737 vs 777 landing performance question

[Guest]

Hi,

 

I remember the 737 took " ages " before reaching taxi speed after landing after having idled the throttles, applied reverse thrust, auto speed brakes set on 2, manual brakes etc.

 

Now the 777 despite its weight can land on a carrier deck, it can slow down in 1/4 or 1/3 of runways! medium-long ones.

 

Question : in the reality, does it really slow down in blink of an eye like pmdg 777 ( auto brakes 2 f.i. ) or is it slightly exagerate now vs the 737 that was slightly exagerate in the opposite way i feel ?

 

Thanx in advance above all to real pilots answers but all opinions are appreciated

[phastings 1]

Not a real pilot, but after a search for the landing perfomance for both types, Boeing's info says a landing field length at MLW of 1,550m to 1,840m for 777 variants and 1,300m to 1,600m for 737 variants.

 

Commercial aircraft land at the same airports, so I'd expect the published maximum specs to be similar.  In the real world, I'd suppose that longer roll outs save wear and tear on brakes and engines.

[tcas_climb 45]

This has been discussed many times. The only way to tell is if we had some sort of utility that measured the distance the aircraft travels from wheels touch the runway to a complete stop, and compare this distance to the autobrake stopping distances in the manuals.

[scandinavian13 5,192]

It's worth mentioning that the 737 has brakes on four wheels (two on each of the mains) to slow it down.  The 777 has three times as much, with twelve.  Sure, it's much larger and heavier, but with twelve wheels with stopping power, you have a lot of energy absorption potential.

 

It's all broken down to the basic F = m * a.  Increases in m result in a larger F to be absorbed.  The 737's autobrake level two gets you about 5 ft/s² of deceleration.  At MLW (-700), that's an absorbed F of 670000 lbf/s².  Assuming four brakes share this load equally, that's 167500 lbf/s² per brake.

 

For the 777, assuming the same 5 ft/s² of deceleration, at MLW (-200LR) that's an absorbed F of 2460000 lbf/s².  Assuming twelve brakes share this load equally, that's 205000 lbf/s² per brake.

 

In the end, the difference is "only" about 37500 lbf/s².  Solving for the equivalent mass, that's only about 7500 pounds of difference per brake.  So, despite the 737 and 777 themselves being 358000 lbs apart (MLWs of 134000 lbs and 492000 lbs, respectively), the individual brakes only see them as 7500 lbs apart.

 

Add to this fact that the 777 brakes have an extra disc compared to the 737 (5 and 4, respectively), you could break it down like this: 737 is 41875 lbf/s² per disc, and the 777 is 41000 lbf/s² per disc.  Since the rim diameter is only about an inch different between the two planes, you could assume the brake disc size is relatively similar enough to dismiss surface area differences.  That 875 lbf/s² difference amounts to an equivalent mass of 175 pounds of difference per pad...or just about the weight of myself.  Not too different when you brake (har har) it all down.

 

 

 

Thanks for asking that question, by the way.  Working through it all refreshed a little of my math/science side that doesn't see much challenge in my currently IT-centric world.

 

(This post really accentuates the need for people to understand the fundamental difference between 'break' and 'brake' - haha - I just corrected two or three of my own...despite leaving one as a terrible pun.)

[Guest]

Yes, IT stuff and flight simulation must have a some sort of intriguing relationship ... also Sid Meier if I recall it right started his marvelous career with flight simulations...

 

Back to us, thanks a lot for the time dedicated to the answer, a very professional answer btw. This kind of approach within an answer boosts my passion for flight simulation, you know what I mean...and it ain't the first time that I carefully read you Scandinavian13, I am impressed by your deep knowledge and passion, we are lucky to have you here !

 

Too much compliments are not necessary I know, sorry ... ok ok you can Paypal me later as agreed ... )

 

 

 

I also thought about the increased number of wheels and therefore brake power into the 777 and you have confirmed my initial thoughts about the possible reason for a strong deceleration, what I missed in my train of thoughts was the increased ft/s of the T7 and you perfectly described it and I thank you for that.

