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Faro
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Why Have Thrust Reversers At All?

Sat Jan 12, 2008 10:08 am

If thrust reversing is not taken into account in computations of certification/regulatory landing distance requirements and airliners can be safely dispatched with thrust reversers unserviceable, why have them at all? They add weight, complexity and maintenance costs and some aircraft like the VC-10 and A380 have been designed with only 2 T/R out of 4 engines. They don't seem to be terrible necessary...

Faro
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futureualpilot
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 10:15 am

Shorten up the landing distance and give the aircraft an added safety margin. They probably can also help shorten the amount of time an aircraft takes on the runway, increasing the efficiency of any given runway.
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Faro
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 10:24 am



Quoting Futureualpilot (Reply 1):


Shorten up the landing distance and give the aircraft an added safety margin. They probably can also help shorten the amount of time an aircraft takes on the runway, increasing the efficiency of any given runway.

In my cost-benefit analysis, does this really justify hauling around all that added weight and cost over the 20+ year useful life of my aircraft? With the cost of fuel being what it is, I believe there may be a serious case to be made for the reverserless airliner if safe landing distances are catered to 100% by the brakes alone...

Faro
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thegeek
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 11:12 am

Only thing I can suggest is that they save wear on the brakes. Also, not relying nearly 100% on the wheel brakes builds in some limited redundancy.
 
oly720man
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 12:28 pm



Quoting Faro (Reply 2):
In my cost-benefit analysis, does this really justify hauling around all that added weight and cost over the 20+ year useful life of my aircraft?

What's the cost and time out of service to change the brakes more often?
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Buzz
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 1:53 pm

Hi Oly720Man, Buzz here. To change a 737 brake, which is not intended for continuous duty high speed stops takes a couple mechanics 2 -3 hours. Only about 2 hours for an A320... it can be done faster by experienced mechanics, but I was factoring in getting the tools, parts, drive over to the airplane. Most MRO's charge 50-60 dollars per hour

And I recall seeing what was left of some L-1011 tires and brakes after a rejected takeoff near V1. #3 engine had a compressor stall, and the flight crew dragged the airplane to a stop on the runway rather than fly SFO-OSA. So they were heavy. Since #3 engine was misbehaving, the flight crew used the RH brakes "heavily". All 4 tires on that bogie absorbed so much energy they had melted the fuse plugs and deflated by the time they taxied back to the gate. And what I saw of the tires... they had melted off the rims, brakes were stuck inside the rims. But that was an extreme case.

Of course you could have drag chutes, they're "always" fun to pull off the runway, re-pack , and install. Thrust reversers are a reasonable way to dissipate the excess kinetic energy. After you slow to 60 knots or so the brakes can easily absorb the remaining energy.
 
KensukeAida
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 1:54 pm

It's an added safety margin, and it reduces wheel brake wear. Plus, during wet runway conditions or when accidentally landing long, the thrust reversers would be more effective than wheel breaks alone. We saw that with AF 358 and WN 1248 (delayed reverser activation were contributing causes to a runway overrun). And with airport congestion, they help get the damn plane off the runway faster.

Quoting Faro (Thread starter):
A380 have been designed with only 2 T/R out of 4 engines

Incidentally, that was because the FAA told them they had to (the original design called for none). But they managed to turn this into a positive by making them electrically activated, which saves weight and adds even more redundancy in case of a hydraulic failure.

Bottom line is that Airbus managed to do something neat, and they are still seen as necessary by regulatory agencies.

- John
 
cptspeaking
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 2:28 pm



Quoting Thegeek (Reply 3):

 checkmark 

Another thing to consider with thrust reversers is that even when you don't spool the engines and only put them in idle reverse, you are creating a near zero thrust situation. This alone can decrease the stopping distance from a forward idle thrust setting, then when you spool the engines in reverse, you decrease it even more. As stated, reversers are not required for safe flight, but they give an added safety margin and reduce wear on other parts.

