B737 Glide Performance

[capt waffoo]

Had a conversation with a microlight pilot today and got onto the subject of glide angles. Of course he knew all about his, but I had to admit I did'nt have much idea about the 737. Always been too busy to look at the ROD whan doing relights in the sim.

Can any 737 Perfo expert with the gen let me know so I can redeem myself ps?

Ideally glide angles/ROD at min drag clean speed at high, medium and low altitudes.

Thanks!

[Tinstaafl]

Did an aviation physics course a few years ago.

As I recall the L/D of most jet transports was around 18 or 20 to 1 at best L/D angle of attack in a clean configuration

The IAS for this constant. Have to check this as I haven't thought about it for a year or two.

IAS will correspond to min drag speed - whatever that is for the a/c. This would equate to best range speed (not LRC, although that is close)



[This message has been edited by Tinstaafl (edited 23 September 2000).]

[quid]

The IAS for best range will be the best specific range speed. Generally 99% of Long Range Cruise. Much faster than L/D max.

The speed for L/D max would be closer to max endurance. Just glide at your best holding airspeed for the weight and you won't be far off.

------------------

[CaptainSandL]

Waffoo,

The best info I can find from Boeing for the classic (3/4/500), is in the All-engines planning tables for a M0.74/250KIAS descent. Idle thrust, zero wind etc.

From 37000ft to MSL takes 23min, uses 300kg and you will cover a distance of 100nm at 35T, 111nm at 45T and 116nm at 55T. Allowances are included for a straight-in approach with gear down and landing flap at the outer marker.

So lots of corrections to the above figures for the true engines-off equivalents, perhaps the main one being that if you were in a glide you would probably want to be at the min drag speed – approx 210.

Gliding in the sim seems to give about 2000fpm but varies with altitude (amongst other things). For glide angle, try looking at the V/B or FPA on the descent page, I don’t know any figures offhand but will have a look next time I am in.

Incidentally a good article on how to handle the double engine flameout written by one of my colleagues can be found at:
http://www.b737.org.uk/lossofthrust.htm

S & L

[CaptainSandL]

Does anybody know the L/D max (the ratio not the speed) for any version of the 737?

[Cough]

Our QRH recons on 2nm/1000ft.

Gives a ratio of about 6:1

[John001]

Hi cough,

without wishing to be a pedant isn't 2nm/1000ft a 12:1 glide ratio?

[Dufo]

12,16 actually. Better sharpen those handling skills!

[LOKE]

I alway used:

2nm/1000 @ high speed
2.5nm/1000 @ 250
3nm @ close to L/D

Of course weight had a substantial impact and naturally wind had to be considered.

Those #s worked well for me as long as you did a few how goes it checks along the way. 6000'/nm if you ant to equate it to glide ratio.

LK

[Cough]

Oops, giva ya that...

[rodthesod]

I alway used:

2nm/1000 @ high speed
2.5nm/1000 @ 250
3nm @ close to L/D

Of course weight had a substantial impact and naturally wind had to be considered.

Those #s worked well for me as long as you did a few how goes it checks along the way. 6000'/nm if you ant to equate it to glide ratio.

LK

Surely your 1:1 is landing config all eng inop?

rts

[ProfChrisReed]

I alway used:

2nm/1000 @ high speed
2.5nm/1000 @ 250
3nm @ close to L/D

Of course weight had a substantial impact and naturally wind had to be considered.

We glider pilots are intimately acquainted with best L/D, though not for Boeings. Weight doesn't change the best L/D, but it does change the speed at which the best L/D is achieved - the heavier you are, the faster you need to fly to be at best L/D.

3 nm per 1000 ft = 18:1, which seems about right from other things I've read.

For greatest distance across the ground you should fly a little slower (between best L/D and min sink) in a tailwind, and somewhat faster in a headwind. The headwind calculations require information about the gliding polar, but a good rule of thumb in gliders is that you achieve the greatest ground distance into a headwind at best L/D plus half headwind. At Boeing glide speeds, if the rule scales up, that might be very little difference.

