bruin787
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Thrust Reversers Open During Taxi

Fri Jul 18, 2008 2:49 pm

I wanted to share a few pictures I took yesterday as it startled the hell out of me. For those of you that are famliar with LAX, these pictures were taken from Imperial Hill and show a LH A346 taxiing all the way from the NORTH side of the airfield to the south with at least two of the four reversers open all the way ( I couldnt clearly see the other two engines). Now, this takes several minutes. He eventually came to a stop, closed one of the two that I could see, but continued taxiing with at least one open. He eventually closed it just before entering the alley for his gate.

I have NEVER seen anything like this before. Is this common? Or even allowed? I know pilots are sometimes slow to stow them away while clearing the runway, but this guy kept them open for several minutes!  Wow!

http://farm4.static.flickr.com/3031/2679436010_d6f5441973_o.jpg

http://farm4.static.flickr.com/3140/2679436080_fcf508152c_o.jpg
http://taxiwayalpha.blogspot.com
 
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jetmech
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RE: Thrust Reversers Open During Taxi

Fri Jul 18, 2008 3:03 pm



Quoting Bruin787 (Thread starter):

Each reverser is controlled by a separate thrust reverser (TR) lever, so they actually move independently of one another. What you probably saw for the extended period, was the engine at idle with the TR shells still extended.

IIRC, most TR levers have two regions. The first region is the slight upward pull which allows the TR to translate to the extended position. Once the TR is fully extended, an inter-lock is released, which allows further upward movement of the lever. This subsequent region controls reverse thrust from idle to full power.

I dare say that if the TR lever is in the first region no damage will be caused. The TR will stay extended for as long as the lever is in the first region and there is no system to forcibly retract the reversers.

For maintenance purposes, we often extended the TR's with the lever, and they stayed there for as long as we needed. With the situation you observed, the engine was running, but I'm pretty sure this would have been at idle only. Idle reverse thrust is unlikely to cause any damage.

Nice photos BTW, particularly the first one  Smile .

Regards, JetMech

[Edited 2008-07-18 08:08:18]
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
SlamClick
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RE: Thrust Reversers Open During Taxi

Fri Jul 18, 2008 3:32 pm

Lots of airplanes have more residual thrust at ground idle than they need to sustain a safe taxi speed. They will accelerate the whole time, unless taxiing up a significant hill. Some ill-advised pilots will counter this by dragging the brakes. Most of us will let the speed build up on a long taxi, then brake down to a slower speed than we want, and let it accelerate again. Some taxi routes might be a couple of miles or more.

Another speed control measure, where permitted might be to use reverse thrust in moderation.

Quoting JetMech (Reply 1):

Correct.

Think of it this way.

> With the reverse levers stowed, you can move the thrust levers into the forward thrust range, all the way to TOGA power. The reverse lever is locked DOWN while the thrust lever is off the idle stop.

> With the thrust lever at the idle stop, the reverse lever is unlocked and can be raised. If you raise the reverser lever you can no longer move the thrust levers forward.

> If you raise the reverse lever, you unstow the reverser doors, either sliding sleeve linked to blocker doors, or old clamshell type. Once these devices have moved by either hydraulic or pneumatic power, to their reverse position, you can continue to raise the reverser handles and they become (reverse) thrust levers.

There is no real problem being at reverse idle. It is about the same as forward idle, just a little different airflow around the engines. If you go into higher thrust settings in reverse at slow forward speeds then you tend to recirculate the air and the engine starts re-ingesting exhaust air. Breathing this air with high temperatures and low oxygen content is not a good situation and eventually can lead to compressor stalls and other unwanted results.

Now in the description above I've been talking about one type of reverser control. There are two basic models. Some jets have a thrust lever and a separate reverser lever attached on the forward side of that, hinged upward to increase reverse thrust. Others you raise a lockout lever, even raise the thrust lever out of a detent, then you pull aft on the thrust lever to throttle up in reverse.

