the effect of reversers on landing distance question

[Ivo de Colfmaker]

Hi  ,
I tried for fun some landing practice at  Toncontin  airport , Runway 02, 2021 meters long ,the RNAV 02 approach.
Landing weight 200 tons, no wind. According to Topcat calculations  autobrakes 4 and autobrakes MAX authorized with max landing weight of 252245 kg.
autobrakes 4 gives a margin of 266 meters, autobrakes MAX 577 meters.
But I  can not find any info  on the  effect of the reversers. for instance, how much shorter will it be to full stop with  reversers idle  and  full reverse?
Is there any info available
ivo

[Hardy Heinlin]

Hi Ivo,

the brake pressure control in the autobrake system is a function of groundspeed. Each of the five autobrake settings aims at a certain aircraft deceleration (see manual, chapter Landing Gear). That is, when speedbrakes or reversers are added, the controller removes some brake pressure to keep the selected deceleration value constant.

The autobrake system gets groundspeed data from the IRS. That's why the autobrakes are inoperative when the IRS is inoperative.


Regards,

|-|ardy

[Will]

...and putting it all together, it means that the effect of the reversers isn't to shorten the landing roll, but to extend the life of the brakes and the tires. They save money, not runway.

[Ivo de Colfmaker]

Hi Hardy and Will,
Thank you for the answer,
Found it  on page 475.
I thought that  applying max brake and then adding full reverse would create more drag, hence a shorter stop , did not think this more drag is used to releave the pressure on the brakes  to prolonge there live!
Ivo

[Ivo de Colfmaker]

Ok,
Next question then, I don't use the autobrakes, but let us say for some reason I want to stop as quickly as possible,
Now , I use manual braking,  slam the pedals down to the floor,when using full reverse, this must shorten my stopping distance , no?
Ivo

[emerydc8]

I ran an OPS for landing at 652,000 lbs (max ldg wt) on RW23R at LGG. The first OPS is run without taking credit for the reverse. The second OPS is calculated with four reverse. As you can see for yourself, when using reverse in this case, it is the difference between getting stopped within the 7677 runway length or not getting stopped.

Without reverse, using max autobrakes, it will take 7794' to stop. With reverse it will stop in 6784.

No reverse:
https://drive.google.com/file/d/0B-WRh0Hf7VdZWk9mdGo5WXUxaEU/view?usp=sharing

Reverse:
https://drive.google.com/file/d/0B-WRh0Hf7VdZc3RlT0kxTUo1aWM/view?usp=sharing

[Will]

Hi Ivo,

Each setting on the autobrakes selector sets a targeted rate of deceleration that goes from 1 to MAX. As you dial up the autobrakes to higher numbers, you'll get more deceleration, and a shorter landing rollout.

Meanwhile, without the autobrakes, your feet can do the same thing. The more you press down on the brake pedals, the more your deceleration and the shorter your landing roll.

It's my understanding that maximum manual braking (which you get by stomping down as hard as humanly possible on the brake pedals) gets you more powerful braking than MAX on the autobrake selector. So yes, stomping hard on the brakes would shorten your landing roll.

[John H Watson]

Meanwhile, without the autobrakes, your feet can do the same thing. The more you press down on the brake pedals, the more your deceleration and the shorter your landing roll.

Within the limits of the anti-skid and brake torque-limiting systems, of course 

[Ivo de Colfmaker]

autobrake selector. So yes, stomping hard on the brakes would shorten your landing roll.

That is not what I ment, when manual braking the autosystem is not used, so this would to me mean that the decelaration logic  that is used on the autobrake setting also is inop.
In this case  reverse thrust shortens my stopping distance  I would think.

Here is a stupid scenario, but to make my point valid, I want  to land on a runway with  autobrakes max, which leaves me  200 feet  runway left at full stop. Not realistic, but for the sake of argument.
Now I screw up, and touch down 300 feet  further, so I am not gonna make it, then applying full reverse , as in the example by emerydc8 , the stopping distance is 1000 feet shorter.

And that brings me by my initial question,  how much shorter will my landing distance be with reversers, and now I know , a big difference it seems
Ivo

[Will]

Hi, sorry if I misunderstood you, Ivo. You're right, once you move off of autobrakes and into manual braking, then for a fixed application of the brakes, the presence of reverse thrust would shorten the landing roll. At max braking, limited by anti-skid and torque limiters, reverse thrust would make the landing distance shorter.

[Hardy Heinlin]

Ok,
Next question then, I don't use the autobrakes, but let us say for some reason I want to stop as quickly as possible,
Now , I use manual braking,  slam the pedals down to the floor,when using full reverse, this must shorten my stopping distance , no?
Ivo

This will shorten the distance, yes.

But the difference won't be big as your deceleration is so high that your speed will be quickly so low that the aerodynamic effect of the reversers will be nearly zero before the reverser sleeves are even extended. Below 80 knots the reverser effect tends to zero.


|-|ardy

[emerydc8]

I can't speak for the 744, but I can tell you that I have powered back the DC-8 with JT3D-3B, JT3D-7 and CFM56 engines and it works well (just make sure there's nothing on the ramp in front of you that can get blown away and never touch the brakes while you are moving backwards). So, while I agree it tends towards zero, I doubt that it ever gets there. Otherwise, there is no way you could ever power back. It's not something you are allowed to do, but sometimes you do what you have to in order to make it work.

