 100 feet ? Itís 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 shouldnít they ask. The only stupid thing is not asking the question.
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Old 4th Nov 2020, 22:33
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reamer,

There is NO such thing as a stupid question in aviation!!!

McHale.
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Old 5th Nov 2020, 01:44
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Originally Posted by excrab View Post
So why isnít there something to prevent the rudders being fully deflected by the pilot at 100 feet ? Itís 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 shouldnít they ask. The only stupid thing is not asking the question.
Because you need full rudder deflection in the event of a single-engine go-around at 100 feet...
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Old 5th Nov 2020, 02:02
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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...
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Old 5th Nov 2020, 12:03
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Originally Posted by Derfred View Post
Because you need full rudder deflection in the event of a single-engine go-around at 100 feet...
From the FCOM:

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.
I think you're confusing full rudder pedal input with full rudder surface deflection?

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...
No but bad habits creep in. I have had quite a few pilots get in the habit of moving their hands from the thrust levers onto the reversers in the flare after they'd closed them and start to squeeze upwards. A karate chop usually ensued and a friendly chat on the taxi in about the dangers of having one reverser open and potentially one not open at 10' (I had reversers fail to open from time to time) and the runway excursion that would likely occur. As with most bad habits they were unaware they were doing it and no one had told them.
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Old 5th Nov 2020, 12:24
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Re: Wheel spin up

Originally Posted by FlyingStone View Post
Not on the 737.

If RA reads below 10ft (correct or false), you can deploy the reversers.
Correct I was getting mixed up with the auto speed brake logic. Previously discussed here .
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Old 5th Nov 2020, 14:01
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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. Canít remember who/where. But, I do remember it was the typical unstable approach to a wet runway...canít 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. (Donít 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 (Iím guessing it was the advanced model...donít know.), and I donít 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
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Old 6th Nov 2020, 04:37
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Originally Posted by PantLoad View Post
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. Canít remember who/where. But, I do remember it was the typical unstable approach to a wet runway...canít 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
Pretty darn good memory in fact!

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.

Last edited by Dave Therhino; 6th Nov 2020 at 06:19.
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