Posts: 58
Joined: Thu May 27, 2004 8:51 am

Glide Ratio Of Airline Jets Compared To A Light Ac

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
Posts: 11243
Joined: Wed Jan 28, 2004 4:27 am

RE: Glide Ratio Of Airline Jets Compared To A Light AC

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.
Hey that guy with the private jet can bail us out! Why? HE CAN AFFORD IT!

nearord
Posts: 184
Joined: Thu Jun 17, 2004 11:16 am

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

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
Posts: 7865
Joined: Thu Jul 29, 1999 3:50 am

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

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]

PhilSquares
Posts: 3371
Joined: Sun Mar 28, 2004 6:06 pm

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

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.
Fly fast, live slow

N766UA
Posts: 7865
Joined: Thu Jul 29, 1999 3:50 am

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

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]

Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: Glide Ratio Of Airline Jets Compared To A Ligh

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.
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

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

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

Cheers,
QantasA332

bohica
Posts: 2379
Joined: Tue Feb 10, 2004 3:21 pm

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

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
Posts: 324
Joined: Thu May 20, 2004 9:09 am

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

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]
Thermodynamics and english units don't mix...

Areopagus
Posts: 1327
Joined: Thu Sep 20, 2001 12:31 pm

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

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.

Posts: 58
Joined: Thu May 27, 2004 8:51 am

RE: Glide Ratio Of Airline Jets Compared To A Ligh

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
Posts: 3371
Joined: Sun Mar 28, 2004 6:06 pm

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

My mistake, you are correct. On an average weight 400 it is 15:1 a heavier weight 400 it approaches 20:1
Fly fast, live slow

SailorOrion
Posts: 1960
Joined: Tue Feb 06, 2001 5:56 pm

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

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

SailorOrion

vzlet
Posts: 818
Joined: Thu Mar 11, 2004 10:34 am

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

Glide ratio doesn't vary with weight changes, but at a heavier weight a given ratio will be achieved at a higher speed.
"That's so stupid! If they're so secret, why are they out where everyone can see them?" - my kid

707CMF
Posts: 4698
Joined: Thu Mar 14, 2002 5:39 pm

RE: Glide Ratio Of Airline Jets Compared To A Ligh

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
Posts: 3371
Joined: Sun Mar 28, 2004 6:06 pm

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

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.

Fly fast, live slow

ben
Posts: 1369
Joined: Fri Aug 06, 1999 9:27 pm

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

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.

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos