jadedmonkeys
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Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 12:23 pm

I was reading this article on Netscape about a pilot who lost his job and wrote a book called "Ask the Pilot: Everything You Need to Know About Air Travel." Anyway one of the questions is:

Q. If a large commercial jet loses total engine power, can it glide to a landing, or is it all over?
A. Complete engine loss "is about as probable as a flight attendant volunteering to give you a shoe shine." He says that a large jet that has no power actually performs better than a light Piper or Cessna without power since it has a glide ratio that is almost double that of small planes.

I know the glide ratio for the Cessna 172 I fly is 9:1 which is for every 1,000 feet of altitude I will cover about 9,000 feet which is 1.7 miles. so let's say a 737 has a glide ratio of almost double 9:1 like 17:1, if it's 10,000 feet in the air and 32 miles away from LAX, it will actually glide right onto a runway, I find that hard to believe. Are there any pilots out there who can confirm the glide ratio of any commercial jet?

BTW I got that calculation by 10,000X17/5280
 
NIKV69
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RE: Glide Ratio Of Airline Jets Compared To A Light AC

Tue Jun 22, 2004 12:30 pm

This argument comes up a lot with my friends. Commercial airliners need thrust to create lift on the wings. The air flow over the wings lifts the aircraft in the sky. I feel a commercial airliner would stall after if lost power. Also if a commercial airliner did lose all engines and was gliding to a safe landing what would happen if it needed to maneuver while it came in? Sounds to me that a pilot that lost all engines would have to get it down fast and hope he didn't have to do much.
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nearord
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 12:34 pm

You could find out by searching for the air canada stories from the 90's about the 767 that ran out of gas cause the fuel guys forgot to do the conversion. It glided to safety at an abandoned air force runway that was having a car race at the time. I don't really know the details, but I'm sure there were lots of stories about it.

Btw, that sounds about correct at between 16-20:1 for the larger jets. It wouldn't stall as long as the nose start pitching down and altitude is traded for airspeed. Most airliners have a best glide speed of about 200-230 kts and normally cruise a lot faster than that.

[Edited 2004-06-22 05:38:40]
 
N766UA
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 12:34 pm

There have been at least 2 or 3 large jets that have glided to safety. I think they have a glide ratio is like 12:1 or something like that. One was a 767 and the other I know of an A330.

[Edited 2004-06-22 05:35:19]
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PhilSquares
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 12:41 pm

A typical commercial airliner will glide at a ratio of about 3:1. So if I were at 10000 feet and lost all my engines, I could expect to glide about 30 miles.

On the aircraft I fly, the 744, at high gross weights, the ratio approaches almost 4:1, the aircraft doesn't want to come down at all.

There is another problem in that on the two engine aircraft, you have a RAT which when extended will give hydraulic and or electrical. On most two engine aircraft the APU can be started in flight so you have another power source. On the 747/747-400, the apu can't be started in flight, although I would sure try, but there is no RAT either. So, you could glide some distance, but as you slowed down the engines would windmill at a slower speed. The flight controls are all hydraulic and the volume of fluid the pump puts out at low engine speeds would probably be insufficient to control the aircraft.

Likely scenario, I don't think so. My guess is, just like the BA and KLM aircraft that encountered volcanic ash, is you'd get at least one engine restarted.
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N766UA
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 12:43 pm

A clean 727-200 has a glide ratio of 17:1. A clean MD-80 has a glide ratio of 28:1. A clean 747-200 has a glide ratio of 17:1 as well.

http://www.gte.se/userfiles/129/GTE_Economical_Paper.pdf

[Edited 2004-06-22 05:44:40]
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Ralgha
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RE: Glide Ratio Of Airline Jets Compared To A Ligh

Tue Jun 22, 2004 12:43 pm

This argument comes up a lot with my friends. Commercial airliners need thrust to create lift on the wings. The air flow over the wings lifts the aircraft in the sky. I feel a commercial airliner would stall after if lost power. Also if a commercial airliner did lose all engines and was gliding to a safe landing what would happen if it needed to maneuver while it came in? Sounds to me that a pilot that lost all engines would have to get it down fast and hope he didn't have to do much.

You are dead, dead wrong. Airliners, just like any airplane, do not need thrust to create lift, they need angle of attack (not too much), and airspeed. If you lose power, then you sacrifice altitude to keep airspeed, i.e., you GLIDE.

Airliners are generally much more streamlined than many GA propellor airplanes. They also don't have the windmilling prop after the engine dies, which is the equivilant of putting a solid disk on the front of the airplane as far as drag goes. When you feather a prop, you can feel the drag go away and the airplane goes a lot farther. Even if the prop doesn't feather (most single engine variable pitch airplanes), you can still feel the drag drop dramatically when you pull the prop control all the way out.

