Happy-flier
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Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 6:30 pm

I wonder if anyone else has taken note of just how unbelievably quiet the reverse thrust is on the GE-powered 777-200LR? I flew this aircraft type with Air Canada and thought that the pilot must have just used wheel braking on landing, even though the braking was quite noticeable. But then on the return legs of the flight, which comprised two distinct flights, this same sensation was noticed: powerful braking, but none of the audible engine reverse sound that's so commonplace even on the GE-powered 763.

What gives? Have the engines really become that quiet? Or is AC using wheel braking, even just after touchdown? The latter seems unlikely.
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bond007
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 7:01 pm



Quoting Happy-flier (Thread starter):
Or is AC using wheel braking, even just after touchdown? The latter seems unlikely.

Most probably wheel braking and idle reverse. With carbon brakes, policies have changed somewhat over the years, and idle reverse is commonly used. Carbon brakes are also more effective, the hotter they get.


Jimbo
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PhilSquares
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 7:02 pm



Quoting Happy-flier (Thread starter):
What gives? Have the engines really become that quiet? Or is AC using wheel braking, even just after touchdown? The latter seems unlikely.

Most likely just idle reverse and auto brakes. On the 744 that tends to be the most common technique for stopping during normal operations.
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speedracer1407
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 7:20 pm



Quoting Happy-flier (Thread starter):
thought that the pilot must have just used wheel braking on landing, even though the braking was quite noticeable.

A common misconception that appears on this forum just about weekly is that reverse thrust the primary means of decelerating after touchdown. This is not the case; wheel brakes are the primary stopping force and RT helps out when standard operating procedure requires it or the pilot elects to use it.

As I understand it, in the vast majority of situations, reverse thrust is not factored in to the calculated landing distance. Only in certain circumstances, on certain aircraft, are landing distances calculated with the assumption that thrust reverse will contribute to stopping performance. Even then, RT only contributes a small percentage to the total braking effort.

It's also my understanding that thrust reverse was used more often in the previous era of cheap fuel and steal brakes, which wear in proportion to the amount of energy dissipated in each stop. Expensive fuel and carbon brakes, fitted to many modern planes, wear in proportion to number of appilcations, so a bit of help from the reverser has little or no advantage

I've experienced plenty of landings without the use of RT on a wide range of types, and when the reversers are deployed, the engines are often left at idle.

O
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PhilSquares
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 8:17 pm



Quoting Speedracer1407 (Reply 3):
A common misconception that appears on this forum just about weekly is that reverse thrust the primary means of decelerating after touchdown. This is not the case; wheel brakes are the primary stopping force and RT helps out when standard operating procedure requires it or the pilot elects to use it.

Not quite true. Reverse thrust is very effective at higher airspeeds, such as you have right after touchdown. As the IAS decreases, wheel brakes become more and more effective. If you watch what happens on a aircraft with autobrakes when reverse thrust is used you will see if you use a moderate amount of reverse thrust with the autobrakes in 1 or 2 or a Low setting you will see the autobrakes don't start to modulate until you are down in the 100 KIAS range.

There is really a tradeoff in stopping. I have flown for carriers that have "power by the hour" and there is no hesitation to use reverse thrust. Conversly, carriers who own their engines rely more on wheel brakes rather than anything more than idle reverse. In addition, as has been pointed out carbon brakes are now more effective than the old steel brakes.

Finally, fuel is a minor factor; you might burn 100kgs on a med twin and perhaps 200kgs on a 400. You aren't going to get much more than 70% N1 with full reverse and most likely a lower reverse thrust would be selected.
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dl767captain
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 8:27 pm



Quoting Speedracer1407 (Reply 3):
A common misconception that appears on this forum just about weekly is that reverse thrust the primary means of decelerating after touchdown. This is not the case

It's definitley the primary means here in SAN, I was on a DL 763 that landed the other night and they almost always use a large amount of thrust reversers, i'm sure it has to do with the short runway and not wanting to take any chance but it sure is loud.
 
kimberlyRJ
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 8:57 pm

I have also flown on AC's 777's and other airlines LR's and noticed the reverse thrust sound is still there, depending on if its used.