Also, most importantly, they sound cool.  Wink
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lowrider
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 2:52 pm

Also, if your aircraft has brake cooling issues, generous use of reverse reduces the energy the brakes must absorb, leading to cooler brakes and shorter turn times.

Quoting Faro (Reply 2):
In my cost-benefit analysis, does this really justify hauling around all that added weight and cost over the 20+ year useful life of my aircraft?

Look at it this way, they only have to save the aircraft from going off a slippery runway once and they have paid for themselves, be it on a landing or a reject. Further, not taking them into account in performance calculations and then using them builds in an extra margin for error.
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OldAeroGuy
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 4:36 pm



Quoting Faro (Thread starter):
If thrust reversing is not taken into account in computations of certification/regulatory landing distance requirements and airliners can be safely dispatched with thrust reversers unserviceable, why have them at all?

Current takeoff performance regulations for both the FAA and EASA allow the use of reverse thrust when calculating balanced field length requirements from a wet runway. Under these conditions, reverse thrust provides a significant performance benefit.

Quoting KensukeAida (Reply 6):
It's an added safety margin, and it reduces wheel brake wear. Plus, during wet runway conditions or when accidentally landing long, the thrust reversers would be more effective than wheel breaks alone.

Agree and you can add the real world stopping capability reversers provide on runways with snow, slush and/or ice.
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nema
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 5:15 pm

I find that fewer flights i take use the reversers on landing these days.
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pilotpip
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 5:37 pm

I'd much rather have them available to me on a wet/icy runway than not have them. They help quite a bit even though they are not taken into landing performance data. Remember performance data is usually worst-case scenario.
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barney captain
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 5:48 pm



Quoting Faro (Thread starter):
If thrust reversing is not taken into account in computations of certification/regulatory landing distance requirements

It is on the 737-700. AFAIK, it's the only airliner that was certified using TR's in the landing data computations. All other a/c that I'm aware of, the TR's on landing are just a bonus. We can land without them under certain conditions, but you take a performance penalty for it.
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jhooper
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 9:35 pm

In the C-5, you actually need to use them for emergency descent purposes, because you can't use spoilers for that purpose in-flight.
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2H4
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 9:37 pm



Quoting Jhooper (Reply 13):
In the C-5, you actually need to use them for emergency descent purposes, because you can't use spoilers for that purpose in-flight.

Does having one or more inop reversers limit the aircraft's maximum cruising altitude?

2H4
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OldAeroGuy
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 10:46 pm



Quoting Barney Captain (Reply 12):
It is on the 737-700. AFAIK, it's the only airliner that was certified using TR's in the landing data computations.

Not true. The 737-700 was not certified using reverse thrust as part of its basic landing performance.

The 737-700 was certified at Part 25 Amendment 77. The FAR 25.125 Landing regulation in effect was at Amend. 72 as shown by the link below:

http://rgl.faa.gov/REGULATORY_AND_GU...DD0E5852566720049A5A0?OpenDocument

The pertinent part of the regulation is quoted below:

(3) Means other than wheel brakes may be used if that means--
(i) Is safe and reliable;
(ii) Is used so that consistent results can be expected in service; and
(iii) Is such that exceptional skill is not required to control the airplane.

This language goes all the way back to Amend 0 given in the following link:

http://rgl.faa.gov/REGULATORY_AND_GU...B1183852566720049A04E?OpenDocument

(3) Means other than wheel brakes may be used if that means--
(i) Is safe and reliable;
(ii) Is used so that consistent results can be expected in service; and
(iii) Is such that exceptional skill is not required to control the airplane.

While this language seems to allow the use of reverse thrust, the FAA have consistently ruled that thrust reversers do not qualify under this wording. This has been true for the airplanes certified both before and after the 737-700.

It's possible that your airline has operational rules for determining landing field length with and without reverse thrust for the 737-700, but the certified data is not based on the use of reverse thrust.
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jhooper
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RE: Why Have Thrust Reversers At All?

