Gliding with a Boeing 737

[Bluescan]

Hello everybody,

I've tried some searching around on the web, but not finding answares for my questions. As a glider pilot I'm a bit interested in how the airliners glide etc.

For a Boeing 737-700 with wing lets:
- What is the best L/D ratio?
- What are the best speeds for best L/D at given weights?

Would it be possible to save fuel, descending earlier at a lower speed?
Are there any requirements of how long an typical 737 engine has to run before max power (take-off)?

Sorry if there questions have been asked earlier, I'm new here...

Best regards,
Bluescan

[BelArgUSA]

I do not have the 737 numbers for you -
xxx
For the 747-200 Classic - we are close to 20:1 ratio.
We could glide some 220 km from FL 360, assuming "no wind".
Typical glide speed would be some 200 KIAS.
I often mention the above to people who say that "jets glide like a brick"...
xxx
:D
Happy contrails

[ix_touring]

jets glide like a brick"...



Its all relative, the first single seater (glider) I flew had a ratio of about 18:1 :sad: K-8 (http://en.wikipedia.org/wiki/Schleicher_Ka_8)

The plastic twin seater I lfirst flew in had a ratio of 36:1 :) Grob 103 (http://en.wikipedia.org/wiki/G103_Twin_II)

As for a metal glider of 74 classic vintage, the Blanik (http://en.wikipedia.org/wiki/LET_L-13) has a ratio of ca. 28:1. On a tangent, it also has forward swept wings...

iX

[Denti]

The classic was somewhere cited to be around 1:22, the NG seems to be quite a bit better, its a goddamn gliderplane really, cant slow it or get down fast enough (glides too good), especially if heavy.

Learned to fly on an old Bergfalke 2, then Ka 8, Ka6 and ASW 15, the old stuff :) Sadly no time anymore to fly gliderplanes (except the NG of course).

[stackedup]

You should have tried the Grunau Baby, it could even sink in a thermal !http://static.pprune.org/images/smilies/wink2.gif

[rogerg]

Wot about the T31!!

[Bluescan]

Thank you BelArgUSA and Denti!

So, what I see here is that the L/D is quite good, up to 22 on a newer 737. BelArgUSA, you expected the best L/D speed to be quite low. Then I wonder what a realistic speed would be on a 737.

And second question, do the jetengines need to run for x minutes before going full? If you started your engines on the runway, just completing the checklists and then full throttle. How healthy would it be?

-----
We also have a Ka8 in our club, a fun glider to fly, but don't it have a l/d closer to 26-27?
You may watch my gliding videos here (http://www.youtube.com/seilfly)

[YakAngel]

About the same amount of healthiness as you waking up from a deep sleep, immediately jumping out of bed and being forced to run a sprint 200m run. How does that sound to you?????

[Flambards]

As close as damit, rule of thumb -

V2 plus 20 KIAS gives you the best L/D ratio on a 737.

Green dot on the Airbus

Therefore the best glide speed for still air.

[BelArgUSA]

Bluescan -
xxx
Best glide speed obviously lower for a 737 than a 747.
I hate to give uneducated numbers, but certainly could be 160-180 KIAS.
Wish airliners had an "angle of attack" indicator. Learjets have one.
With such instrument, you do not even worry about approach speeds.
Heavy or light, fly the attitude...!
xxx
:8
Happy contrails

[Yipoyan]

My B747-400 has FPV on the PFD. The angle of attack is the difference between aircraft symbol and the FPV (flight path vector).

Likewise, Airbuses with FBW are all FPV equipped.

[Nightrider]

On the classic the glide speed is around 210kn, on the NG it is around 200kn. Also the classic has a higher descent rate, it is close to 1:20; the NG does 1:22; that is the -800 I am talking about.

[Tee Emm]

V2 plus 20 KIAS gives you the best L/D ratio on a 737.


Does that assume you are gliding with flaps down since V2 is usually associated with a take off flap setting. I would have thought that any drag systems would increase the glide angle. For dead stick approaches in the 737 Classic we normally use 210 knots clean to reach vicinity of the airfield then flaps 5 and gear as required to fine tune the approach. This is pure "about yea" guesswork and not based on any specific figures from a manual

[Intruder]

747 clean would be V2+80 KIAS.

