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B737 Series use of reverse thrust during landing roll

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B737 Series use of reverse thrust during landing roll

Old 31st Jan 2020, 07:07
  #21 (permalink)  
 
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Originally Posted by Skyjob View Post
I was always in the understanding that it was mandatory, to get rid of the forward thrust vector, even at idle.
That's it Skyjob
How many accidents have there been because even idle reverse was not selected, which gets rid of the considerable forward thrust from the engines at idle? I can think of so many.
If you don't select reverse idle, you are in fact braking against engine thrust which is going to increase brake wear and heating, even if performance is not an issue.
And even Reverse idle contributes some additional deceleration.
Crank it up to full reverse especially at high speed say >100 its and it really kicks in. Much more effective at high airspeeds hence the need to select right after touchdown. the 737-200 which could reverse almost the entire jet flux being low by pass, using giant buckets, and could stop on reverse alone in about 1800 metres. Landing in Jersey on a wet runway at 1650 metres it was a godsend.
Many of the later High By Pass engines seem to just be big noise machines but even so you are probably reversing 40% of the max thrust - just an estimate. Review any good video of reverse in operation on a wet runway during testing and you can see it is still a big player.
The 737-200 had 15000 lbs of thrust, most of that was directed forwards but because of the angle of about 45 degrees deflection we used to reckon that about 8-10000 lbs was reversed. Thats a lot.
When we got the 737-400 with the CFM engines at 24000 lb thrust, we reckoned we were reversing about half the jet flow through cascades, but the main fan is not reversed and that is a big component. So we reckoned that the -400 was far inferior to the -200 in stopping power under reverse and so it proved to be.
Of course all of this is taken into account in landing performance so even the RJ with no reversers can still land!! But it is severe limited on icy / slick runways where the brakes do very little and the reversers become the dominant retarding force.
R Guy
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Old 31st Jan 2020, 07:09
  #22 (permalink)  
 
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Originally Posted by FlyingStone View Post
Why wouldn't you want to use at least idle reverse?
Hi Flying Stone
You wouldn't is the short answer. Lots of cons and no pros!
R Guy
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Old 31st Jan 2020, 07:29
  #23 (permalink)  
 
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Originally Posted by Centaurus View Post
Re use of reverse thrust in the Boeing 737 series.

Edited for brevity from the FCTM:
"Maintain reverse thrust as required, up to maximum, until the airspeed approaches 60 knots...

...The thrust levers should be positioned to reverse idle by taxi speed then full down after the engines have decelerated to idle. Reverse thrust is reduced to idle between 60 knots and taxi speed to prevent engine exhaust re-ingestion and reduce the risk of FOD.... The PM should call out 60 knots to assist the PF in scheduling reverse thrust."

Question: If landing into a (say) 30 knot HW component, 60 knots IAS means a 30 knot ground speed. The slower the aircraft is on the ground while using high reverse thrust, increases the risk of FOD and engine surge.

Would it therefore be true to say that reverse thrust should be positioned to idle at 60 knots ground speed if landing into wind, rather than 60 knots airspeed?
This is a thorny annual Centaurus but has been well discussed.
My understanding is this. But note, my understanding!
The thing you are trying to avoid by cancelling reverse, or more correctly going to reverse idle at 60 knots IAS, is the reversed airflow being projected forward and being re-ingested by the Fan Section and causing instability in engine performance and FOD damage. Otherwise you could keep reverse to a standstill. And every now and again I see that as a passenger when the pilot forgets, with the resultant bang as the engine surges.
Now this is all based on IAS , and GS is entirely irrelevant. The airflow felt by the plane is the relative wind which is a combination airplane forward motion and actual wind.
Say landing in 60 its HW you will be canceling reverse as the plane comes to a physical stop (if you wanted to) but the engines are still feeling that same 60 kt relative headwind so the reversed airflow is still not reaching the front of the engine. And no danger of FOD or surge even though the plane is physically stopped.
Take an extreme example. Light plane and 100 kt headwind. You get airborne at 120 IAS with 20 GS. Reach 50 ft and then abort. Then land immediately at 120 IAS GS 20. You could clearly blast away on your reversers to full reverse until the IAS reaches 60 before you would have a problem.
Not sure if that makes sense.
Best wishes
R Guy

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Old 31st Jan 2020, 11:38
  #24 (permalink)  

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Take an extreme example. Light plane and 100 kt headwind. You get airborne at 120 IAS with 20 GS. Reach 50 ft and then abort. Then land immediately at 120 IAS GS 20. You could clearly blast away on your reversers to full reverse until the IAS reaches 60 before you would have a problem.
Not sure if that makes sense.
it is not approved to reverse the A/C (negative GS) with reverser use.
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Old 31st Jan 2020, 12:37
  #25 (permalink)  
 