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Photo © Werner Horvath
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Photo © Ismael Jorda


First type on a Boeing left: Lever and smaller lever bottom center.
Second type on an Airbus right: Little tabs on forward side of thrust levers (lower left) are reverse lockout levers. Lift them and you can pull back into the reverse thrust range.
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bomber996
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RE: Thrust Reversers Open During Taxi

Fri Jul 18, 2008 3:33 pm

I've seen multiple private jets use this practice at the small airport where I work. I always assumes that they would deploy the TR for extended periods to slowly slow the aircraft down instead of using the brakes. If you're going to use the engine in idle might aswell use idle TR as you're going to burn the fuel anyways. But thats just my speculation.

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SlamClick
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RE: Thrust Reversers Open During Taxi

Fri Jul 18, 2008 4:12 pm



Quoting Bomber996 (Reply 3):
But thats just my speculation.

Well, I think I agree. If the airplane manual and the company ops manual permit it, I think there are advantages over use of wheel brakes. Even the fuel consumption argument against is debatable. Especially in high-bypass fans you do get forward thrust with revese idle. The core exhaust typically does not get reverse vectored. So fuel burn is about the same. Offsetting that, you would have to know how much extra fossil fuel is used in the manufacture transport and installation of brakes which would be increased a lot by use during taxi.

Further, accelerate-stop distance as part of the takeoff performance calculations is based on full braking authority. If you have heated the brakes a significant amount in a long taxi-out, perhaps you don't still have full brake capacity.

I've even heard of brake overtemps during taxi out causing the wheel fuse plugs to let go after retraction. Not to get too technical here but - bad thing.
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avioniker
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RE: Thrust Reversers Open During Taxi

Fri Jul 18, 2008 5:10 pm



Quoting SlamClick (Reply 4):
Further, accelerate-stop distance as part of the takeoff performance calculations is based on full braking authority. If you have heated the brakes a significant amount in a long taxi-out, perhaps you don't still have full brake capacity.

I've even heard of brake overtemps during taxi out causing the wheel fuse plugs to let go after retraction. Not to get too technical here but - bad thing.

With the SepCarb brakes on the bus fleet, the hotter the better; but the fuse plug factor still comes into play.
It's normal procedure to taxi with the reversers deployed after a heavy landing taking the temps above a nominal level so you don't pop the fuses.

At TWA a not small fortune was saved by taxiing with one or more reverser deployed thus saving the brakes. Of course we didn't have the new brake technology then so cooler was better. When the results of a one year study were compiled on the MD fleet it was found that keeping one reverser open most of the way to the gate was worth over 30 landings. Amazing the things that Icahn caused to be discovered in the name of savings.

 Smile
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SlamClick
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RE: Thrust Reversers Open During Taxi

Fri Jul 18, 2008 11:57 pm



Quoting Avioniker (Reply 5):
on the bus fleet, the hotter the better

Within reason of course.  Smile I know of at least one airline that has a published maximum brake temperature for takeoff on its Airbus fleet. In that case it is also keyed to takeoff performance calculation.
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DashTrash
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RE: Thrust Reversers Open During Taxi

Sat Jul 19, 2008 3:25 am



Quoting Bomber996 (Reply 3):
I've seen multiple private jets use this practice at the small airport where I work. I always assumes that they would deploy the TR for extended periods to slowly slow the aircraft down instead of using the brakes. If you're going to use the engine in idle might aswell use idle TR as you're going to burn the fuel anyways. But thats just my speculation.

I pop one bucket open almost every time I taxi (Citation X). The airplane has so much thrust at idle many times I don't even spool an engine to get it rolling. Since we don't have single engine taxi procedures, this or riding the brakes is the only option to keep the speed down. It's not really effective to use reverse to stop the aircraft. We're also only approved to use one reverser during taxi, except for doing the TR check on the first flight of the day.

Not aware of any airliners that are approved for this, but obviously some are. I've been in a CRJ jumpseat where the crew taxied with one open, but it was a no-no for them to do it.
 
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HAWK21M
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RE: Thrust Reversers Open During Taxi

Sat Jul 19, 2008 7:51 am

Normally attempted to reduce the Taxi speed,without riding the brakes.
regds
MEL
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DeltaGuy
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RE: Thrust Reversers Open During Taxi

Sat Jul 19, 2008 1:41 pm



Quoting Bomber996 (Reply 3):
I've seen multiple private jets use this practice at the small airport where I work. I always assumes that they would deploy the TR for extended periods to slowly slow the aircraft down instead of using the brakes.

