Glide ratio

[West Coast]

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.

[john_tullamarine]

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.

[island_airphoto]

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.

[Capn Bloggs]

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

[West Coast]

The 15-17:1 value falls in line with the values I'm finding as well.

[bigduke6]

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

But straight turbojet should be a bit higher that modern turbofan......

[PBY]

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

[john_tullamarine]

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.

[sierra_mike]

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

[West Coast]

John is correct, but all input is appreciated.

15:1 at green dot. Does green dot represent best L/D, or close to it?

[tdracer]

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).

[misd-agin]

B767-200 17.9:1 AFM had it.


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).

[henra]

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.

[john_tullamarine]

Aerodynamically the correct glide Ratio is sink rate/IAS

Might we ask you to amplify this a little .. ?

[henra]

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!

[Desert185]

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.

[West Coast]

I'm afraid SOP and industry practice won't allow power off descent to touchdown anymore.

[Desert185]

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.

[flyingchanges]

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


That would require actual flying skills...

[Desert185]

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.

[Mozella]

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.

Explain please.

All glider pilots know that best no-wind gliding range is achieved at best L/D and the range is indipendent of weight. Counter to intuition, the heaver aircraft will glide the same distance, but it will do so at a higher speed. That's why racing sailplanes load up with water ballast. They can glide the same distance at higher speeds.

It's not immediately obvious to me why a Classic 747, unrestricted by ATC considerations, would obey a different set of aerodynamic laws. Perhaps you're flying the heavy version at best L/D (or more likely at some speed above best L/D) and comparing that to the light version being flown at the same high speed; i.e. even further above the best L/D for the light version. That would indeed cause the light aircraft to glide a shorter distance. Could that be it?

[Brian Abraham]

Mozella, the reason is they fly a fixed Mach/Speed schedule irrespective of weight, they have no interest in the L/D.

[Owain Glyndwr]

So while sinking at the same rate per second, the distance travelled will be greater. The 'apparent' glide Ratio will be greater, while L/D still remained the same

But the sink rate is also proportional to TAS so the glide angle remains the same at 1/(L/D) no matter what the altitude.

[ATC Watcher]

As a glider pilot allow me a few remarks : pure Gliding ratios ( or Finesse max as we call it) in modern gliders can reach high levels but the most important other factor is the minimum sink rate and at which speed range it can keep that , That what is interesting, not the gliding ratio per se.
For info the ASH 25 ( which I sometimes fly) with winglets and 26m wingspan has a finesse of 62 with 0,45m/s and will keep a good sink rate through a large speed range. You will never get that sort of perfection in a jetliner because it is build for speed , not gliding.

A modern jet liner (e.g.B767, A330 B787 ) have "official" gliding ratios around or slighly above 20:1 but only at a very specific speed , outside of that exact speed, they will probably start dropping like stones, (i.e close to 10.1) . If I remember correctly the Gimli B767 got something like 12:1 with a Glider pilot at the commands.

[Mozella]

Mozella, the reason is they fly a fixed Mach/Speed schedule irrespective of weight, they have no interest in the L/D.

"They" used to be "me" before I retired. I'm quite familiar with flying airliners having done so for 33 years. I've also done my fair share of sailplane racing. But fixed mach/speed procedures were not part of the discussion. At least I didn't think so. The post said, "............planning a min fuel burn descent without ATC interference."

It seemed to me that the situation involved stretching the glide using minimum power (since the head office, not to mention the passengers, get all upset if you shut all four down). That would be done the same way in a 747 as it would be in a sailplane; i.e. best glide ratio or best L/D. That's how you go the most distance for a given altitude, ignoring wind.

Of course, airliners have other pressing interests, cost being a big factor. That's why all speeds, climb, cruise, and decent, are faster than optimum from an aerodynamic standpoint. Each time the flight attendants got a raise we flew faster and each time the pilots took a pay cut, we flew slower as directed by the company bean counters. :ugh:

Cost also explains why we didn't make decents at best L/D. It's simply too slow. But sometimes I flew slower even than that when, for example, I got instructions to hold at a fix for an extended period of time. In that case I immediately slowed to max endurance speed (minimum sink speed for glider pilots) and sauntered toward the holding fix at which time I accelerated a bit to holding speed. Efficient flying isn't hard, but you can't save fuel and/or time by being lazy which explains why some pilots raced toward the holding fix.

In our company, we didn't ever fly a fixed Mach/Speed schedule. Nearly every aspect of selecting cruse, climb, and decent speeds (and the associated power settings) took into consideration such things as the aforementioned cost, aircraft weight, wind, our schedule, leg length, other airlines published schedules, and even maintainance items which wore out based on time in service.

But I got the idea the theoretical 747 example was free of such constraints as well as ATC restrictions, but I could have misunderstood what was being said. That's why I asked.

