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View Full Version : Can a 777, or 747 airliner, Glide? If so how far?


Phlosphr
05-06-2003, 12:09 PM
Just wondering. If I am taking off from JFK and I am below 10K feet, I'd think the plane would drop like a sack of Lead. But I wonder about when we are up around 30K feet. Can those huge planes glide? If they can't how do they land on the water? example, the safety movies before flight.

Odieman
05-06-2003, 12:15 PM
Yes they can....see here (http://www.wadenelson.com/gimli.html) .

Keith

Padeye
05-06-2003, 12:16 PM
Virtually any plane that can fly can glide. Of course the kinetic energy to overcome drag and move forward has to come from somewhere so without an engine it comes from the potential energy of altitude. The ratio of altitude lost to distance flown is called glide ratio. Of course a big heavy airliner will have a poorer glide ratio than a purpose made glider but it won't fall out of the sky unless the pilot does something stupid. Something stupid might be trying to maintain altitude. It can be maintained for a bit but at the expense of airspeed. When airspeed falls too low for a given angle of attack the wings stall, lose lift, then the plane falls. To a certain extent this is the correct procedure on landing. The rate of descent of a steady state glide may be too fast for a safe landing so the plilt will lift the nose or flare to slow the rate of descent and airspeed for a safe landing.

Grey
05-06-2003, 12:33 PM
There was an Air Transat flight (236?) that glided for about 180 km to land in the Azores. I would imaging a 7x7 would do just as well.

Phlosphr
05-06-2003, 12:43 PM
Padeye - if you are in Phoenix right now you may understand this.

I took lessons down at Estrella Jump school south of Chandler. Those gliders have what, 75 foot wings? I never got a solo flight in but the tandam's I flew were very fun. We stayed up for quite a while, and I remember the instructor - this was in 94' - saying something to the effect that a large airliner would have difficulty in the lower altitudes...

bernse
05-06-2003, 12:43 PM
The Gimli Glider, a 767 (iirc) that glided to a landing back in the early 80s.
http://www.wadenelson.com/gimli.html

Shagnasty
05-06-2003, 12:51 PM
A 747 has a glide ratio of about 17:1. This means that for every 1 foot loses in altitude, it can travel 17 feet forward. If a 747 loses all 4 engines at 35,000 feet, it will be able to glide over 110 miles before landing (or crashing). Provided that there is an airport in this range, the pilot could probably set it up for a safe landing.

Terminus Est
05-06-2003, 01:14 PM
There's also the bizarre hijacking of Ethiopian Airlines Flight 961 (http://www.cnn.com/WORLD/9611/24/ethiopia.hijack/). The hijackers wanted to go to Australia, but would not let the plane (a 767) refuel. The plane eventually ran out of fuel and ditched off the Comoro Islands.

bernse
05-06-2003, 01:15 PM
Of course, if I would actually read the very first post, I would have seen what I mentioned was already covered.

Xema
05-06-2003, 01:43 PM
Originally posted by Phlosphr
If I am taking off from JFK and I am below 10K feet, I'd think the plane would drop like a sack of Lead.

It's a popular misconception that "the engines make it fly" so if you lose power you drop like a stone. In fact, airliners are rather good gliders -- a typical best glide ratio might be 18:1. The reason is not really that the designers are contemplating engine-out landings, but simply because a "clean" airplane is more efficient.

But I wonder about when we are up around 30K feet.

It would not be much different from lower altitudes. The best glide ratio doesn't change much with altitude (though the true airspeed at which it happens increases).

Rayne Man
05-06-2003, 02:18 PM
A few years ago a British Airways 747 flew through a volcanic dust cloud in the Pacific which stopped all four engines. The plane then glided down from nearly 40,000 feet down to 10,000 feet before they could re-light the engines. It then landed safely even though the windshield had been sand blasted by the same dust cloud.

Padeye
05-06-2003, 02:21 PM
Originally posted by Phlosphr
Padeye - if you are in Phoenix right now you may understand this.

I took lessons down at Estrella Jump school south of Chandler. Those gliders have what, 75 foot wings? I never got a solo flight in but the tandam's I flew were very fun. We stayed up for quite a while, and I remember the instructor - this was in 94' - saying something to the effect that a large airliner would have difficulty in the lower altitudes...