The Gimli glider, a Boeing 767, apparently achieved only 12:1 (but maybe didn't need to fly best L/D). It helped that the captain was also a glider pilot.

http://en.wikipedia.org/wiki/Gimli_Glider

[squeaker]

If the B737-3/4/500 is the Classic, does that make the -200 the Veteran?
Anyway, the min drag speed was about 250 IAS. At 290 or 210 you could just hang on to a 3 degree slope, but not at 250.
Is the runway threshold going up or down the windscreen? Who needs VNAV....

[misd-agin]

Does anybody know the L/D max (the ratio not the speed) for any version of the 737?

Fact - 767-200 is 17.9 per Boeing manual

Experience - 757 is cleaner. 767-200 at 20 DME needs about 200' more altitude(energy) than the 757 for a clean approach, with idle power to <1000' AFL than the 757.

Experience - S-80 needs slightly more and the 727 even more altitude(energy) to accomplish the same. I was told the 727 was 17:1 or slightly better.

737? IDK.

Almost all airliners have very similar glide ratios. For simplicities sake let's call it 18:1. 30,000' is 5 n.m. high. 5 n.m. x 18 = 90. Your altitude in thousands x 3 = gliding distance. 40,000' = 120 miles.

Speed? Holding speed. Per Boeing that is L/D speed. That's the speed I would use for my glide speed. At high altitudes the FMC generated airspeed is to low (KIAS/ECAS/MACH issues).

I've had several emails on this topic with Boeing engineers.

Sim experience - 250 kts at 7 mile final on the glideslope gets you to the runway. Perfect for 727 but more energy than you need for the 757/767(cleaner, ie better gliders). Maybe a slower speed (KIAS) is perfect but 250 has always worked in the sim. More energy vs not enough is *always* the answer when you're a glider. ;)

Air Canada achieved 12:1? I believe they also had tons of energy, which could have been used, in an optimum situation, to achieve a better glide ratio.

And what speed was used in the descent? Makes a big difference in your glide distance. Can be checked by doing the math on TAS/sink rate at various speeds and glide ratio(w/idle power) can be computed.

[PantLoad]

Yes, the 737-200/300/400 were pretty much the same. If you were descending straight in (no downwind or base turn to speak of), you needed to be 35 miles from touchdown at 10,000 ft AFE and 250 kts. That gave you a 3:1 plus an extra five miles for deceleration and dirty up.

This is the rule I always used, and it worked fairly well (no wind). Of course, if you were having to turn around and land the other direction (downwind/base/final) you could be 10,000 ft AFE and 250 kts at 25 miles. If you were doing a base entry, 10K/250/30 miles worked.

Descent profiles above 10,000 (or above your speed limit altitude) was (no wind) maybe 2.5:1...depending on how far down you pushed the nose.

Of course, all of the above assumes idle thrust and no wind. If you have to keep a bit above idle for pressurization...or if you're running engine A/I...or if you've got like maybe a 100 kt average tailwind...you'll have to add a bit (like maybe 10 miles or more....if you have to do all three...add maybe 20 miles).

Also, this assumes spool up at 500 ft AFE (VMC situation).

PantLoad

[misd-agin]

[quote=PantLoad]Yes, the 737-200/300/400 were pretty much the same. If you were descending straight in (no downwind or base turn to speak of), you needed to be 35 miles from touchdown at 10,000 ft AFE and 250 kts. That gave you a 3:1 plus an extra five miles for deceleration and dirty up.
***********************************************************

Guys said that 250kts @ 10,000' @ 30 DME worked for the 727. Well, yeah it did, but you needed drag (ie early flap extension) eventually. For the 727 I used to use 33 or 35 DME to the runway (it's been a while and I havn't stopped drinking so my memory's faded :p ). S-80 was something like 37-38 DME. 767 around 41 DME, 757 around 43 DME @250 kts @ 10,000' with typical winds.

Perfect every time? Nah. Plus your dealing with Mother Nature but it's close.

767/757 FMC descent data (VNAV) is pretty good. You need to play with KIAS in FLCH in make it work out perfectly.