Sometimes pilots will select reverse idle, so while the engines look they are reversing they are producing no reverse thrust...

Which airports were you landing at? Many airports don't like the noise that is produced!

Kimberly.
 
bond007
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 9:25 pm



Quoting PhilSquares (Reply 4):
Reverse thrust is very effective at higher airspeeds, such as you have right after touchdown. As the IAS decreases, wheel brakes become more and more effective.

Right, but on a dry runway the overall extra braking effort obtained is only something like 10-15% of total braking force.

Most of the perception that the braking effect of reverse thrust being huge, is no doubt based on the amount of noise produced when anything but reverse idle is used. The braking effect is certainly not proportional to the noise output  Wink


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osiris30
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 9:43 pm



Quoting Bond007 (Reply 7):
The braking effect is certainly not proportional to the noise output

Perhaps not, but some of us love the roar  Wink
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PhilSquares
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 10:10 pm



Quoting Bond007 (Reply 7):
Right, but on a dry runway the overall extra braking effort obtained is only something like 10-15% of total braking force.

??? I don't quite follow you. But, I can assure you, at higher speeds even selecting a idle reverse will generate a fairly good deceleration rate. Anything above idle reverse will give you better braking. At higher speeds reverse thrust can be upwards of 80% of the deceleration force. And the proof of this is using autobrakes while using reverse thrust. At the lower autobrake settings the antiskid won't even cycle if reverse is used.
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vikkyvik
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 10:55 pm



Quoting DL767captain (Reply 5):
It's definitley the primary means here in SAN, I was on a DL 763 that landed the other night and they almost always use a large amount of thrust reversers, i'm sure it has to do with the short runway and not wanting to take any chance but it sure is loud.

Saying it's the "primary means" might be a bit misleading.

First off, SAN doesn't have a short runway. It has 7591 feet of landing length starting at the displaced threshold (and assuming operations are heading west). That's plenty of room to land most airliners, without having to slam on the brakes.

Secondly, given that reversers don't even get used below somewhere around 80 kts, brakes provide most of the stopping force from there to taxi speed.

Quoting PhilSquares (Reply 9):
??? I don't quite follow you. But, I can assure you, at higher speeds even selecting a idle reverse will generate a fairly good deceleration rate. Anything above idle reverse will give you better braking. At higher speeds reverse thrust can be upwards of 80% of the deceleration force. And the proof of this is using autobrakes while using reverse thrust. At the lower autobrake settings the antiskid won't even cycle if reverse is used.

That may be true....But far as I know, selecting autobrakes gives you a predetermined deceleration rate. So if the thrust reversers are able to provide said deceleration rate, then of course you won't be using any brakes.

However, the brakes are likely capable of providing a deceleration rate far exceeding the autobrakes (at least at low or medium setting). If you touch down, use full reverse, and slam on the brakes, I'd wager that the vast majority of your stopping force will come from the brakes.

Anyway, feel free to correct me if I'm wrong.
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PhilSquares
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 11:16 pm



Quoting Vikkyvik (Reply 10):
That may be true....But far as I know, selecting autobrakes gives you a predetermined deceleration rate. So if the thrust reversers are able to provide said deceleration rate, then of course you won't be using any brakes.

Please re-read the post. The poster made the comment that wheel brakes are the "primary stopping force" during landings where reverse and brakes are used. Assuming normal braking, at high speeds, reverse thrust will result in very little braking and as you slow down the brakes will become more and more active due to the reduced effectiveness of reverse thrust.

Yes, selecting autobrakes will give you a predetermined deceleration rate. However, the thread had been talking about normal braking and I never mentioned anything about a max energy stop.

Interestingly enough, a maximum energy stop is accomplished by using manual braking and full reverse.

