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Testing of idle reverse thrust before takeoff. A wise precaution?

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Testing of idle reverse thrust before takeoff. A wise precaution?

Old 24th Mar 2019, 04:46
  #21 (permalink)  
 
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Never tired of saying don't make your own procedures. If too tempted ask the manufacturer. This becomes an addiction. And on aircraft run on software can have other effects which are not obvious.
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Old 24th Mar 2019, 05:00
  #22 (permalink)  
 
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What a good idea! I might check the gear operation too in case those pins were left in.

Don't invent procedures, don't deviate from the FCOM. At least not without good, compelling reason to do so. A misplaced sense of caution does not satisfy that condition. And my sarcasm aside, where do you stop? Eventually you'll be cycling every switch and doing a compass swing.
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Old 24th Mar 2019, 07:17
  #23 (permalink)  
 
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Originally Posted by stilton View Post

707-436 RR Conway testing R/T before TO v JT3D engines

Ok, I’ll bite, why is that ?
I was not aware of that on the BOAC RR 707's, but if you open the reversers on a PW JT3D they have fan cold and hot end sliding cowls, and very often one would not fully close up after landing.
The RR engine on the 707 had cascade type reverser built by RR
(the DC8 43 Conway application was different with a Douglas transiting ring thrust reversers and exhaust silencers)
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Old 24th Mar 2019, 07:58
  #24 (permalink)  
 
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Originally Posted by A37575 View Post
But could be a seriously big deal on a high speed rejected take off if runway limited..
Reject executed at V1 will only give you a 50% chance of stopping in the remaining distance anyway with a critical propulsive loss at engine failure recognition speed with a balanced field.

Its calculated using Gross not
Net data.as others say, do a proper walk around. All you will do by cycling each and every flight is put extra wear on components and increase the likelihood it wonít work when you need it most.
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Old 24th Mar 2019, 08:47
  #25 (permalink)  
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Reject executed at V1 will only give you a 50% chance of stopping in the remaining distance anyway

Its calculated using Gross not Net data.


You wouldn't like to expand on these points, would you ?
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Old 24th Mar 2019, 08:48
  #26 (permalink)  
 
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Thrust reversers fail to deploy for a number of reasons - the numbers are better now than they used to be, but figure roughly once in every 5,000 to 10,000 deployments a T/R will fail to properly deploy due to some fault, and someone forgetting to remover the lockout pins is pretty low on the probability list of failures.
As VinRouge notes, deploying the T/R prior to every takeoff just wears them out quicker, and makes it a little more likely one will fail when you really need it.
Besides, what are you going to do if one fails to deploy on a pre-takeoff check? Most likely you're go to go back, take a delay while the T/R gets deactivated and locked out, then depart with a reverser that doesn't work - which is what you're would have done anyway except now you're doing it several hours later...
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Old 24th Mar 2019, 12:32
  #27 (permalink)  
 
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Checking reversers with the aircraft stationary at the end of the runway or on the taxiway not a good idea as the reversed exhaust gas flow could blow stones or any other FOD up which could get ingested into the fan or core and cause you an even more significant problem. Not sure what flight crew operating procedures say about stowing reversers on landing at what speed to prevent this (and I'm pretty sure you don't use T/Rev to come to a complete stop), but as an Engineer we do not usually carry out an engine running reverser check on ramp for this reason. We usually have a facility through the CMS to do a cycling check using electrics or hydraulics. I can only speak for what I work on (Airbuses and B744), and there could be exceptions to this.......and yes I do know cascade vanes are designed to mitigate the ingestion of blown up FOD, but still good practice I'm sure.
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Old 25th Mar 2019, 02:29
  #28 (permalink)  
 
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But could be a seriously big deal on a high speed rejected take off if runway limited..
Not really......

A37575, you have to remember that there is very little, if any, reverse thrust. Most reversers only affect the fan air, not the flow from the core. Whilst the fan flow might generate some reverse, the core is still pushing you along. All reverse really does in an airliner is cancel out the core thrust, and not much more. It does destroy the lift generated by the wings and may dissipate any water that might be on the runway - both help sit the aircraft down on the ground a bit more firmly - but that's about it.
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Old 25th Mar 2019, 02:40
  #29 (permalink)  
 
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Well, the ground crew will love you, after they finish picking everything up.
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Old 25th Mar 2019, 04:20
  #30 (permalink)  
 
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Originally Posted by rog747 View Post
I was not aware of that on the BOAC RR 707's, but if you open the reversers on a PW JT3D they have fan cold and hot end sliding cowls, and very often one would not fully close up after landing.
The RR engine on the 707 had cascade type reverser built by RR
(the DC8 43 Conway application was different with a Douglas transiting ring thrust reversers and exhaust silencers)

Interesting, thanks for that !
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Old 25th Mar 2019, 08:00
  #31 (permalink)  
 
