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Gliding with a Boeing 737

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Gliding with a Boeing 737

Old 3rd Nov 2008, 19:11
  #21 (permalink)  
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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.
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Old 3rd Nov 2008, 21:50
  #22 (permalink)  
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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?
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Old 3rd Nov 2008, 23:16
  #23 (permalink)  
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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?

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Old 3rd Nov 2008, 23:59
  #24 (permalink)  
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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.
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Old 4th Nov 2008, 00:44
  #25 (permalink)  
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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.


Last edited by NSEU; 4th Nov 2008 at 01:00.
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Old 4th Nov 2008, 02:13
  #26 (permalink)  
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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)...
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Old 5th Nov 2008, 13:13
  #27 (permalink)  
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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.
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Old 5th Nov 2008, 23:47
  #28 (permalink)  
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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.
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Old 6th Nov 2008, 05:13
  #29 (permalink)  
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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.
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Old 6th Nov 2008, 07:49
  #30 (permalink)  
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Some first principles

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.
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Old 6th Nov 2008, 22:47
  #31 (permalink)  

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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).
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Old 6th Nov 2008, 22:59
  #32 (permalink)  
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Can you post a link to your book, perhaps Amazon.co.uk?

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Old 7th Nov 2008, 02:48
  #33 (permalink)  
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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

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Old 7th Nov 2008, 06:26
  #34 (permalink)  
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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 .....
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Old 7th Nov 2008, 09:45
  #35 (permalink)  
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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'
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Old 7th Nov 2008, 10:07
  #36 (permalink)  
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A319 figures from QRH give approx 2.5nm per 1000' at Green dot speed, and then 800ft per nm fully configured.
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Old 7th Nov 2008, 10:15
  #37 (permalink)  
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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.
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Old 7th Nov 2008, 10:20
  #38 (permalink)  
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V2 + 20 is a clean speed
Can you quote an authoritive source (Boeing FCTM) for that figure?
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Old 7th Nov 2008, 10:47
  #39 (permalink)  

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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!
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Old 10th Nov 2008, 15:29
  #40 (permalink)  
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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
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