Quoting Vikkyvik (Reply 10):
I'd wager that the vast majority of your stopping force will come from the brakes.

I think you'd find the energy is not quite as much as you think again, because of the fact reverse thrust is extremely effective at higher airspeeds.
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vikkyvik
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Thu Oct 30, 2008 11:34 pm



Quoting PhilSquares (Reply 11):
Please re-read the post. The poster made the comment that wheel brakes are the "primary stopping force" during landings where reverse and brakes are used. Assuming normal braking, at high speeds, reverse thrust will result in very little braking and as you slow down the brakes will become more and more active due to the reduced effectiveness of reverse thrust.

OK, I see what you were responding to.

Quoting PhilSquares (Reply 11):
Interestingly enough, a maximum energy stop is accomplished by using manual braking and full reverse.

Maximum manual braking can give you more braking power than full autobrakes, correct?

Quoting PhilSquares (Reply 11):
I think you'd find the energy is not quite as much as you think again, because of the fact reverse thrust is extremely effective at higher airspeeds.

Fair enough. You'd know better than I. I'd just assume that the airplane would have slowed down quite a bit before the reversers even deployed and the engines spooled up.

Thanks for the clarifications.
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tdscanuck
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Fri Oct 31, 2008 5:52 am



Quoting Vikkyvik (Reply 12):
Maximum manual braking can give you more braking power than full autobrakes, correct?

Depends on the setting and airplane. On the ones I know of, autobrakes 1-max are preset deceleration rates, and you can go higher using max manual. However, the RTO setting on the autobrakes should be equivalent to max manual.

Quoting Vikkyvik (Reply 12):
Fair enough. You'd know better than I. I'd just assume that the airplane would have slowed down quite a bit before the reversers even deployed and the engines spooled up.

Typical deploy time for a reverser is ~4 seconds. So the plane may be slowing, but it hasn't had a whole lot time to do so.

Tom.
 
PhilSquares
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Fri Oct 31, 2008 7:55 am



Quoting Vikkyvik (Reply 12):
Maximum manual braking can give you more braking power than full autobrakes, correct?



Quoting Tdscanuck (Reply 13):
However, the RTO setting on the autobrakes should be equivalent to max manual.

Yes, manual braking gives you more braking than MAX on the autobrakes.

The FCTM on the 744 says that with manual braking you get the best braking when compared to any mode of the autobrake system.
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Fri Oct 31, 2008 9:30 am

What is the AC SOP in such cases...Do they give priority to Brakes with Idle reverse or increase the brake overhaul life by using more reverse thrust & save on the brakes until needed.
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kimberlyRJ
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Fri Oct 31, 2008 9:30 am

I wonder why the policy over reverse thrust has changed over the years!?

When I was a kid I used to love landing at airports, Greece, Africa, Spain – all over where the pilots would use so much reverse thrust you would and most of the town around the airport knew you had arrived.

Being cabin crew I don’t get to see the engines during take off and landing, but I can tell you that reverse is hardly ever selected at the larger airports – even when the runway is wet.

I was on a Boeing 777 last year coming in from JFK early in the morning at LHR and we touched down normally, started to break normally when there was an odd vibration felt, I knew I was not the only one to feel it as the PAX went to look out their windows (always a good sign something is up)… Way after reverse is normally selected (if used) the pilots revved both engines into what sounded like full reverse – it sounded glorious!

It was must unusual for early morning ops at LHR I can tell you…

So has the GE engines on a Boeing 777-200LR got quieter? I don’t think so, I think the pilots are just in idle reverse  Wink

Kimberly.
 
bond007
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Fri Oct 31, 2008 12:25 pm



Quoting PhilSquares (Reply 9):
But, I can assure you, at higher speeds even selecting a idle reverse will generate a fairly good deceleration rate. Anything above idle reverse will give you better braking. At higher speeds reverse thrust can be upwards of 80% of the deceleration force. And the proof of this is using autobrakes while using reverse thrust. At the lower autobrake settings the antiskid won't even cycle if reverse is used.