I agree that at lower speeds the aerodynamic drag effects would go to zero, but I think there is still a reverse vector when max reverse is applied, even when the aircraft is stopped.

[John H Watson]

Did the DC8 just have clamshell buckets which reversed all the exhaust, unlike the turbofans which just reverse the coldstream?

[emerydc8]

The P&W JT3Ds had the clam shells and the CFM56 just reversed the fan with blocker doors.

I understand that Eastern Airlines was the first to power back a 757, but I hear they stopped it due to FOD.

[Jeroen D]

Did the DC8 just have clamshell buckets which reversed all the exhaust, unlike the turbofans which just reverse the coldstream?

I think there is a fundamental,difference here. The clamshell buckects really revert the exhaust gasses forward. On the modern turbofans the air cold or otherwise leaves the engine near perpendicular tomthe engine axle. I seem to recall a thread where the actual working (braking effect if you like) was explained in detail.

Jeroen

[Ivo de Colfmaker]

Hi,
Thanks for the info, all very interesting.
Maybe it is not the right place to say in this topic, since there is no thank you section going to do it anyway.
PSX is developed mostly for professional 747 pilots and engineers as a computer based training aid, and not so much for the " Microsoft simmer"
And it shows if one looks at the forum members, many of them pilots, instructors, engineers, traffic controllers, Flight dispatchers etc.
Many of the topics are way  above my comprehension , that's why this is such an educational place here.

I do not have any flight experience or training , I had a technical education, spent 40 years polishing diamonds .something completely different.

But whenever I ask a question, how trivial it may be for these pro's to answer, I always get one,
This forum is one of the best there is, always politely, friendly, helpful, it is never to much to ask!
On top of this a bunch of people that spend lots of free time to enhance this simulator even more, for the benefit of all who like to use it.

I have seen a few forums, and  there is none like this one.
So thanks to all of you.

Regarding the topic itself, my scenario is of course unrealistic, planning a runway to land on with such a small margin is not done , and missing the touchdown point with 300 feet  neither. unless it's me that's flying !!
As always though very educational.
Ivo

[emerydc8]

I think there is a fundamental difference here. The clamshell buckects really revert the exhaust gasses forward. On the modern turbofans the air cold or otherwise leaves the engine near perpendicular to the engine axle.

I agree with you that the clam shells were more effective at redirecting the thrust compared to the Venetian blinds on the CFM56 engines, but I personally did it in the DC-8-73 with the CFM56. It's not something you would do unless you were in a jam because the inboard engines are only a few feet off the ground. Also, a compressor stall on that engine is a bigger deal than on the JT3D. If you do a search you will find a number of references that Frank Borman (former president of the old Eastern Airlines) had written a biography where he stated that Eastern did powerbacks with the 757 as a cost-cutting measure. I don't have the book, but next time I am in Barnes & Noble I will see if it's on the shelf.

Ivo, check your PM.

Jon D.

[torrence]

Um ... Chaps

Just for the record, current PSX does move backward from a full stop when max reverse thrust is applied (for a light aircraft config anyway - I used the St. Maartin's Take off situ).  No funny business with high winds or saving the brakes (RTO armed) for the exercise.  This may be just the effect of the various tweaks that Hardy has made to the dynamic model etc. to get the 'feel' of the ground handling as realistic as possible when the aircraft is handled in a normal manner.  I figure there must be a neg  thrust component somewhere in the model or this wouldn't happen.  Haven't heard any of the 744 pilots say they have tried it in a real plane 

Cheers,
Torrence

[Hardy Heinlin]

The discussion on whether the 744 can move backward by reverse thrust or not has been started 20 years ago and the opinions keep varying ... :-)

When you test it in company sims: In some sims it works, in others it won't.

I don't think it's engine specific. As far as I recall, in Cathay sims it works, in BA sims it doesn't. Both use RR models.

In PSX it works (again). Persuaded by the majority ...


Cheers,

|-|ardy

[John H Watson]

Haven't heard any of the 744 pilots say they have tried it in a real plane

Quite some time ago in the PS1 community, we heard of one pilot who did try to use reverse thrust to push back a 747 (or to at least assist the pushback process) and destroyed several, if not all, engines in the process (I assume by EGT exceedance). Henceforth, he became known as the "6 million dollar man" (I assume the repair bill was close to this). I can't recall if actually helped the pushback process in this case, but I vaguely recall that there was a procedure in some 747 AOMs that allowed the use of reverse thrust to assist with pushback, at least on icy surfaces.

At my local airport, grooves were added to the tarmac to give the tugs added traction to assist with the initial pushback. Most of our bays (gates) also have sloped surfaces so that the aircraft will roll away from the terminal buildings... given a sufficient initial push.