Testing of idle reverse thrust before takeoff. A wise precaution?
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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.
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.
Most thrust reversers do create a significant amount of net reverse thrust, particularly at higher airspeed (i.e. over ~100 knots) due to the ram drag of all that fan air going into the inlet. While it's true the core thrust cancels some of the reverse fan thrust, it comes no where near cancelling all the reverse thrust (it is why you'll note that max reverse N1 is typically fairly low - in the 80-85% N1 range - above that the reverse fan thrust doesn't increase as fast as the core forward thrust so a higher N1 doesn't stop you any quicker.)
Even statically, there is enough reverse thrust that you can power back a 767/CF6-80C2 (been there, done that - during flight testing). Obviously not an approved procedure though - very, very hard on the engines due to all the re-ingestion.
During the development phase of the 777, I ask the question - part in jest but also part seriously - why don't we just get rid of the reversers. They're heavy, maintenance intensive, and a T/R malfunction was blamed for the recent Lauda 767 crash. I was told the use of reverse reduces brake wear so much it saved between $50 and $100 on every landing...
tdracer,
How come then many airlines SOP is idle reverse and use the brakes to stop?
We were told the opposite - reverse costs way more than brakes due temp cycles and fuel burn. We plebs flying the beast have no idea, it's their train set so we play with it their way.
How come then many airlines SOP is idle reverse and use the brakes to stop?
We were told the opposite - reverse costs way more than brakes due temp cycles and fuel burn. We plebs flying the beast have no idea, it's their train set so we play with it their way.
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The always green dilemma reverse thrust / brake wear / landing CONF does not come with a one size fits all solution. It is very much dependant on the Operator's types of operations (legacy, low cost, charter, short-medium-long haul etc..) and the related cost analysis.
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tdracer,
How come then many airlines SOP is idle reverse and use the brakes to stop?
We were told the opposite - reverse costs way more than brakes due temp cycles and fuel burn. We plebs flying the beast have no idea, it's their train set so we play with it their way.
How come then many airlines SOP is idle reverse and use the brakes to stop?
We were told the opposite - reverse costs way more than brakes due temp cycles and fuel burn. We plebs flying the beast have no idea, it's their train set so we play with it their way.
During the development phase of the 777, I ask the question - part in jest but also part seriously - why don't we just get rid of the reversers. They're heavy, maintenance intensive, and a T/R malfunction was blamed for the recent Lauda 767 crash. I was told the use of reverse reduces brake wear so much it saved between $50 and $100 on every landing..
tdracer
I remember reading a really interesting response to a thread by a man (I’m sure his surname was Green) on an aviation forum in the nineties. The conversation was about how best to deal with a reverser deploying after take off, as it’s a possible failure in the simulator. He had been( maybe still was) a Boeing test pilot. He gave me the impression that unless you were very very lucky, this was one failure that would end in a crash. I seem to remember him saying that they’d investigated the 737-400 and saying that the results of a failure of a reverser on that variant was pretty frightening. It of course had the sleeve type reverser, rather than the bucket type on the -200, which was even worse.
edit: He said that to have any chance you’d have to get the engine shut down very quickly, as well as getting above a certain speed (V2 + ?).
Perhaps it has been mentioned but selecting reverse idle during taxi, say, as I did once, to help keep brake temps down, dumps the derates on the AT.
Do that on the 75/76 and selecting TOGA gives you full thrust for takeoff if you forget to reselect D 1 or D2, fantastic acceleration on a light aircraft though !
Reversers were checked on Concorde during taxi out.
Do that on the 75/76 and selecting TOGA gives you full thrust for takeoff if you forget to reselect D 1 or D2, fantastic acceleration on a light aircraft though !
Reversers were checked on Concorde during taxi out.
Call it overkill but after reading about a fatal accident where the leading edge devices for some reason failed to extend during the before takeoff checks, I must admit to always after that, looking through my side window to confirm the LED's on my side were indeed extended in the 737-200. Strangely enough I didn't ask the occupant of the other seat to check his side. But it gave me peace of mind if nothing else. The use of idle reverse on the ground in the 737-200 was frequently "cracked" to keep taxi speed within commonsense limits.without having to ride the brakes so at least we knew they worked before take off. These little quirks were considered good airmanship by some and and superfluous nonsense to others.