I am in Phoenix but not sure why that would help me understand.

Anyway, the statement is a pretty vague one. Does he mean if an airliner lost engines at low altitude? Did he mean AGL or sea level altitude? I don't think anyone is saying it would be a cakewalk gliding a loaded 777 compared to a lightweight glider with high aspect ratio, low drag wings but that nothing really prevents it from gliding as long as it's within its flight envelope.

pipper
05-06-2003, 03:38 PM
Hmmm- Xema are you correct in saying that the glide ratio doesn't change with altitude? The glide ratio is a result of, among other things, lift generated by the wings, right? And I know that at some airports (Phoenix, for example) when it gets really hot, the air becomes less dense and some of the larger planes have difficulty in taking off (from the shorter runways at least). So am I correct in putting these two items together to say that lift (assuming the same speed) decreases with altitude and that the glide ratio should also decrease with altitude

Broomstick
05-06-2003, 06:39 PM
In general, denser air makes the airplane fly better. You can generate more lift at a slower true air speed* in dense air. This has a lot to do with why it takes my Cessna or Piper twice as much runway to get airborne on a 90 F degree day as opposed to a 10 F degree day at the same airport.

So it baffles me that folks would say an airliner would have more difficulties gliding at a lower altitude than a higher one. Now, manuvering a big airliner in tight quarters - such as low to the ground - could be difficult but the wing should be just as happy slicing through thick air as thin.

With the stated 17:1 or 18:1 glide ratios of the big airliners they actually do glide pretty well. Much better than the small airplanes I fly, which range from 7:1 to maybe 9:1 glide ratios.

The big difference comes in the actual landing. A 7x7 has to maintain an airspeed of well over 100 mph (I'm not sure of the exact figure, and it would also depend on whether you could deploy things like flaps or not). Let's say 150 mph is required to make the wing generate enough lift to keep it in the air under control. It's also huge. If you're without power you have to make hundreds of tons of stuff moving at over 150 mph touch the ground gently - on the first try.

Now, my dinky Cessnas/Pipers/ultralights, which weigh 2400 lbs or less (in some cases 1600 lbs, or even as low as 800 lbs) touch down at under 50 mph in an emergency landing. That's more like a car accident than a building falling over (a 7x7 easily weighs as much as building). Which has a lot to do with why small aircraft, whether powered or pure gliders, are easier to land successfully (that is, the humans aboard survive or even walk away) without power.

Another factor in the landing is the surface vs. the weight of the aircraft. A Cessna 150 can land in backyard without damage(Unfortunately, I actually got an opportunity to demonstrate this in real life, once upon a time). Land a 7x7 in a field and it will sink under it's own weight, after which the landing gear rips off as the rest of the airplane keeps going, plowing a furrow as it goes.

But, problems with landing aside, a big airliner - 777, 747, whatever - glides just fine, provide you have a competant pilot at the controls.

* The whole topic of speed in flight can get quite complicated, what with ground speed, true airspeed, indicated airspeed, calibrated airspeed.... I am not discussing this under the assumption the detail is not required for the question at hand, but did want to let the nitpickers know that yes, I do understand there are some details and complications

MLS
05-06-2003, 07:03 PM
Wasn't there a case a few years ago where a pilot brought down a commercial airliner in a field, mostly successfully, considering everything? I think it was in Ohio, but am probably wrong. The plane and the ground were pretty messed up, and I think there were some passenger deaths, but not as many by far as there would have been if he had not been so skillful. He was a very experienced pilot, and not too long after that had to retire due to the airline's (or the FAA's?) mandatory retirement policy. Sorry this is so vague; maybe somebody else recalls more details.

Rick
05-06-2003, 09:13 PM
Perhaps you are thinking of Captain Al Haynes (http://www.snowcrest.net/marnells/haynes.htm) Who was the pilot of the United DC-10 that lost ALL hydraulics and crashed in Sioux City.

hobbes730
05-06-2003, 09:37 PM
I think most gliders have something like a 30:1 glide ratio, which isn't terribly better than the 17:1 ratio of big jets. All other things being equal, it seems a 747 wouldn't have to make a much harder landing than a glider.

Terminus Est
05-06-2003, 10:12 PM
In the Sioux City accident, the plane still had engine power. That is, in fact, how they controlled the plane: by adjusting the throttle to the left or right engine. A superb bit of flying.