Sat Jan 12, 2008 11:34 pm



Quoting 2H4 (Reply 14):
Does having one or more inop reversers limit the aircraft's maximum cruising altitude?

#1 and #4 - No (Can't use them in-flight anyway)
#2 and $3 - Yes (FL250)
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tdscanuck
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RE: Why Have Thrust Reversers At All?

Sun Jan 13, 2008 12:06 am



Quoting Faro (Thread starter):
If thrust reversing is not taken into account in computations of certification/regulatory landing distance requirements and airliners can be safely dispatched with thrust reversers unserviceable, why have them at all?

Airlines like 'em for the insurance and flexibility.

Quoting Faro (Reply 2):
In my cost-benefit analysis, does this really justify hauling around all that added weight and cost over the 20+ year useful life of my aircraft?

It's an outlier problem. As noted earlier, they only have to save you once in 20+ years and they more than pay for themselves.

Quoting Thegeek (Reply 3):
Only thing I can suggest is that they save wear on the brakes. Also, not relying nearly 100% on the wheel brakes builds in some limited redundancy.

This works to reduce wear for steel brakes, but not carbon. Steel brakes wear based on the amount of energy absorbed. Carbon brakes wear based on number of applications.

For wheel brakes, you've got some redundancy because you've got multiple brakes (you are unlikely to lose all of them at once) but having a method that doesn't rely on ground friction certainly increases your options.

Quoting Barney Captain (Reply 12):
Quoting Faro (Thread starter):
If thrust reversing is not taken into account in computations of certification/regulatory landing distance requirements

It is on the 737-700.

Nope. OldAeroGuy is correct. It is factored into balanced field length and wet runway performance, but not certification landing distance.

Tom.
 
oly720man
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RE: Why Have Thrust Reversers At All?

Sun Jan 13, 2008 10:26 am



Quoting Buzz (Reply 5):
Most MRO's charge 50-60 dollars per hour

Parts on top of that presumably?

So, if desired, this could be done overnight during dead time.

As a a matter of interest, do a/c like the 146 without reversers need brakes changing more regularly than, say, an A320 or 737? Or are they designed to use (thicker?) brakes that will last a certain number of cycles?
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FlyASAGuy2005
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RE: Why Have Thrust Reversers At All?

Sun Jan 13, 2008 10:38 am



Quoting CptSpeaking (Reply 7):
Also, most importantly, they sound cool.

You got that right! Nothing better than landing in some Caribbean destination on a 757!  Big grin
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Viscount724
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RE: Why Have Thrust Reversers At All?

Mon Jan 14, 2008 2:58 am



Quoting Tdscanuck (Reply 17):
It is on the 737-700.

Nope. OldAeroGuy is correct. It is factored into balanced field length and wet runway performance, but not certification landing distance.

My guess is that the Southwest 737-700 that overran the snow-covered runway at MDW a couple of years ago and struck a car, killing a passenger in the car, would probably have been more serious if the 737 lacked thrust reversers.
 
cptspeaking
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RE: Why Have Thrust Reversers At All?

Mon Jan 14, 2008 4:55 am

Quoting Viscount724 (Reply 20):

Yes, however if I remember correctly, the crew couldn't/didn't get the TRs deployed for something like 14 seconds after touchdown (with a tailwind on a runway with poor braking action), which ended up being a contributing factor. So even though they were installed, they didn't really help much in that instance...

http://ntsb.gov/ntsb/brief.asp?ev_id=20051213X01964&key=1

Quoting NTSB Probable Cause report:
The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

The pilots' failure to use available reverse thrust in a timely manner to safely slow or stop the airplane after landing, which resulted in a runway overrun. This failure occurred because the pilots' first experience and lack of familiarity with the airplane's autobrake system distracted them from thrust reverser usage during the challenging landing.


[Edited 2008-01-13 20:56:44]
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Qantas744er
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RE: Why Have Thrust Reversers At All?