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All reverse really does in an airliner is cancel out the core thrust, and not much more. It does destroy the lift generated by the wings and may dissipate any water that might be on the runway - both help sit the aircraft down on the ground a bit more firmly - but that's about it.
The above argument would suggest having a reverse thrust system at all is a very expensive exercise in poor cost effectiveness. Why haven't the manufacturers realized this and built their aircraft without reverse thrust? Save millions of $$$ in maintenance costs. OK - reverse might be handy on slippery runways when wheel braking efficiency is reduced but it could be cost effective to accept that risk.
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Old 25th Mar 2019, 08:36
  #32 (permalink)  
 
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Originally Posted by Atlas Shrugged View Post
A37575, you have to remember that there is very little, if any, reverse thrust. Most reversers only affect the fan air, not the flow from the core. Whilst the fan flow might generate some reverse, the core is still pushing you along.
Almost all of the thrust generated by a large turbofan comes from the fan, so it's illogical to suggest that the absence of a core reverser makes much difference.

Early RB211s, for example, had both but the core reverser was subsequently deleted because it was a PITA for very little benefit.
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Old 25th Mar 2019, 09:04
  #33 (permalink)  
 
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Centaurus,
Look at the A380 - only reversers on #2 & #3. Just enough to claim the "major means of deceleration" for certification but really a WOFTAM.
The concept of testing reverse thrust prior to each takeoff is simply risible - where do you stop testing? Reversers and brakes worked fine last landing or there would be a tech log entry so just fly the aircraft FFS and not try to do block overhaul each T/R.
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Old 25th Mar 2019, 09:09
  #34 (permalink)  
 
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Originally Posted by Centaurus View Post
The above argument would suggest having a reverse thrust system at all is a very expensive exercise in poor cost effectiveness. Why haven't the manufacturers realized this and built their aircraft without reverse thrust? Save millions of $$$ in maintenance costs. OK - reverse might be handy on slippery runways when wheel braking efficiency is reduced but it could be cost effective to accept that risk.
Not sure if this true or not but heard it few times now......the A380 was originally designed with no T/Rev, but it was the regulators who said that it had to have it as the design spec for that standard of aircraft dictated it and so it was installed on the inboards to satisfy them.

Last edited by Tom Sawyer; 25th Mar 2019 at 09:21.
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Old 25th Mar 2019, 09:20
  #35 (permalink)  
 
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Originally Posted by Atlas Shrugged View Post
Not really......

A37575, you have to remember that there is very little, if any, reverse thrust. Most reversers only affect the fan air, not the flow from the core. Whilst the fan flow might generate some reverse, the core is still pushing you along. All reverse really does in an airliner is cancel out the core thrust, and not much more. It does destroy the lift generated by the wings and may dissipate any water that might be on the runway - both help sit the aircraft down on the ground a bit more firmly - but that's about it.
80% of a modern, high by-pass engine's thrust comes from the fan. The core engine is really only there to drive the fan, the gearboxes for the ancillary pumps/generators etc. , and provide bleed air for air conditioning/anti-icing. On that basis Fan T/Rev thrust significantly cancels out Core Thrust during application of of T/Rev.
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Old 25th Mar 2019, 14:08
  #36 (permalink)  
 
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Originally Posted by Centaurus View Post
The above argument would suggest having a reverse thrust system at all is a very expensive exercise in poor cost effectiveness. Why haven't the manufacturers realized this and built their aircraft without reverse thrust? Save millions of $$$ in maintenance costs. OK - reverse might be handy on slippery runways when wheel braking efficiency is reduced but it could be cost effective to accept that risk.
All true and revisited years ago in regulatory discussions.

In the end there was a realization that there was a money making business to offer the pilots what they wanted
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Old 25th Mar 2019, 16:11
  #37 (permalink)  
 
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Re post #34, Airbus once proposed at one of the A3XX airline working groups back in the 1990's to delete all thrust reversers to save weight and complexity however the airline working group recommended at least 2 engines have reversers. The main reason for rejecting the no reverser proposal was the estimated required runway length at MTOW for a slippery/contaminated runway if I recall correctly..
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Old 25th Mar 2019, 23:57
  #38 (permalink)  
 
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A reverser test after maintenance seems to makes sense -- by someone at least. One of the major missteps in the the report was that the engineers skipped a required functional test. Of course, that doesn't mean that the flight crew should repeat it just in case . . .
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Old 26th Mar 2019, 01:32
  #39 (permalink)  
 
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On that basis Fan T/Rev thrust significantly cancels out Core Thrust during application of of T/Rev.
I know....
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Old 26th Mar 2019, 01:52
  #40 (permalink)  
 
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Originally Posted by Centaurus View Post
The above argument would suggest having a reverse thrust system at all is a very expensive exercise in poor cost effectiveness. Why haven't the manufacturers realized this and built their aircraft without reverse thrust? Save millions of $$$ in maintenance costs. OK - reverse might be handy on slippery runways when wheel braking efficiency is reduced but it could be cost effective to accept that risk.

Those savings will be gone pretty quickly
after you slide off the end of the runway in
dodgy braking conditions and bend your kite


Reverse is invaluable in those conditions, Iíll take all the deceleration devices there are, sometimes you need all of them
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