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Originally Posted by FlightDetent View Post
it is not approved to reverse the A/C (negative GS) with reverser use.
You got there before me Flight Detent. Was just composing correction!
Clearly in that very unlikely situation, which was purely illustrative, you would have to be going backward to achieve 60 kts IAS. But that was not really the point which is that it is IAS that governs the point at which you go from full reverse to reverse idle.
That said we did reverse back in Berlin for a period to evaluate not using the pushback vehicles. It didn't catch on due to the number of broken windows in then terminal due to FOD being thrown forwards but it works!
And I have used it when we were not near any buildings or people on a clean surface (while it was still permitted in the Vol 1 Manual.
Thanks for picking that up.
R Guy
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Old 31st Jan 2020, 12:44
  #26 (permalink)  
 
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Originally Posted by Centaurus View Post
Over the years of Boeing 737 simulator training with crews from various airlines we see quite a few variations in use of reverse for landing. .


Finally, a word about use of reverse thrust for landing on slippery runway operations with a significant crosswind component. Readers can study the appropriate Boeing FCTM advice at their leisure. But read it carefully especially the advice on reverse idle selection and use of brakes. This relies heavily on accurate directions from the PF to the PNF at a dynamic time. If the aircraft starts sliding sideways on a slippery runway then it is vital the PF take over operation of the reverse levers. Some juggling of brakes in conjunction with reverse thrust will be needed and there is no time to call out instructions to the PNF.
Read the diagram published in the FCTM to see what I mean.
Some operators prefer to have the PNF operating the reverse levers on all landings - dry or slippery.
UNQUOTE

Which is why I guess some operators don't let PNF anywhere near the reversers!
R Guy
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Old 31st Jan 2020, 18:34
  #27 (permalink)  
 
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This may be a good time to point out that Boeing's own FCTM has the wrong thrust levers depicted in this section. Maybe it's the same 767 that they used in the body-contact depictions.
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Old 2nd Feb 2020, 07:44
  #28 (permalink)  
 
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Devil 60 kts of what?

come on gang. I know that SOPs have become pedantic but fly areoplanes not simulators and for the newbies sweat flap speeds and think about where you are when you cancel the reverse. If your being called by the checker for idiotic definitions think about what reference is needed for the environment. If you have a 60 kt headwind you probably can stop without using much reverse so what are you thinking about?
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Old 2nd Feb 2020, 18:55
  #29 (permalink)  
 
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Originally Posted by Busserday View Post
come on gang. I know that SOPs have become pedantic but fly areoplanes not simulators and for the newbies sweat flap speeds and think about where you are when you cancel the reverse. If your being called by the checker for idiotic definitions think about what reference is needed for the environment. If you have a 60 kt headwind you probably can stop without using much reverse so what are you thinking about?
Agreed with technicality, but stick to the unlock of reverse thrust to negate the forward thrust vector in all cases, no need to deploy them beyond that. Saves, noise as at low idle, less then forward idle, reducing noise, too...
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Old 2nd Feb 2020, 20:40
  #30 (permalink)  
 
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Originally Posted by retired guy View Post
That's it Skyjob
Crank it up to full reverse especially at high speed say >100 its and it really kicks in. Much more effective at high airspeeds hence the need to select right after touchdown. the 737-200 which could reverse almost the entire jet flux being low by pass, using giant buckets, and could stop on reverse alone in about 1800 metres. Landing in Jersey on a wet runway at 1650 metres it was a godsend.
Many of the later High By Pass engines seem to just be big noise machines but even so you are probably reversing 40% of the max thrust - just an estimate. Review any good video of reverse in operation on a wet runway during testing and you can see it is still a big player.
Few reversers do better than about 25% effectiveness - but as noted getting them out early is a big benefit due to the ram drag of all that air going in the inlet at higher speeds. Very early high bypass engines (JT9D/747) had core exhaust 'spoilers' - they didn't really reverse the core exhaust but pretty much eliminated the forward thrust component. However keeping the actuators and such working in the hot exhaust stream was difficult and expensive, and the benefit was minor, so they went away early on.
Most high bypass reverse systems limit N1 to around 80-85% - above that the forward core thrust increases faster than the reverse fan thrust, so there is no benefit to going higher.
The theoretical benefit of not using the reversers is it saves the wear and tear of the deployment actuators and related mechanism (MTBF of a reverser is in the 5000 cycle ballpark). That being said, back during the initial 777 development, I'd just been involved in the Lauda crash investigation (one of the most difficult, painful things I did during my career), and I semi-seriously asked the question 'why don't we just get rid of the reversers' - it was a lot of weight and complexity, they were potentially dangerous, and we didn't get any direct cert credit for them. I was told that on a 777 sized aircraft, using reversers saved ~$100/landing in brake wear and maintenance...
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