Mon Jan 14, 2008 7:44 am

What many don't know is that on planes like the 744 737NG a340 etc. etc. When a autobrake setting is selected the addition use of thrust reverse will not decrease landing distance but simply decrease brake pressure applied to mantain the autobrakes defined decelleration rate. Only on the RTO setting were max physical brake pressure is applied and no fixed decelleration rate the use of T/R will decrease stopping distance.

Leo  Smile
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iwok
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RE: Why Have Thrust Reversers At All?

Mon Jan 14, 2008 10:52 am



Quoting Faro (Reply 2):
In my cost-benefit analysis, does this really justify hauling around all that added weight and cost over the 20+ year useful life of my aircraft? With the cost of fuel being what it is, I believe there may be a serious case to be made for the reverserless airliner if safe landing distances are catered to 100% by the brakes alone...

I think its intended that for the one time you really need the RT for what ever reason, they are there to help you.

Quoting Tdscanuck (Reply 17):
It's an outlier problem. As noted earlier, they only have to save you once in 20+ years and they more than pay for themselves.

The problem comes when one is INOP and you have a wet runway to land on: it seems to cause many problems. It also seems to add additional workload for the flight deck which is manageable under 99.999% of the time, but which causes problems during very tough landings.

I personally think that not having them is actually safer, because it could help the flight deck be more conservative on landing approaches. But of course the bean counters don't like this at all.

iwok
 
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zeke
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RE: Why Have Thrust Reversers At All?

Mon Jan 14, 2008 12:07 pm



Quoting Tdscanuck (Reply 17):
This works to reduce wear for steel brakes, but not carbon. Steel brakes wear based on the amount of energy absorbed. Carbon brakes wear based on number of applications.

 checkmark  and using reverse with carbon brakes can increase wear as well. Carbon brakes like to work in their medium temperature range for highest performance, using a low auto brake setting and reverse takes the carbon brakes longer to heat up.
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OldAeroGuy
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RE: Why Have Thrust Reversers At All?

Mon Jan 14, 2008 5:04 pm



Quoting CptSpeaking (Reply 21):
Yes, however if I remember correctly, the crew couldn't/didn't get the TRs deployed for something like 14 seconds after touchdown (with a tailwind on a runway with poor braking action), which ended up being a contributing factor. So even though they were installed, they didn't really help much in that instance...

http://ntsb.gov/ntsb/brief.asp?ev_id=20051213X01964&key=1

Quoting NTSB Probable Cause report:
The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

The pilots' failure to use available reverse thrust in a timely manner to safely slow or stop the airplane after landing, which resulted in a runway overrun. This failure occurred because the pilots' first experience and lack of familiarity with the airplane's autobrake system distracted them from thrust reverser usage during the challenging landing.

And this pretty much sums up why reverse thrust still has value in real operations as opposed to pure regulatory performance.
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roseflyer
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RE: Why Have Thrust Reversers At All?

Mon Jan 14, 2008 9:29 pm



Quoting Qantas744er (Reply 22):
Only on the RTO setting were max physical brake pressure is applied and no fixed decelleration rate the use of T/R will decrease stopping distance.

The highest Autobrake setting is full brake pressure. You have reduced braking pressure settings of 1, 2 and 3. But you also have max and RTO settings on the Autobrake selector. You will get the same max pressure setting when using this setting. Reverse thrust will help decrease stopping distance.

Quoting Qantas744er (Reply 22):
What many don't know is that on planes like the 744 737NG a340 etc. etc. When a autobrake setting is selected the addition use of thrust reverse will not decrease landing distance but simply decrease brake pressure applied to mantain the autobrakes defined decelleration rate.

On the 737NG, the autobrake selector corresponds to a specific deceleration rate, which then has a varied pressure applied to the brakes.

Setting/Brake Pressure
1: 4 ft/sec/sec
2: 5
3: 7.2
max: 3000psi
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