Yes, I understand, but in terms of the overall braking effect of both reverse thrust and autobrakes or manual braking, the contribution from the reverse thrust on a dry runway is relatively small ... like less than 15% extra braking effort. I don't have the numbers to hand, but performance tables will show reduced landing distances of only a few percent less for dry runways when using reverse thrust ... but up to 20% or more for slush/ice contamination.

Jimbo
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daviation
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Fri Oct 31, 2008 12:59 pm

Just from a passenger's point of view, I've noticed that reverse thrust also depends on the activity at the airport, and the location of the runway exits. For example, in the landings at EWR, all of my flights have used reverse thrust -- my assumption is that they must use specific runway exits so that three planes are ready at the same time to cross the active takeoff runway on their way to the terminal. The 737s would have no trouble landing on a dry 11,000 ft runway, but they are instructed by ground control to be at a certain place at a certain moment. Another example is SWF with a 12,000 ft runway (albeit with a displaced threshold), a CRJ should have no trouble stopping in plenty of time with brakes alone. Yet most pilots use reverse because the runway exits are spaced so far apart. If they pass the exit by the crosswind runway, they have to roll out to the end of runway 27, and then it's a long taxi back to the terminal. I was very surprised recently that on my 777 flight to TLV, no reverse was used, not even idle. We just rolled out to the end, probably because the runway is 11,000 ft, and there is little traffic. When we returned to EWR, again no reverse, but our landing was at 5 a.m., so there was little traffic as well. Am I correct, or is this simply my own theory?!
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CanadianNorth
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Fri Oct 31, 2008 1:01 pm



Quoting Bond007 (Reply 7):
only something like 10-15% of total braking force.



Quoting PhilSquares (Reply 9):
At higher speeds reverse thrust can be upwards of 80% of the deceleration force.



Quoting Bond007 (Reply 17):
like less than 15%



Quoting Bond007 (Reply 17):
but up to 20% or more for slush/ice contamination.

In the gas turbine section of the AME course we were taught that thrust reversers would normally contribute approximately 20% of the braking force, but can be kicked up to ~50% in emergencies or if the runway is really slippery or wet.


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PhilSquares
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Fri Oct 31, 2008 2:54 pm



Quoting Bond007 (Reply 17):
Yes, I understand, but in terms of the overall braking effect of both reverse thrust and autobrakes or manual braking, the contribution from the reverse thrust on a dry runway is relatively small ... like less than 15% extra braking effort. I don't have the numbers to hand, but performance tables will show reduced landing distances of only a few percent less for dry runways when using reverse thrust ... but up to 20% or more for slush/ice contamination.

First of all, the performance charts don't show any effect of reverse thrust. It's not taken into consideration when computing landing distances.

You really need to go back and re-read the post. I never talked about the overall braking just about "normal" ops. However, if you want to go there, your 15% is too low. At higher airspeeds, especially hi-bypass fans, the reverse effect is much greater that 15%. If you look at the effectiveness with respect to KIAS, you would see a dramatic drop off of reverse thrust effectiveness as you get into the 100KIAS range. That has been true for all the Hi-bypass aircraft I've flown (747/DC-10/747/744). Granted on the older aircraft, like the 727 the effect of reverst thrust was much less.
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bond007
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sat Nov 01, 2008 3:52 pm



Quoting PhilSquares (Reply 20):
You really need to go back and re-read the post. I never talked about the overall braking just about "normal" ops.

I just commented on the effect and contribution to overall braking. I wasn't starting an argument, or even disagreeing .... purely stating some facts, that's all.

Just because folks are discussing slightly different aspects of reverse braking doesn't mean we are disagreeing with you, or that we need to re-read your post.

Quoting PhilSquares (Reply 20):
However, if you want to go there, your 15% is too low. At higher airspeeds, especially hi-bypass fans, the reverse effect is much greater that 15%.