[henra]

But the sink rate is also proportional to TAS so the glide angle remains the same at 1/(L/D) no matter what the altitude.

Ouch. :}
Serious Blond moment on my side. :O
Obviously the difference between IAS and TAS does not only apply in the horizontal plane.
Proving the point that with wrong assumptions taken as a given (constant descent rate) one can re-define physics. Not. :{

[Desert185]

Mozella

But I got the idea the theoretical 747 example was free of such constraints as well as ATC restrictions, but I could have misunderstood what was being said. That's why I asked.

Not theoretical...real world. No speed brake, power off descent compensating for wind, weight and airport arrival pattern, adding power at ~1,000' AGL, if left alone by ATC, which obviously wasn't always the case.

If flying an airplane where the variety of broad landing weights was not possible, then the weight factor for descent planning was/is negligible and unnoticeable.

To be clear, at same Mach/airspeed descent schedule: heavier weight...longer glide. Lighter weight...shorter glide.

[Mozella]

Not theoretical...real world. No speed brake, power off descent compensating for wind, weight and airport arrival pattern, adding power at ~1,000' AGL, if left alone by ATC, which obviously wasn't always the case. .......... snip .........
To be clear, at same Mach/airspeed descent schedule: heavier weight...longer glide. Lighter weight...shorter glide.

OK, that makes sense. I took the post to mean a minimum fuel decent which involves operating at best L/D. That's why it didn't make sense. Now I see the examples were idle decents using the same speeds at widely varying weights; quite different from a minimum fuel decent.

For reasons discussed earlier, such things are never done in the airline business except (perhaps during a fuel emergency) since airliners usually have sufficient fuel and not much extra time or money. However, back when I was one third my age and one half my weight I flew fighters for the U.S. Navy where we used a best L//D decent frequently. We had plenty of time and money but we were always out of fuel. :ok:

[john_tullamarine]

back when I was one third my age and one half my weight

.. now, that evokes whimsical memories for most of us older folks ... or was it half my age and a third of my weight ? .. whichever ... :)

[Brian Abraham]

To be clear, at same Mach/airspeed descent schedule: heavier weight...longer glide. Lighter weight...shorter glide.I've read that an emergency descent in a light weight 747 is somewhat eye watering with respect to the nose down attitude. True??

[LeadSled]

Brian,
True, and, of course, something like 15 or more degrees nose down appears far steeper than it really is.

[sierra_mike]

15:1 at green dot. Does green dot represent best L/D, or close to it?

yes, green dot corresponds to speed with best L/D

[Hobo]

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

Slight thread drift, but doesn't 'glide' mean no thrust, ie all engines out?

I agree with John_tullamarine I have seen 2nm/1000 feet work to perfection several times (in the (real) simulator :O) with both engines out on various series of 737.

[john_tullamarine]

I have seen 2nm/1000 feet work to perfection several times (in the (real) simulator :O) on various series of 737.

Indeed .. on the 733 it was an infrequent, but regular, end play to be left very high by ATC, or one chose to be very high due wind related turbulence until entering the circuit (over CNS at 10,000ft was typical with a strong westerly) .. 25/gear/idle/limit speed and somewhat better than 2/1000 was achievable .. however, the mental arithmetic machine was running hot all the way down ... let it get away ever so slightly and there was nothing left in the toolkit to claw it back. Naturally one had briefed the passengers on PA prior ..

SOP proscription prevented our investigating 40 in lieu of 25.

Great satisfaction, though, when it all worked out well .. and, of course, one nonchalantly would give the impression that it was all a routine walk in the park ...

However, I guess this sideline is not what the OP was after ..

Mind you, the Electra at 2/1000 still needed a bit of power to avoid going low .. but that is an another tale ..

[latetonite]

John, you forgot the electra had no wings..

[misd-agin]

Sim demonstrations - dual engine failure at 5000'/250kts/20 DME and 10,000'/250kts/35-40 DME.

Establish L/D speed and glide to the airport.

18:1 would give a range of 15 nm at 5000' and 30 nm at 10,000'.

Trading airspeed and flap extension at the latest possible moment allows you to reach the runway.

2nm/1000' would have you at 10 DME at 5000' or 20 DME at 10,000'. That's more than enough energy and no where near the max capability of the a/c.

[pattern_is_full]

Doesn't GROUND SPEED play a role in figuring glide ratio?

I get that it plays no role in best L/D speed and such, but if you want to figure out whether you can make the Azores or whatever from 37,000 feet, don't you need to know if you have a tail or headwind component?

Seems to me TAS is irrelevant. IAS tells you how to fly for the best range/glide ratio possible, and GS tells you what that ratio will be (and whether it will be enough!)