727 PDCS data was about 10 miles off from mid 30's. It would say 117 miles from 37,000' (why I remember that I don't know) but 107-108 would be perfect. If you started down at the PDCS generated point you needed power in the end game. I think it might have been based on gear/full flaps at the outer marker which isn't how most folks fly the jet.

[john_tullamarine]

.. or, then again, if you have a turbulence driven aerodrome, the 73 will come nicely overhead at 210 kt and 10,000 ft AGL on crosswind ... and do a normal tight circuit, dirtying up along the way.

Much more comfortable than thumping into the circuit at normal speeds and shaking the customers up far more than is necessary.

[hawk37]

Misd-agin,

any chance you could expand on too low FMC generated speed, or the Boeing emails?


Speed? Holding speed. Per Boeing that is L/D speed. That's the speed I would use for my glide speed. At high altitudes the FMC generated airspeed is to low (KIAS/ECAS/MACH issues).

I've had several emails on this topic with Boeing engineers.



Hawk

[misd-agin]

Misd-agin,

any chance you could expand on too low FMC generated speed, or the Boeing emails?



Hawk

For minimum speeds at altitude we use Vref +80 below FL250 and Vref +100 above FL 250. It works. Obviously it's just a swag but Vref +80 gets below 1.3(?) Vs at the higher altitudes. So they came up with a easy to remember number that covers all situations. I'd be B.S. ing you if I tried to explain the problem but I believe it's a KIAS/KEAS problem. Discovered by experience when guys tried to slow to FMC holding speeds at high altitude and reported back that it wasn't a very good idea! :eek:

It was obvious if you ever tried it. I remember holding at FL410(?) (10 n.m. box pattern, expanded to 20 n.m. box pattern). FMC hold speed was probably 210 +/- range, plane wasn't happy below the low 220's so we held using high 220's. Based on later policy (Vref +100) the higher speed was correct and appropriate.

Boeing emails? I asked some questions regarding various speeds and their relationships to each other - L/D max, best glide, best rate, min sink, best angle, best holding speeds, etc.

My memory of their answer ("FMC data is correct") is that L/D max = best glide = FMC holding speed = best angle.

Next question I asked was why in the 757 FMC holding speed wasn't the same as the best angle speed(prior email said it was). "It wasn't?" and havn't heard from them again. (nothing sinister just lost touch...)

"we don't use min sink speeds".

[hawk37]

Misd-Agin. thanks for the info. Sound like the fmc data is true minimum drag speed, in which case holding at said speed seems likely quite a handful.

Do the fmc holding speeds (I presume they are in KIAS) increase with altitude? This would make sense as minimum drag KCAS is also expected to increase with altitude.

Hawk

[misd-agin]

Misd-Agin. thanks for the info. Sound like the fmc data is true minimum drag speed, in which case holding at said speed seems likely quite a handful.

Do the fmc holding speeds (I presume they are in KIAS) increase with altitude? This would make sense as minimum drag KCAS is also expected to increase with altitude.

Hawk

Yes, they're in KIAS. No, FMC holding speeds do not increase with altitude. That's the gist of the problem. They researched acceptable speeds and apparently realized the Vref +80 (+100 > FL250) was always an acceptable holding speed.

[SlamBam]

For those flying 737NGs with winglets--how much of a difference do they make in glide performance?

[hawk37]

Based on some generic numbers, you can make an estimate. Lets confine our thoughts to low altitude (ie less compressibility), and a rough figure of 6 % better specific range at max range speed (you should be able to get an exact figure from the performance sections for the aircraft with and without winglets). Assume a flat thrust curve vs velocity.
You've now got approx 6 % less total drag, even though the winglets add parasite drag, and the added weight gives added induced drag. However that is at max range speed about 1.3 times min drag speed. So at min drag speed, expect about 5% less drag (induced drag will be higher at the slower speed, but the reduction of parasite drag will be even more since it increases with TAS squared).
So, expect a 5 % increase in glide distance. It it was 20:1, expect about 21:1