Perhaps you need to reread my post  Wink

You just mentioned that I was talking about overall braking .... and that's exactly what I meant. It's also almost non-effective at speeds less than 60kts. The effective of reverse thrust on deceleration and the decrease in total landing distance is a small percentage.

Jimbo
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tdscanuck
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sat Nov 01, 2008 6:10 pm



Quoting Kimberlyrj (Reply 16):
I wonder why the policy over reverse thrust has changed over the years!?

Three major factors:
1) Fuel cost
2) Noise concerns
3) Carbon brakes

Quoting PhilSquares (Reply 20):
First of all, the performance charts don't show any effect of reverse thrust. It's not taken into consideration when computing landing distances.

Yes, it is. It's not taken into consideration when computing *dry* landing distances. It can be (depending on when the aircraft was certified) for wet landing distances.

Tom.
 
bond007
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 02, 2008 8:18 pm



Quoting PhilSquares (Reply 20):
First of all, the performance charts don't show any effect of reverse thrust

Well, the ones I have in front of me all do. They clearly show landing distance adjustments with/without reverse thrust on each engine, on dry runways, and good/med/poor braking action for contaminated runways. Whether it's taken into account or not is another matter.


Jimbo
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PhilSquares
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 02, 2008 8:49 pm



Quoting Bond007 (Reply 23):
Well, the ones I have in front of me all do. They clearly show landing distance adjustments with/without reverse thrust on each engine, on dry runways, and good/med/poor braking action for contaminated runways. Whether it's taken into account or not is another matter.

If you're looking at the QRH, that's another issue. Those are airline specific and not part of the certification process. The performance data in Vol 2 should have the with and without reverse thrust. Under the JAA certification and the FAA reverse thrust is not considered.

What aircraft are you looking at and is it the FCOM charts or QRH?
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tdscanuck
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Mon Nov 03, 2008 1:46 am



Quoting PhilSquares (Reply 24):
Under the JAA certification and the FAA reverse thrust is not considered.

I feel like I'm banging my head on a wall...FAA takes reverse thrust into account for *wet* runway certification (yes, there's a wet runway certification, has been since the late 90's).

Neither FAA nor JAA include reverser thrust for the dry runway certification.

Tom.
 
PhilSquares
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Mon Nov 03, 2008 7:29 am



Quoting Tdscanuck (Reply 25):
I feel like I'm banging my head on a wall...FAA takes reverse thrust into account for *wet* runway certification (yes, there's a wet runway certification, has been since the late 90's).

You fail to realise I'm talking about an aircraft, the 777 and the 744 that was certified prior to that change being made.

The 737NG that WN operates have been certified under that criteria the 380 is also.

Now how does that head feel????
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tdscanuck
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Tue Nov 04, 2008 2:19 am



Quoting PhilSquares (Reply 26):
You fail to realise I'm talking about an aircraft, the 777 and the 744 that was certified prior to that change being made.

Now how does that head feel????

Better.  ashamed 

Although it does feel so good when I stop...

Tom.
 
FlyDeltaJets
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Tue Nov 04, 2008 2:57 am



Quoting Speedracer1407 (Reply 3):
As I understand it, in the vast majority of situations, reverse thrust is not factored in to the calculated landing distance. Only in certain circumstances, on certain aircraft, are landing distances calculated with the assumption that thrust reverse will contribute to stopping performance. Even then, RT only contributes a small percentage to the total braking effort.

Im no pilot but I have done weight and balance and when the thrust revesere are MEL'ed on 737's espically the payload is almost cut in half. I think that speaks to the the importance of them. Then on the other side of the coin one of the largesst jets in the world have only 2. Also many ERJ have none installed so I see where that may make it secondary.
I personally think when you get that smooth touchdown with little or no (felt) stopping effort that pilot "did his/her thing"

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bond007
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Tue Nov 04, 2008 3:52 am

Quoting PhilSquares (Reply 26):
You fail to realise I'm talking about an aircraft, the 777 and the 744 that was certified prior to that change being made.