[Port Strobe]

737 clean speed is at Vref40 + 70, so perhaps V2 + 40 give or take?

Yipoyan - wouldn't you also have to consider the wing incidence to the fuselage before you can use that method? I'm running on the assumption it's non zero for a 747 else it would eat up a hell of a lot more runway at 400t than it does no?

[Flambards]

V2 + 20 is a clean speed

[FullWings]

Do you mean an 'idle power descent' or a 'no-engine glide'? Many of the glide angle figures being quoted are more appropriate at flight idle and the 'real gliding' L/D will be substantially less, I would hazard in the 15-18:1 range.

Normal holding speed is a good point to start but in reality, the speed for best glide (under power) will reduce as the residual thrust increases.

[B737NG]

V2 on a B737 is not a speed when the AC is in clean configuartion..... +20 you might get when you Take Off with F1 + 20 kts.... could be clean, with F5 + 20kts you are not clean for sure.

Fly safe and land happy

NG

[františek dobrota]

Time limit for higher power setting (extract from 737CL AMM):

After the engine start, operate the engine at low idle power
for two minutes minimum before you accelerate the engine above
idle power.
NOTE: Taxi time at or near idle power can be part of the two
minute warmup period.

And some useful doc. for "heavy" gliders...:)

http://www.casa.gov.au/fsa/2003/jul/22-27.pdf (http://www.casa.gov.au/fsa/2003/jul/22-27.pdf)
http://www.wadenelson.com/gimli.html (http://www.wadenelson.com/gimli.html)
http://www.rvs.uni-bielefeld.de/publica ... ransat.pdf (http://www.rvs.uni-bielefeld.de/publications/compendium/incidents_and_accidents/Ladkin-AirTransat.pdf)
http://www.moptc.pt/tempfiles/20060608181643moptc.pdf (http://www.moptc.pt/tempfiles/20060608181643moptc.pdf)
http://versa.bmvit.gv.at/uploads/media/ ... 0_2_01.pdf (http://versa.bmvit.gv.at/uploads/media/A310_GZ._85007_vom_12._Juli_2000_2_01.pdf)

[Flambards]

Understood, 'V2 is a speed you fly at with flaps down.'

Ignore the flaps thing, just take the V2 figure the manual gives you at the appropriate weight then completely forget it happens to be called V2.

Then add 20 kts

Then fly that number on the speed dial in the CLEAN CONFIG.:)

Best L/D occurs at approximately whatever the V2 figure is in Knots plus twenty - (this gives you the 737-300 CLEAN best L/D approximately).

Other jets, configs etc will all be slightly different.

[Yipoyan]

Port Strobe,

We do not fly using the FPV on our B744. Takeoff rotation, climb, cruise, descent, etc. are all based on aircraft pitch. The FPV, which is normally switched off, is generated by the flight management system using data from the IRS. As such, the FPV does not really present an attitude, but a trajectory. The angle of attack thus is the difference in pitch between the aircraft symbol and the trajectory. The incidence of the wing to the fuselage does not come directly into play. The same goes for the Airbus 330 which I had flown.

The interesting thing about the FPV on our Boeing is that there is virtually no write up in the manuals on how to use it. It was fortunate for me to have come from the A330, and therefore understand its usage. I understand that very few B744s have FPV installed.

[wbryce]

Now, I may be wrong here as its been a while since I done the theory, VMD gives the best glide as well as the best ROC, so would I be correct in saying, if you went into the FMC climb page and got the speed for best rate, wouldn't this also be your best glide speed?

[FlyingOfficerKite]

This is interesting - but I would be surprised if the engine-out performance of a B737 is anything near that of a glider.

Having flown wooden gliders years ago - the most efficient being a Skylark 4 with a L/D ratio of 1:36 - and then having carried out a manual reversion in the sim with both a B737-300 and -700 I was amazed by the glide angle on the Boeing. I remember looking ahead for the airfield but was surprised to find it was out of sight under the nose!