 

Btw Scandinavian, there must be an aspect of the T7 that you know only in a superficial way... hey forumers, we have to find it out ))))................. I'm joking

 

Thanks a lot

 

Cheers

[kjjj11223344 48]

Maybe it's just me, but I don't see this problem you are having. My T7 tends to take a little longer than my NG (even the 800 with sometimes 150+ approach speeds) in order to reach taxi speeds. Maybe you are carrying a light load (I use a pretty full plane for all my flights)?

[phastings 1]

And Kyle, the 777 will have a larger speedbrake area and a bigger reverse thrust effect compared to the 737 - but also more momentum from the extra weight!  Not sure how you can add those factors into the equation!

 

Don't published minimum landing distances only take into account the wheel braking effect, so in 'real life' operations the aircraft will use less runway?

[KevinMM 22]

Also, remember your winter driving classes.  Brakes stop the wheels from turning.

 

Now, say it again with me " Brakes stop the wheels from turning"

[PHXMD-11 86]

 

 

winter driving classes

 

I'm from Phoenix. What is this winter you speak of?

[whoispankaj 51]

Now ww are into car driving lessons eh?

[scandinavian13 5,192]

 

 

And Kyle, the 777 will have a larger speedbrake area and a bigger reverse thrust effect compared to the 737 - but also more momentum from the extra weight!  Not sure how you can add those factors into the equation!

 

Interesting thoughts, but they won't add much to the discussion, and here's why:

 

Reverse thrust is a non-issue because autobrakes are set to slow the aircraft down at a specific deceleration rate.  So, if you use reverse thrust, the plane will actually use less braking power to yield the same rate of deceleration.

 

The effect of the speedbrakes is also somewhat negligible as well, and that's because they're not actually "speedbrakes" - they're "spoilers" (at least on these two planes).  I know that sounds pedantic, but it's actually really important to the point here.  Sure, they do go up at enough of an angle that it creates a force against forward movement, but contrary to what most people think, the major reason spoilers are there is to spoil the lift from the wing to place more weight on the wheels.  The more weight on the wheels, the more potential grip, which means the more potential braking action.  In other words, if you're looking at the four forces of flight, spoilers are more of a gravity vector device than a drag vector device.

 

Finally, momentum and force are intimately related.  They're so intimately related, in fact, that we've been addressing momentum this whole time without even directly mentioning it.  Momentum is the amount of energy an object is currently carrying, at a constant velocity (p = m*v).  Force is the amount of energy to affect a change to an object's momentum (F = m*a).  Therefore, the topic at hand - braking, or "the change in object velocity through a decelerative force" - is best addressed by directly referring to force, which indirectly addresses momentum.

 

Maths Proof:

F = m * a   ||   p = m * v

F = m * ∆v/∆t

F * ∆t = m * ∆v (remembering p = m * v)

F * ∆t = p

...or force * time = momentum.

[tcas_climb 45]

Momentum is the amount of energy an object is currently carrying, at a constant velocity (p = m*v). Force is the amount of energy to affect a change to an object's momentum (F = m*a).

 

Maths Proof:

F = m * a || p = m * v

F = m * ∆v/∆t

F * ∆t = m * ∆v (remembering p = m * v)

F * ∆t = p

...or force * time = momentum.

It's cool statements like this that makes me regret not doing physics in school.

[scandinavian13 5,192]

It's cool statements like this that makes me regret not doing physics in school.

 

haha - I'm pretty sure an actual physics major would cringe.  I just always like maths/physics, so I took courses here and there.  Maths and physics always have some sort of logical answer.  History was just memorizing things, and English made me write more and more and more and more...ever wonder where the damage came from that makes me so wordy and terse in typing?

 

College English...