Quoting FlyDeltaJets (Reply 28):
Im no pilot but I have done weight and balance and when the thrust revesere are MEL'ed on 737's espically the payload is almost cut in half. I think that speaks to the the importance of them.

Well, up to now we've been talking landing distances. If we now switch to accelerate-stop distances, FAR 25 specifies that reverse thrust can be used as an extra means of deceleration on wet runways. Now I don't disagree that this can be significant, but on runways with poor braking action only ... as opposed to the almost negligible decrease in landing distances on dry runways with T/Rs.

Here is a Boeing chart to demonstrate my earlier point (for 737-800). Note that the red/blue lines are almost equal until the braking action decreases.

http://www.airliners.net/uf/view.fil...=536889115&filename=phpe7fqdO..gif

Jimbo

[Edited 2008-11-03 19:59:30]
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mpdpilot
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Fri Nov 07, 2008 5:59 am

I thought I might chime in here. Currently being in a CRJ systems class, the explanation that I have been given is that both the thrust reversers and the spoilers assist in breaking mostly by putting more and more weight on the wheels of the aircraft. Though the reversers can cause significant slowing power, I have been told that they also cause major disruptions in the airflow over the wing allowing the weight to be on the wheels sooner making more effective wheel braking. On a side note when the aircraft is on a relatively slippery runway with a cross-wind, the reversers can pull the aircraft to the left or right. Just some additions to the answers that have been given here.

To be honest that feeling of being stopped probably came from the pilot applying the brakes him or her self and applying them relatively hard. At least more so than the thrust reversers.

[Edited 2008-11-06 22:01:39]
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longhauler
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sat Nov 08, 2008 8:17 pm

Another factor is that Air Canada owns the aircraft/engines and fuel (so to speak) but only leases the brakes on a per-landing basis. For that reason, it is SOP to use idle reverse thrust (less wear on the engines, and less fuel) and maximum brakes as in the long run, it costs the airline less. As previously mentioned, carbon brakes become more and very effective the hotter they get.

There is of course the caviat that one would expect from a carrier that routinely operates on snow covered and slippery runways. That is maximum reverse thrust must be selected and used on anything other than a dry or long runway.
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vikkyvik
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 09, 2008 5:58 am



Quoting LongHauler (Reply 31):
Another factor is that Air Canada owns the aircraft/engines and fuel (so to speak) but only leases the brakes on a per-landing basis. For that reason, it is SOP to use idle reverse thrust (less wear on the engines, and less fuel) and maximum brakes as in the long run, it costs the airline less. As previously mentioned, carbon brakes become more and very effective the hotter they get.

Not to get too off-topic here, but how does that work? What's the idea behind leasing the brakes? I would have thought they came with the aircraft.
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scxmechanic
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 09, 2008 12:46 pm



Quoting Bond007 (Reply 21):
First of all, the performance charts don't show any effect of reverse thrust. It's not taken into consideration when computing landing distances.

On the Boeing 737NG Reverse Thrust IS considered when landing distances are computed.

See quote from accident report below....

http://www.ntsb.gov/recs/letters/2006/a06_16.pdf


"The flight crew used an on-board laptop performance computer (OPC) provided in the
cockpit of Southwest Airlines’ airplanes to calculate expected landing performance. For landing performance calculations, flight crews enter flight specific data into the OPC, including the expected landing runway, wind speed and direction, airplane gross weight at touchdown, and the reported runway braking action.

The 737-700 OPC is programmed to assume that the engine thrust reversers will be deployed on touchdown ( Boeing 737-300 and -500 model airplanes are not programmed with this assumption) and to calculate the stopping margin (the runway distance remaining from the front of the nose wheel to the end of the runway pavement after the airplane comes to a stop). The OPC then alerts flight crews if the remaining runway distance is not sufficient for the airplane to land and completely stop on the runway under the selected weather and runway conditions.

