# Can a 777, or 747 airliner, Glide? If so how far?

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.

Yes they can…see here .

Keith

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.

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.

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…

The Gimli Glider, a 767 (iirc) that glided to a landing back in the early 80s.

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.

There’s also the bizarre hijacking of Ethiopian Airlines Flight 961. 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.

Of course, if I would actually read the very first post, I would have seen what I mentioned was already covered.

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.

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).

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.

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.

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

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

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.

Perhaps you are thinking of Captain Al Haynes Who was the pilot of the United DC-10 that lost ALL hydraulics and crashed in Sioux City.

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.

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.

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…

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…