Most people (even pilots) are surprised at how far an airplane will glide. I pulled the engine on one of my students and suggested we try for the airport over there. He said no way we could make that. I told him to give it a shot. Guess what, we made it easily, even had to throw out the flaps so we didn't overshoot.
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QantasA332
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 12:55 pm

It all depends on the L/D (lift to drag) ratio of the aircraft in question...

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bohica
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 12:59 pm

There was an Air Transat flight which lost all power after a fuel leak over the Atlantic a few years ago. It managed to land in the Azores. There was a program on TV about it recently but I can't remember which network it was on.
 
Vorticity
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 1:16 pm

Tan (glide_angle) = 1 (Lift/Drag)

easily enough ...

Tan (glide_angle)min = 1 (Lift/Drag)max

The velocity for L/D max varies with altitude, but you can continue to derive basic performance equations and come up with the velocity needed for best glide slope at any altitude. A Gulfstream IV can make 82 miles by performance equations, at an angle of 3.964 degrees from 30,000 ft. Ideal velocity varies from 374 knots at 30,000 ft to 229 knots at sea level.

[Edited 2004-06-22 06:16:56]
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Areopagus
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 2:13 pm

Philsquares: A typical commercial airliner will glide at a ratio of about 3:1. So if I were at 10000 feet and lost all my engines, I could expect to glide about 30 miles.

I think you mistyped. 10000 feet altitude is almost 2 miles, so if you glide about 30 miles, you have a 15:1 glide ratio.

 
jadedmonkeys
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RE: Glide Ratio Of Airline Jets Compared To A Ligh

Tue Jun 22, 2004 2:31 pm

I knew that they glided because if they didn't then they probably wouldn't even fly, but i didn't realize they really did have a glide ratio better than a light single engine airplane. 28:1 on an MD-80? WOW, that's really good. That means if I calculated correctly that if an MD-80 lost all power at FL 350 they would glide for 185 miles! That means if the aircraft is at FL 380 it can glide from Las Vegas to Los Angeles.
 
PhilSquares
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 4:38 pm

My mistake, you are correct. On an average weight 400 it is 15:1 a heavier weight 400 it approaches 20:1
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SailorOrion
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 6:22 pm

Most commercial airlines are at around 15:1 to 20:1 these days.

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vzlet
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 8:25 pm

Glide ratio doesn't vary with weight changes, but at a heavier weight a given ratio will be achieved at a higher speed.
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707CMF
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RE: Glide Ratio Of Airline Jets Compared To A Ligh

Tue Jun 22, 2004 9:37 pm

A typical commercial airliner will glide at a ratio of about 3:1. So if I were at 10000 feet and lost all my engines, I could expect to glide about 30 miles.

On the aircraft I fly, the 744, at high gross weights, the ratio approaches almost 4:1, the aircraft doesn't want to come down at all.


And you are a 747 captain ?
Wow.

I am but a lowly kilofox pilot (small prop), but let me remind you something : glide ratio has next to nothing to do with the load. An aircraft at full load will have roughly the same glide ratio as an empty one. First point.

Second point, if you think a typical commercial airliner has a 3:1 ratio. may I ask you when was you last simulator checkride ? Heck, a 3:1 ratio is the ratio of a parachute ! A commercial airliner's ratio will be somewhere between 15 and 20. Nowhere near 4 (fortunately).

That's your story and you stick to it, but heck, just by that post you really sound like an armchair captain.

cheers,

707
 
PhilSquares
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Tue Jun 22, 2004 9:54 pm

707cfm,

Pardon moi, I guess it's very nice to be in a position where you've never made a mistake. I have already apologized for my inadvertent math error.

I have a deal for you, send me your email address, and I will be more than happy to provide you with a copy of my airline ID, License and airline email address. Other than that there is nothing I can do. I suppose I could add the last 90 days flying from the company's computer.

I also realize that as you say, weight has nothing to do with glide ratio, or your "first point" however, I suggest you try descending in a -400 that weighs 300 tonnes vs. one that weighs 220 tonnes. You will find the heavier aircraft takes more distance to descend.


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ben
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RE: Glide Ratio Of Airline Jets Compared To A Light Ac

Wed Jun 23, 2004 12:39 am


707cmf, That was a typo by Philsquares. You probably hadn't seen the correction (reply 12) before you posted your message....


Thanks for all the information, Phil.

You said:

You will find the heavier aircraft takes more distance to descend.

OK just to clarify that point. Are you saying that the aerodynamics are actually better at a higher speed? I would guess that is the case but have not reached that subject in my studies/exams yet. It comes up towards the end I think.

It is virtually impossible to tell a 'lay person' about the relationship between weight, airspeed and glide range. Yes, a 300t aircraft does glide better than a Cessna.. they think you're weird.