If the reverse thrust credit had not been factored into the stopping distance calculations
made by the OPC, it would have indicated that a safe landing on runway 31C was not possible under a braking condition of either fair or poor. The Safety Board is concerned that the landing distance safety margin is significantly reduced on a contaminated runway when the reverse thrust credit is allowed in landing stopping distance calculations. As a result, a single event, the delayed deployment of the thrust reversers, can lead to an unsafe condition, as it did in this accident. The Safety Board concludes that the safety margin must be restored to those airplanes for which the reverse thrust credit is currently allowed in landing performance calculations. "
 
bond007
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 09, 2008 3:25 pm



Quoting Scxmechanic (Reply 33):
Quoting Bond007 (Reply 21):
First of all, the performance charts don't show any effect of reverse thrust. It's not taken into consideration when computing landing distances.

On the Boeing 737NG Reverse Thrust IS considered when landing distances are computed.

That quote wasn't from me .... but regardless of whether it's taken in to consideration or not, the charts show adjustments for landing distance with/without reverse thrust.

We appeared to get off track, and discuss details on whether it was considered or not, where it was documented etc., etc., when my point was that on a dry runway, the difference in landing distance when using (or not using) reverse thrust is measured in hundreds of feet... in most cases a couple of hundred or less.

Jimbo
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OldAeroGuy
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 09, 2008 4:20 pm



Quoting PhilSquares (Reply 26):
You fail to realise I'm talking about an aircraft, the 777 and the 744 that was certified prior to that change being made.

Depends on what version of the 777 you're considering.

The 772LR and the 773ER use reverse thrust credit for takeoff stopping distance on wet runways.

And so will the 748.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
tdscanuck
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 09, 2008 6:54 pm



Quoting Vikkyvik (Reply 32):
Not to get too off-topic here, but how does that work? What's the idea behind leasing the brakes? I would have thought they came with the aircraft.

The brakes do come with the aircraft, but they're consumable (like tires and, in some sense, engines). If you lease engines they also come on the aircraft.

Quoting Scxmechanic (Reply 33):
On the Boeing 737NG Reverse Thrust IS considered when landing distances are computed.

Only on wet runways (for certified distances). Another thread went totally off the rails over this distinction...there is only one set of certified distances for most aircraft (dry runway). Anything certified after the late 90's has certified wet (using reverse thrust) and dry (not using reverse thrust) numbers. Beyond that, there's a whole ton of advisory data that's not certified, but is valid, like contaminated runways.

Tom.
 
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longhauler
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 09, 2008 9:40 pm



Quoting Vikkyvik (Reply 32):
Not to get too off-topic here, but how does that work? What's the idea behind leasing the brakes? I would have thought they came with the aircraft.

It is a novel idea. The brakes are owned by a third party, and Air Canada pays a fee to this third party on a per-landing basis. In return, this third party supplies and maintains the brakes. As a result, AC pays a fee whether the brakes are used or not, per landing. So it makes sense, where safe, to use as much brakes as possible.
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!
 
Viscount724
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 09, 2008 10:33 pm



Quoting LongHauler (Reply 37):
Quoting Vikkyvik (Reply 32):
Not to get too off-topic here, but how does that work? What's the idea behind leasing the brakes? I would have thought they came with the aircraft.

It is a novel idea. The brakes are owned by a third party, and Air Canada pays a fee to this third party on a per-landing basis. In return, this third party supplies and maintains the brakes. As a result, AC pays a fee whether the brakes are used or not, per landing. So it makes sense, where safe, to use as much brakes as possible.

A month or so ago there was an interview in Aviation Week magazine with the CEO of Goodrich, one of the major manufacturers of aircraft landing gear, brakes, wheels etc. One question and answer related to this subject, excerpt below.

Aftermarket parts are becoming more and more reliable. Does that impact your aftermarket business because things are lasting longer?