We routinely taxi with both buckets out on the Gulfstream. The jet has so much thrust that it requires very little breakaway power on the initial roll, and on a long taxi the jet will speed up...keeping both buckets out (no reverse thrust, just reversers deployed) will keep your speed managable and brake temps low.

I've seen some Gulfstream drivers forget to dis-arm the Ground Spoiler switch (aft of the throttles). The ground spoilers are actuated by a contact switch in the throttles, both levers must be at idle (with the weight on wheels). When you forget to do this, you'll be taxiing around with buckets out, and spoilers going up and down as you jockey the throttles.

We were watching FoxNews one day and they were showing this criminal being extradited back to the US, arriving in a GIV, with the spoilers going up and down....we knew who forgot to do something.

DeltaGuy
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phxpilot
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RE: Thrust Reversers Open During Taxi

Sat Jul 19, 2008 9:50 pm

When I was at Mesa flying the CRJ-900 I used to do this all the time. One great example is taxiing to 25R at LAS. It's downhill most of the way. I would leave it in idle reverse and never have to touch the brakes.

This did not work too well on the -200. Those reversers are pneumatically powered and if you left them at idle reverse for any length of time, you could not stow them again. Unless you powered up above idle reverse.
 
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jetmech
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RE: Thrust Reversers Open During Taxi

Sun Jul 20, 2008 7:39 am



Quoting Phxpilot (Reply 10):
This did not work too well on the -200. Those reversers are pneumatically powered and if you left them at idle reverse for any length of time, you could not stow them again. Unless you powered up above idle reverse.

That's very interesting! I know it is a completely different beast altogether, but a contrasting phenomenon could be observed on RR powered 747's. RB211-524D4 and G2 TR's are pneumatically powered also. The TR's used on these engines are of the translating shell type, with internal blocker doors.

As you can image, it is highly dangerous to enter the cold air stream duct due to the fact you are in close proximity to the blocker doors. One of the "safe guards" to prevent accidents during maintenance was to ensure that the pneumatic supply from the APU was shut off from the wings. Mind you, this was an assumed safe way to enter the TR duct if one could not be bothered to fit the locking pins.

A group of LAME's experimented with the TR's with APU air supply switched off. To their horror, it was found that there was enough residual pressure in the wing pneumatic ducts to cycle the TR's several times! So much for the TR's being "safe" when APU air was shut off to the wings!

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
KAUSpilot
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RE: Thrust Reversers Open During Taxi

Sun Jul 20, 2008 8:06 am

I will say on the EMB-145/135, it is prohibited, at least at my company.

The trust reversers are required to be at idle no later than 60 knots on the landing rollout and the buckets stowed no later than 40 knots (barring emergency situations, such as aborted takeoffs or brake failures).

The reason given is the potential for FOD damage to the engines. Then again, it is a relatively light aircraft by jet transport standards so the brakes are always more than adequate for stopping power, especially during single engine ops.
 
SlamClick
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RE: Thrust Reversers Open During Taxi

Sun Jul 20, 2008 3:12 pm



Quoting Phxpilot (Reply 10):
One great example is taxiing to 25R at LAS.

That is one place I was thinking about as I was writing reply #2. It is rather deceptive until you get to know the airport, but there is a fair downhill run out that way. One airline I didn't fly for once ran a 737 off into the desert, unable to make the turn or stop there. Most embarassing.
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bio15
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RE: Thrust Reversers Open During Taxi

Tue Jul 22, 2008 8:52 pm



Quoting SlamClick (Reply 2):
If you go into higher thrust settings in reverse at slow forward speeds then you tend to recirculate the air and the engine starts re-ingesting exhaust air. Breathing this air with high temperatures and low oxygen content is not a good situation and eventually can lead to compressor stalls and other unwanted results.

Hi Slamclick. Does this also happen when cold bypass air (cascade type reversers) operate on above-idle reverse thrust?
 
tdscanuck
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RE: Thrust Reversers Open During Taxi

Wed Jul 23, 2008 12:05 am



Quoting Bio15 (Reply 14):
Does this also happen when cold bypass air (cascade type reversers) operate on above-idle reverse thrust?

Yes. Even "cold" bypass air is in the 150-200 degF range thanks to all the energy dumped in by the fan.

Tom.

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