The rule of thumb for the engine out performance in a B737 was clean speed (210kts in the -300) with a glide ratio of 1nm per 1000ft - so approximately 1:6. Nothing, I would suggest, anywhere near the performance of the most inefficient glider (the Air Cadet T31 did better than that!).

So where these figures for engine-out performance for the B737 being comparable with a glider come from I don't know?

FOK :confused:

[OPEN DES]

Now, I may be wrong here as its been a while since I done the theory, VMD gives the best glide as well as the best ROC, so would I be correct in saying, if you went into the FMC climb page and got the speed for best rate, wouldn't this also be your best glide speed?

Vmd would give you the best ANGLE of climb as opposed to best ROC. Best glide speed on a jet would equate to Vmd=green dot=clean maneuvering speed. Best ROC speed is typically around 20 knots higher than Vmd/max angle.

[NSEU]

My B747-400 has FPV on the PFD. The angle of attack is the difference between aircraft symbol and the FPV (flight path vector).

"angle of attack is used to describe the angle between the chord line of the wing of a fixed-wing aircraft and the vector representing the relative motion between the aircraft and the atmosphere."

Sorry to be pedantic, but the airplane symbol is body pitch angle, not wing chord angle ;)

I understand that very few B744s have FPV installed.

Perhaps on older 744's. I believe it's now standard out of the factory. You'd have to pay to have it removed.

Rgds.
NSEU

[wbryce]

Open Des, that makes more sense, I managed to confuse them with each other. best ROC (Vy) should then roughly be 1.32 greater than VMD (Vx)...

[Port Strobe]

Thanks Yipoyan, I hardly ever use the FPV either save for using weather radar in the climb, pitch attitudes have always worked for me, and numerous people before me still. All I was getting at is I'm not so sure your method is entirely accurate, as NSEU has also suggested the wing incidence to the fuselage is the missing element in that equation. Whilst it may be quite small I'm sure two numbers on the same order of magnitude could produce noticeably different results when it comes to flying or measuring AoA. I think what you wrote is a good idea but it doesn't appear robust enough other than to show a change in AoA since the wing incidence is constant for any given configuration. However in the scenario referred to in the opening post I'm with you in that I'd rather set an attitude for clean speed and deal with the numerous more important things that ensue than trying to figure out how to get best AoA.

[-438]

SOP's on the 738's I fly are 5 mins between 2nd engine start (no1) and start of take off roll for first flight (cold) or 3 mins for subsequent starts. As for angle of attack indicators and FPV, we have both on our 738's, as well as HUD. I think the FPV is standard and the AOA is an option. Our 744's have FPV. I use the FPV for accelerating one engine inop and on visual approaches for 3 degree reference, but don't use the AOA indicator.

[pattern_is_full]

The Air Transat that had to glide 65 nm to Lajes(Azores) after fuel exhaustion did about an 11:1 ratio (from 34,500 feet) but also arrived high and fast and had to do 360s to lose altitude before final approach. 200 kt final approach speed and around 2000 fpm descent from the published reports.

Airbus 330, so in between 73 and 74 in size/weight - although obviously "light" in that the fuel tanks were empty. :eek:

[mathy]

For the B737 I have nothing to hand.

The B767-300ER has, by the best estimates that I have seen a zero-lift drag coefficient in the range 0.0130-0.0135 and an Oswald wing efficiency factor of 0.835.

Its wing area is 3050sq ft and the unfactored aspect ratio is 7.98. Taking account of the Oswald coefficient the factored aspect ratio is 6.67. Thus induced drag coefficient is CL^2/20.95

At speeds for which compressibility can be discounted the best L/D ratio to be expected is when zero-lift drag and induced drag are to all intents and purposes equal.

For this to be the case CL = .53 and zero-lift drag coefficient is 0.00134. Suppose the weight of the aircraft is 310000lbf then under ISA conditions at 33000ft pressure altitude and M0.708 we have the necessary conditions and can assume compressibility drag to be vanishingly small. L/D = 19.8 and the airspeed is 238keas.