Flying_Monk
05-06-2003, 10:53 PM
Just for the record, my hang glider has a glide ratio of 10.5:1 and the best rigid-wing hang gliders get around 17:1...of course, my sink rate is much better than a 747's....

Flying_Monk
05-06-2003, 11:00 PM
Just for the record, my hang glider has a glide ratio of 10.5:1 and the best rigid-wing hang gliders get around 17:1...of course, my sink rate is much better than a 747's....

Xema
05-07-2003, 12:15 PM
Originally posted by pipper
are you correct in saying that the glide ratio doesn't change with altitude? The glide ratio is a result of, among other things, lift generated by the wings, right? And I know that at some airports (Phoenix, for example) when it gets really hot, the air becomes less dense and some of the larger planes have difficulty in taking off (from the shorter runways at least). So am I correct in putting these two items together to say that lift (assuming the same speed) decreases with altitude and that the glide ratio should also decrease with altitude

The explanation could get rather technical, but the simple way to say it is that the glide ratio is the ratio of lift to drag, and when air is less dense, both lift and drag decrease, in the same way (and for the same reason -- fewer air molecules per cubic meter). So the glide ratio changes very little with altitude.

It would be slightly more accurate to say that if you want the same lift -- usually, the weight of the aircraft -- then when the air is less dense you must fly at a higher true speed to get it, other things being equal. And when you fly faster, drag increases back to what it was when lower and slower, generating the same lift. A consequence of having to fly at a higher true speed is the need for longer runways at high/hot locations, as you note.

It's incorrect to say that glide ratio doesn't change at all with air density, because of effects like changing Reynolds numbers. But those are minor.

Xema
05-07-2003, 12:32 PM
Originally posted by Broomstick
The big difference comes in the actual landing. A 7x7 has to maintain an airspeed of well over 100 mph (I'm not sure of the exact figure, and it would also depend on whether you could deploy things like flaps or not). Let's say 150 mph is required to make the wing generate enough lift to keep it in the air under control. It's also huge. If you're without power you have to make hundreds of tons of stuff moving at over 150 mph touch the ground gently - on the first try.

I think "over 150 mph" is about right for the touchdown speed of a 7x7.

Assuming that without power you can cause the aircraft to arrive at the right spot, the actual touchdown should not be much different with or without engines. Large aircraft have emergency systems to power their hydraulics in case of engine failure, so the controls (ailerons, elevators, flaps) should work about as normal. The plane is certainly huge, but its control surfaces are sized accordingly -- a near-normal touchdown should be possible.

You are quite correct that for any hope of a decent landing, a big aircraft requires a runway (long enough and rated for the aircraft's weight).

I'm impressed that you were able to land a C-150 in a "backyard" -- I suspect that few power pilots could do this on the first try (it was your first try, wasn't it?).

Xema
05-07-2003, 12:37 PM
Originally posted by hobbes730
I think most gliders have something like a 30:1 glide ratio

About right. The range is from around 18:1 (60-year-old open-cockpit antiques) to 70:1 (The Eta, a 101-ft-wingspan super-exotic). Most modern designs are around 40 - 45:1.

Broomstick
05-07-2003, 08:56 PM
Originally posted by Xema
Large aircraft have emergency systems to power their hydraulics in case of engine failure, so the controls (ailerons, elevators, flaps) should work about as normal. The plane is certainly huge, but its control surfaces are sized accordingly -- a near-normal touchdown should be possible.Depends on the circumstances. The Gimli Glider, for instance, was flown to a landing in a slip - apparently there wasn't enough power to deploy everything and they decided getting the landing gear down took precedence over the flaps.

The size required for control surfaces is, in part, dependent on speed. A C150 has a huge rudder in relation to the rest of it when compared to the rudder and fuselage of a 7x7. Sure, the Boeing has a bigger asolute rudder, but porportionally the Boeing rudder is smaller. The higher the airspeed the less control surface needed. The only problem is if you have a big, high speed airplane that utilizes various deployable devices for (relatively) slow-speed control but can't deploy them. This requires the pilot to land at a higher speed than normal. Which can complicate things considerably.

