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# Glide ratio

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# Glide ratio

9th Apr 2014, 04:55

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Glide ratio

What minimum glide ratio, if any is established for transport category aircraft?
From deep in the recesses, I remember 12:1, yes/no?

Yes, I did a cursory google search, didn't find what I was looking for.
9th Apr 2014, 06:47
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I have never seen any "requirement" per se.

However, commercial reality dictates that a design have good L/D characteristics (or it will fall by the bean counters' wayside).

A reasonable present rule of thumb for heavies is that they glide (nil engines) somewheres around 2nm/1000ft .. which approximates 12:1.

As design and manufacturing technical capability improves .. so will the glide performance as a sideline consequence of improving L/D values.
9th Apr 2014, 18:00

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Only requirement I ever heard of was for motor-gliders to keep people from selling bog-standard powered airplanes as motor-gliders to get around medical requirments.
10th Apr 2014, 03:45

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100-odd nm from 30,000ft at well above best glide speed at idle. More like 15-17:1, methinks.
10th Apr 2014, 03:47

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The 15-17:1 value falls in line with the values I'm finding as well.
11th Apr 2014, 12:57

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20:1 was what I was initially taught to expect.

But straight turbojet should be a bit higher that modern turbofan......
11th Apr 2014, 13:08

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With A320 I am doing at green dot around 200 Knots and descend rate around 1200 fpm.

If you divide 200 by 12 (1200 fpm descend) you come up with 16.6.
That is roughly the glide ratio 1:17
12th Apr 2014, 09:56
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I had presumed that the OP was looking for all engine out gliding performance due to his reference to 12:1.

2nm/1000 is reasonably typical for that situation clean approximating 12:1 .. with quite a bit extra accruing for all engines at idle .. typically 3nm/1000 which approximates 18:1.
12th Apr 2014, 20:32

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A320 QRH states the aircraft can fly up to approximately 2.5NM per 1000ft at green dot in case of dual engine failure. this will make a glide ratio of around 15:1
12th Apr 2014, 20:55

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John is correct, but all input is appreciated.

15:1 at green dot. Does green dot represent best L/D, or close to it?
12th Apr 2014, 23:56

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We always used a rule-of-thumb that you could glide 100 miles after the loss of all engines at 'typical cruise' altitudes. I've never bothered to figure out what sort of l/d that would work out to. IIRC, for all engine out Boeing tells you to maintain an airspeed consistent with the windmill start envelope (which would depend on the aircraft/engine combination). Granted, for fuel exhaustion that won't help...

I don't recall ever seeing or hearing about anything in the FARs regarding l/d or glide ratio - just that the airplane had to be controllable (hence the need for a RAT on many installations).
13th Apr 2014, 01:09

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B727 - supposedly 17:1 (I'm guessing it was just an estimate).

B757-200W and 737-800W - approx. 19.5:1 calculated sink rate/TAS in the airplane.

B777-200 - guessing close to 19:1. It seems to be similar, but slightly less, than the 757W and 737W performance

B787-8 - 21-22:1 per Aviation Week and Space Technology

18:1 is simple math - FL(minus one zero) x 3 = glide distance. Eg FL300 - 30 x 3 = 90.

30,000' = 5 nm. 5 x 18 = 90.

10,000'/250 kts/no wind - 35 nm away should make the runway. 40 nm is tougher and wouldn't be worth the risk in real life(IMO)(simulator profiles).
13th Apr 2014, 08:14

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Originally Posted by misd-agin
B757-200W and 737-800W - approx. 19.5:1 calculated sink rate/TAS in the airplane.
This is a fact that is easily overlooked when discussing Glide Ratio.
Aerodynamically the correct glide Ratio is sink rate/IAS. When calculating how far you can glide with regard to Mother Earth, it becomes sink rate/TAS.
When starting the glide at >40k this will make quite a difference, so with an aerodynamic glide Ratio of 12 an effective glide Ratio regarding the Earth of ~16 - 17 could be expected.
13th Apr 2014, 09:14
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Aerodynamically the correct glide Ratio is sink rate/IAS

Might we ask you to amplify this a little .. ?
13th Apr 2014, 10:29

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Originally Posted by john_tullamarine
Aerodynamically the correct glide Ratio is sink rate/IAS

Might we ask you to amplify this a little .. ?
I'm, afraid I need to, because after re-reading I see it can be a bit misleading/un-precise...

Lift and drag are dependent purely on IAS/CAS. The L/D will thus only be a function of IAS (leaving Mach aside). However, the distance travelled in a certain time will be greater when TAS > IAS. So while sinking at the same rate per second (Edit: Wrong assumption!), the distance travelled will be greater. The 'apparent' glide Ratio will be greater, while L/D still remained the same). So what I was trying to say is that with an L/D of 12 you might (Edit: not) achieve a glide Ratio of 16 to 17 regarding distance travelled from a defined Altitude.
Edit:
Please ignore this calculation. Thanks to @JT (I didn't read the signs and digged deeper, sorry!) and @Owain for pointing this out!

Last edited by henra; 14th Apr 2014 at 22:08.
13th Apr 2014, 17:02

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To contribute a real world perspective...

I once did a demonstration of a DC-8-72 where I reduced thrust to idle from FL240 @ 74 NM out to a straight-in. Field elevation: 1288'. I was able to land without touching power, within 5 kts of on speed at the threshold just by managing energy with gear and flap extension timing (there are no speed brakes on a DC-8, BTW) The landing weight was ~205,000# on a 350,000# TOGW limit.

When flying the Classic 747, one must adjust idle power descent point depending on TOD weight if planning a min fuel burn descent without ATC interference. The heavier the weight, the longer the glide, and the lower the weight, the shorter the glide. The difference in a no wind situation glide between landing weights of 630,000# and 380,000# could be ~25-30 NM. Airplanes with a lesser delta would obviously be affected by weight to a much smaller degree.

From a small airplane standpoint, gliding distance improved very noticeably when I added flap gap seals to my 185. Descent planning to a VFR traffic pattern required a re-evaluation due to the resulting drag reduction.

Engage in speedbrake-free energy management without a green arc or dot. It's an enjoyable game.
13th Apr 2014, 17:06

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I'm afraid SOP and industry practice won't allow power off descent to touchdown anymore.
13th Apr 2014, 17:33

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I know. Mine was accomplished last summer, and done outside an "industry" circumstance. The point perhaps is that the concept is not necessarily encouraged (or, in fact, is prohibited), which is a negative for pilots today. Why not encourage the practice of energy management to 1,000' AGL in VFR conditions to agree with industry stabilized approach criteria? Being fuel efficient and proficient in your craft can't be a bad thing.
13th Apr 2014, 18:21

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Being fuel efficient and proficient in your craft can't be a bad thing.

That would require actual flying skills...
14th Apr 2014, 02:57

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flyingchanges:

As an old guy in this business, I'm probably going to retire for the final time the end of this year. I know that there are young guys around, including my son, who feel the same about flying skills, as apparently you also do. Over emphasis on automation is killing the flying skills, which should exist to support the process when the automation fails or is unavailable. I'm lucky to be finishing my career with a talented group of flight test and research pilots who get it. They not only have confirmed the concept for me, they have been an inspiration.

Automation is a good thing if used properly within a defined CRM relationship between that automation and the pilot's skilled inputs and oversight. Unfortunately, we've headed in a direction that compromises the necessary piloting skills that are required to manage or rescue that automation should the time arise when the guy/gal in the seat needs to be a pilot again.

I hope there's enough folks around to preserve the concept and protect and preserve the title and meaning of "pilot", with and without the assistance of automation. Evolve and adapt, don't devolve.