Bearing in mind the definition of equivalent airspeed and also the roughness of the foregoing assumptions I wonder if Boeing intended the best glide speed of the B767-300ER to be 240keas with an L/D of 20 and therefore a glide slope of 1 in 20. Just a thought.

[John Farley]

I admit to being astonished that FlyingOfficerKite feels a B737-300 would glide only 1nm per 1000ft. That is worse than a Hunter with its gear down. If these numbers came from a sim then I would suggest the sim was in a suspect state of health.

In my book all modern airliners (when clean) are very efficient cruise machines which means they have to be good gliders. Weight of course has no effect on any gliders descent angle – only its best gliding speed which will increase with weight. Which is why during competitions if gliders need to go a long way into wind they load them up with water ballast to help them penetrate (at no detriment to their gliding angle).

[galaxy flyer]

John

Can you post a link to your book, perhaps Amazon.co.uk?

GF

[FlyingOfficerKite]

John

With the greatest respect, that is the rule of thumb used for engine-out approaches when simulating a double engine failure.

The technique is to position approximately 15 miles out at 15,000 ft and plan your approach from there.

I appreciate that the L/D ratio at altitude exceeds this figure - the point being that you aren't going to glide with that performance all the way down.

The idea that you will glide with a L/D ratio approaching 1:20 to a landing is not feasible - unless you plan for a straight-in approach which, considering you're going to have to configure the aircraft and position for the landing, is unlikely.

It's what I've done several times in the sim - with a reputable airline - and it works out fine. I'm surprised no-one else has confirmed this as it's standard SOP for Boeing 737s?

Kind regards

FOK :)

[john_tullamarine]

Interestingly, some Chinese VIP squadron chaps with whom I was working in the sim some years ago all had comparatively different approaches as to how a deadstick should be conducted in the 732 ... all got in very nicely from an initially good height/distance ... somewhat disheartening for those of us who are only mere mortals ...

Although I have not been involved with the 732/3/4 for some years now, my recollections are that 15 at 15 will give you a rather high key circuit to massage. I think I would prefer to glide to an initial maintaining something closer to 2 miles per thousand .. slow and dirty I know I can do better than 1 mile per thousand to lose height .. that being about the best I've seen (without sideslipping) with residual thrust... then, again, having seen it done successfully every which way by my Chinese colleagues .....

[BOAC]

JF - 1 in 1 with gear down and flap running is pretty close for the 737 in the sim. Fortunately I have not had the opportunity to check it for real. I can confirm, however, the Harrier exceeds 1 in 1 by a significant margin 'for real':)

[Right Way Up]

A319 figures from QRH give approx 2.5nm per 1000' at Green dot speed, and then 800ft per nm fully configured.

[fivegreenlight]

Bluescan, you asked;
Would it be possible to save fuel, descending earlier at a lower speed?

Yes, in our 800's we fly a lower speed than we used to and have to descend earlier as a result. Speeds are typically 260kts or less, used to be around 280kts.
I understand we save over 100kg by doing this.

[A37575]

V2 + 20 is a clean speed

Can you quote an authoritive source (Boeing FCTM) for that figure?

[John Farley]

My mistake. I thought FOK's quoted 737 glide performance was clean - not in an approach config.

It was his last remark "So where these figures for engine-out performance for the B737 being comparable with a glider come from I don't know?" that fired me up!

[misd-agin]

I asked this question of our tech folks regarding 757/767's. Digging in a Boeing manual we found 767-200 = 17.9:1 glide ratio.

757 has a better glide profile but that's with engines at idle. Assuming the dead engines on the 757 and 767 create the same amount of drag the 757 should glide better, especially now that they can have winglets installed.

Dual engine flameout checklist lists the best glide speed. It varies with weight. 757/767 a quick rule of thumb is Vref flaps 30 +95 kts. Not perfect, but very, very close for typical weights. Ranges from mid 200's to low 200's KIAS based on weight.

18:1 glide ratio is a great number. That glide ratio allows you to glide 3 miles for every 1,000' of altitude.

Eg, FL300, glide 90 n.m. 30,000/6000' per n.m. = 5 n.m. high. 5 n.m. x 18 = 90 n.m