Weight is a factor - a landing C150 and a landing 7x7 may both have a sink rate of 500 feet per minute, but fer darn sure the Boeing is going to thump the ground harder due to momentum= mass x speed. Which is why they have those big honkin' landing gear, of course.

I'm impressed that you were able to land a C-150 in a "backyard" -- I suspect that few power pilots could do this on the first try (it was your first try, wasn't it?).On that afternoon? Yes, it was my first try - and I almost didn't get that one. And certainly my first in a Cessna. (So far have not had that honor in a Piper)

It was not, however, my first off-field landing. Back in the my ultralight days we used to do that sort of thing for fun (we knew the fields we were landing in - they weren't randomly chosen). So when the Big Moment came I knew what I getting into and there weren't too many surprises, even if I was in an aircraft twice as fast and three times heavier than what I had attempted that sort of thing in before.

I should probably mention that that "backyard" landing (actually, a small hayfield) was under power - it was weather forcing me down, not lack of power. A bit of different circumstance, really, but it does make the point that you can land an airplane in places you wouldn't expect to be able to. In fact, that's what the property owner said to me - "I wouldn't have guessed you could land an airplane there." Truth is, I had my doubts, too, but I figured slamming into a garage had a higher likelihood of me walking away than spinning into the ground under IMC spatial disorientation so there ya go. In the end - no damage to me, the airplane, or anything else (except my pride).

It did delay my private pilot checkride a bit, though, and I got a talking to by the Authorities, but in the end they just seemed happy not to have to do any paperwork on the matter.

I'm a lot more respectful of the weather since then.

jetset707
03-24-2014, 02:54 PM
Just an aside speaking of the Air Canada 767 that ran out of fuel due to a miscalculation by the FO...The 767 has a wind driven turbine that can be deployed and it provides enough power to assist with the flight controls and power cockpit displays.

Remember as in the Sioux city etc... the DC-10, 747, 777 the actual control surfaces are in fact very large and require a lot of force to be manually handled handled thus the hydraulics...
The answer to the question is they glide just fine.
You are correct that temperature is a big factor. a hot dry day like you said, takes a lot of runway to get off the ground. Aircraft fly best at cooler temperatures when the air is more dense. The only difference altitude makes is less time available to find a place to land I.E. USAirways landing in the Hudson.
I think you're right the 777 is what like 17 to 1
There are lots of examples of aircraft gliding just fine...A great one is the Space Shuttle, Not quite as large as a 747 or 777 but, heavy and quite a good glider.

Cheers !

jetset707
03-24-2014, 03:03 PM
The only drawback of the ram air turbine is that it will slow down as the aircraft slows...This can make the aircraft harder to control as less power is provided.
Also the deployment of flaps and or gear will slow the aircraft and make it harder to control.

Musicat
03-24-2014, 03:09 PM
jetset707, you may not have noticed that this thread has been gliding since 2003. Just your luck, though -- most posters are still around, but not the OP, who may have crashed.

the_diego
03-24-2014, 06:32 PM
That Reader's Digest story of a 747 whose four engines turned off because it flew right into the ash column of an erupting volcano over Indonesia ("Mayday, we've lost all four engines!") The pilot reckoned he can glide 175 miles from an altitude of 40,000 feet. That's 22 miles for every 1 mile of altitude lost, about the same performance as a well-trimmed sporting glider.

nevadaexile
03-24-2014, 06:40 PM
The problem with "gliding" is that when the fuel runs out the engines don't shut down at the same time. One does, and then the other. Unless the plane was being controlled or the engines were shut down by the pilots, the plane would bank in the direction of the engine which is off and then crash land in that direction.

Lord Mondegreen
03-24-2014, 06:54 PM
That Reader's Digest story of a 747 whose four engines turned off because it flew right into the ash column of an erupting volcano over Indonesia ("Mayday, we've lost all four engines!") The pilot reckoned he can glide 175 miles from an altitude of 40,000 feet. That's 22 miles for every 1 mile of altitude lost, about the same performance as a well-trimmed sporting glider.

BA 9 (http://en.wikipedia.org/wiki/British_Airways_Flight_9). Captain Moody delivered one of the coolest lines in history, "Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress."

the_diego
03-24-2014, 06:59 PM
^
It's the old Chuck Yeager homespun style, best reserved for dire emergencies.