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Why Have Thrust Reversers At All?  
User currently offlineFaro From Egypt, joined Aug 2007, 1545 posts, RR: 0
Posted (6 years 8 months 1 week 2 days 19 hours ago) and read 4003 times:

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


The chalice not my son
26 replies: All unread, showing first 25:
 
User currently offlineFutureualpilot From United States of America, joined May 2000, 2602 posts, RR: 8
Reply 1, posted (6 years 8 months 1 week 2 days 19 hours ago) and read 4000 times:

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.


Life is better when you surf.
User currently offlineFaro From Egypt, joined Aug 2007, 1545 posts, RR: 0
Reply 2, posted (6 years 8 months 1 week 2 days 19 hours ago) and read 4000 times:



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



The chalice not my son
User currently offlineThegeek From Australia, joined Nov 2007, 2638 posts, RR: 0
Reply 3, posted (6 years 8 months 1 week 2 days 18 hours ago) and read 3990 times:

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.

User currently offlineOly720man From United Kingdom, joined May 2004, 6727 posts, RR: 11
Reply 4, posted (6 years 8 months 1 week 2 days 17 hours ago) and read 3963 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?

What's the cost and time out of service to change the brakes more often?



wheat and dairy can screw up your brain
User currently offlineBuzz From United States of America, joined Nov 1999, 697 posts, RR: 21
Reply 5, posted (6 years 8 months 1 week 2 days 15 hours ago) and read 3930 times:

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.


User currently offlineKensukeAida From United States of America, joined Oct 2005, 217 posts, RR: 0
Reply 6, posted (6 years 8 months 1 week 2 days 15 hours ago) and read 3930 times:

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


User currently offlineCptSpeaking From United States of America, joined Apr 2006, 639 posts, RR: 1
Reply 7, posted (6 years 8 months 1 week 2 days 15 hours ago) and read 3911 times:



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



...and don't call me Shirley!!
User currently offlineLowrider From United States of America, joined Jun 2004, 3220 posts, RR: 10
Reply 8, posted (6 years 8 months 1 week 2 days 14 hours ago) and read 3898 times:

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.



Proud OOTSK member
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3523 posts, RR: 66
Reply 9, posted (6 years 8 months 1 week 2 days 13 hours ago) and read 3845 times:



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.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineNEMA From United Kingdom, joined Feb 2006, 716 posts, RR: 0
Reply 10, posted (6 years 8 months 1 week 2 days 12 hours ago) and read 3822 times:

I find that fewer flights i take use the reversers on landing these days.


There isnt really a dark side to the moon, as a matter of fact its all dark!
User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 11, posted (6 years 8 months 1 week 2 days 12 hours ago) and read 3810 times:

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.


DMI
User currently offlineBarney Captain From United States of America, joined Nov 2001, 965 posts, RR: 13
Reply 12, posted (6 years 8 months 1 week 2 days 12 hours ago) and read 3803 times:



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.



...from the Banana Republic....
User currently offlineJhooper From United States of America, joined Dec 2001, 6204 posts, RR: 12
Reply 13, posted (6 years 8 months 1 week 2 days 8 hours ago) and read 3740 times:

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.


Last year 1,944 New Yorkers saw something and said something.
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 14, posted (6 years 8 months 1 week 2 days 8 hours ago) and read 3740 times:
AIRLINERS.NET CREW
DATABASE EDITOR



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



Intentionally Left Blank
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3523 posts, RR: 66
Reply 15, posted (6 years 8 months 1 week 2 days 7 hours ago) and read 3694 times:



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.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineJhooper From United States of America, joined Dec 2001, 6204 posts, RR: 12
Reply 16, posted (6 years 8 months 1 week 2 days 6 hours ago) and read 3664 times:



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)



Last year 1,944 New Yorkers saw something and said something.
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 17, posted (6 years 8 months 1 week 2 days 5 hours ago) and read 3648 times:



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.


User currently offlineOly720man From United Kingdom, joined May 2004, 6727 posts, RR: 11
Reply 18, posted (6 years 8 months 1 week 1 day 19 hours ago) and read 3519 times:



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?



wheat and dairy can screw up your brain
User currently offlineFlyASAGuy2005 From United States of America, joined Sep 2007, 7004 posts, RR: 11
Reply 19, posted (6 years 8 months 1 week 1 day 19 hours ago) and read 3516 times:



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



What gets measured gets done.
User currently offlineViscount724 From Switzerland, joined Oct 2006, 25311 posts, RR: 22
Reply 20, posted (6 years 8 months 1 week 1 day 2 hours ago) and read 3367 times:



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.


User currently offlineCptSpeaking From United States of America, joined Apr 2006, 639 posts, RR: 1
Reply 21, posted (6 years 8 months 1 week 1 day ago) and read 3336 times:

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]


...and don't call me Shirley!!
User currently offlineQantas744er From United States of America, joined Jun 2005, 1286 posts, RR: 4
Reply 22, posted (6 years 8 months 1 week 22 hours ago) and read 3311 times:

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



Happiness is V1 in Lagos
User currently offlineIwok From Sweden, joined Jan 2005, 1108 posts, RR: 0
Reply 23, posted (6 years 8 months 1 week 18 hours ago) and read 3253 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? 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


User currently offlineZeke From Hong Kong, joined Dec 2006, 9097 posts, RR: 75
Reply 24, posted (6 years 8 months 1 week 17 hours ago) and read 3245 times:



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.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
25 OldAeroGuy : And this pretty much sums up why reverse thrust still has value in real operations as opposed to pure regulatory performance.
26 Roseflyer : 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 setting
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