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tdracer,
How come then many airlines SOP is idle reverse and use the brakes to stop?
We were told the opposite - reverse costs way more than brakes due temp cycles and fuel burn. We plebs flying the beast have no idea, it's their train set so we play with it their way.
How come then many airlines SOP is idle reverse and use the brakes to stop?
We were told the opposite - reverse costs way more than brakes due temp cycles and fuel burn. We plebs flying the beast have no idea, it's their train set so we play with it their way.
I do know for certain that no one considered the amount of landing performance work actually done on contaminated runways during certification.
It has always fascinated me how individual departments in an airline can use very sharp pencils, but when it comes to pulling it all together it is more crayons and markers.
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I joined Ryanair in 2005 and was surprised that their SOP was to use full reverse after coming from a few operators that had used idle reverse for a long time before. So that might imply that they for one believed reverse (and autobrake)was the better option to save money. As you say, we just comply.
tdracer
I remember reading a really interesting response to a thread by a man (I’m sure his surname was Green) on an aviation forum in the nineties. The conversation was about how best to deal with a reverser deploying after take off, as it’s a possible failure in the simulator. He had been( maybe still was) a Boeing test pilot. He gave me the impression that unless you were very very lucky, this was one failure that would end in a crash. I seem to remember him saying that they’d investigated the 737-400 and saying that the results of a failure of a reverser on that variant was pretty frightening. It of course had the sleeve type reverser, rather than the bucket type on the -200, which was even worse.
edit: He said that to have any chance you’d have to get the engine shut down very quickly, as well as getting above a certain speed (V2 + ?).
I remember reading a really interesting response to a thread by a man (I’m sure his surname was Green) on an aviation forum in the nineties. The conversation was about how best to deal with a reverser deploying after take off, as it’s a possible failure in the simulator. He had been( maybe still was) a Boeing test pilot. He gave me the impression that unless you were very very lucky, this was one failure that would end in a crash. I seem to remember him saying that they’d investigated the 737-400 and saying that the results of a failure of a reverser on that variant was pretty frightening. It of course had the sleeve type reverser, rather than the bucket type on the -200, which was even worse.
edit: He said that to have any chance you’d have to get the engine shut down very quickly, as well as getting above a certain speed (V2 + ?).
Ultimately it was determined that it simply was not practical to make an in-flight deployment controllable with the (then new) very high bypass turbofan engines, the only answer was to make sure it never happened. Hence the retrofit of the 'third lock' (aka 'sync lock', although not all installations use an actual lock on the sync cable).
Last edited by tdracer; 27th Mar 2019 at 23:47. Reason: fixed a few typos
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Ultimately it was determined that it simply was not practical to make an in-flight deployment controllable with the (then new) very high bypass turbofan engines, the only answer was to make sure it never happened. Hence the retrofit of the 'third lock' (aka 'sync lock', although not all installations use an actual lock on the sync cable).
You’ve reminded me, this was another point made by Green, trying to make sure it never happened.
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You guys are wearing out the brakes! You guys are putting nicks in the prop!
In general engines care about EGT (time at) and RPM magnitude. The RPM equates to cyclic fatigue of the metal in the rotor-disks. This cyclic life equates mostly to the average flight max RPM (with a goodly amount of margin). So if the RPM in reverse approaches typical takeoff, then it probably counts towards fatigue life of some rotor parts
tdracer,
How come then many airlines SOP is idle reverse and use the brakes to stop?
We were told the opposite - reverse costs way more than brakes due temp cycles and fuel burn. We plebs flying the beast have no idea, it's their train set so we play with it their way.
How come then many airlines SOP is idle reverse and use the brakes to stop?
We were told the opposite - reverse costs way more than brakes due temp cycles and fuel burn. We plebs flying the beast have no idea, it's their train set so we play with it their way.
Whether the aircraft has carbon or steel brakes makes a significant difference in costs estimates per landing.
In my experience it favours using reverse with steel brakes, and idle reverse only for carbon brakes.
As many of you will know, carbon brakes do not like being used "gently" ---- the wear rate is much higher cold than at "normal" working temperature, the reverse of steel brakes
Tootle pip!!