We are migrating to more flight-hour contracts. It’s better if our parts last longer because if we enter into a 10-year contract with an airline, we can reduce their costs and give them a cost-certain outcome. Our brakes will last 1,200 landings and you pay $3 a wheel position per landing. They know they’re never going to pay any more than that for 10 years. But if we can make those things last 1,500 landings, we make more money. We keep making these products better and better. There are certain wear factors on wheels and brakes, thrust reverser actuation, things that are moving all the time. No matter how good they are, they have a limited life. So our objective is to take airline costs down along with our own costs so that our margins aren’t affected and we can get more than our fair market share with those airlines. The last thing we want to do is make a product that doesn’t last long just so we can get an extra overhaul, because in the end we’d put ourselves out of business.
 
vikkyvik
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 09, 2008 11:30 pm



Quoting Tdscanuck (Reply 36):



Quoting LongHauler (Reply 37):



Quoting Viscount724 (Reply 38):

That is very interesting - thanks for the info. Had never heard of that before.
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OldAeroGuy
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Tue Nov 11, 2008 1:26 pm



Quoting Tdscanuck (Reply 36):
Anything certified after the late 90's has certified wet (using reverse thrust) and dry (not using reverse thrust) numbers.

You can use the reversers on dry as well. However, in dry conditions the brakes are so effective that the reversers contribution to stopping distance is insignificant.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
tdscanuck
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Wed Nov 12, 2008 1:12 am



Quoting OldAeroGuy (Reply 40):
Quoting Tdscanuck (Reply 36):
Anything certified after the late 90's has certified wet (using reverse thrust) and dry (not using reverse thrust) numbers.

You can use the reversers on dry as well. However, in dry conditions the brakes are so effective that the reversers contribution to stopping distance is insignificant.

Oh, absolutely. It's just the assumptions behind the calculations...the certified stopping distance on a dry runway assumes you don't use reversers. If you do, you'll stop sooner (albeit probably not that much sooner). The certified distance on a wet runway *does* include reversers...if you don't use them, you'll overrun your expected distance.

Tom.
 
tinpusher007
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 16, 2008 5:21 pm



Quoting Kimberlyrj (Reply 6):
Sometimes pilots will select reverse idle, so while the engines look they are reversing they are producing no reverse thrust...

Not true! Once the reverser is deployed, even if only idle, the fan air is still reversed and thus the engine is indeed producing reverse thrust. It may be quiet and the stopping power may not be noticeable to pax in the cabin, but it is still present.

My company's procedure is to reduce to idle reverse at 80kts (if max or moderate R/T was used) and stow the reversers at 60kts. Between 60kts and taxi speed you can definitely feel the airplane lurch forward a little once the reversers are fully stowed if you're not on the brakes.
"Flying isn't inherently dangerous...but very unforgiving of carelessness, incapacity or neglect."
 
tdscanuck
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 16, 2008 8:59 pm



Quoting TinPusher007 (Reply 42):
Not true! Once the reverser is deployed, even if only idle, the fan air is still reversed and thus the engine is indeed producing reverse thrust.

That depends on the specific engine. Fan thrust varies a lot more with throttle position than core thrust so, at idle, you could get into a situation where core thrust exceeds fan thrust. So, even if you reverser fan thrust, you may not have net reverser thrust (just smaller forward thrust).

Tom.
 
tinpusher007
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RE: Mystery Behind Quiet Reversing On AC's 772LR?

Sun Nov 16, 2008 9:20 pm



Quoting Tdscanuck (Reply 43):
That depends on the specific engine. Fan thrust varies a lot more with throttle position than core thrust so, at idle, you could get into a situation where core thrust exceeds fan thrust. So, even if you reverser fan thrust, you may not have net reverser thrust (just smaller forward thrust).

Point taken! And I honestly didn't consider that. I would think an engine like the GE-90-110 on the 77L having such a large fan, you would have a net reverse even with the core thrust. And my apologies to Kimberylrj  Smile
"Flying isn't inherently dangerous...but very unforgiving of carelessness, incapacity or neglect."

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