What's the terminal velocity of a Boeing 737 in free fall from 12.496km

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Say that all the engines of a boeing 737 failed while it was 12.496km in the air and fell into freefall, what would its terminal velocity be and how long would it take to hit the ground
 

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  • #2
berkeman
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Welcome to the PF. :smile:
Summary:: what's the terminal velocity of a boeing 737 in free fall after the engines have failed, from 12.496km, and how long would it take to hit the ground?

Say that all the engines of a boeing 737 failed while it was 12.496km in the air and fell into freefall, what would its terminal velocity be and how long would it take to hit the ground
What do you mean by "freefall"? Flat spin? Nose down streamlined? Tumbling? What is your motivation for asking this question?
 
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  • #3
Vanadium 50
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And the oddly specific height?
 
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  • #4
Nugatory
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what's the terminal velocity of a boeing 737 in free fall
There’s no such thing - “free fall” and “terminal velocity” are mutually exclusive.

I expect that you actually had a different question in mind, namely what speed will the aircraft reach if placed in an unpowered vertical dive. I would not surprised to find that the airframe breaks up before that terminal speed is reached.
 
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  • #5
Filip Larsen
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And just in case the OP thinks that its the engines that keeps an aircraft flying, let me add that it is the wings (the lift generating device in general) that does that, that it is the wings that allows an aircraft to maintaining forward speed while not gaining too much downwards speed. The engines are simply put just there to provide convenient acceleration and overcome drag.

As an example, gliders are a type of aircraft that can fly completely without engines using uprising natural winds to maintain or gain altitude, capable of traveling thousand of kilometers conditions permitting.

Also, its not uncommon that even passenger jets, which are normally supposed to operate with their engines on at all times, can be glided to a (more or less) safe landing in case of engine outage.
 
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  • #6
russ_watters
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Say that all the engines of a boeing 737 failed while it was 12.496km in the air and fell into freefall, what would its terminal velocity be and how long would it take to hit the ground
The scenario isn't just odd, it is poorly defined and not physically realistic. Just having engines fail doesn't cause an airplane to fall in freefall, and the descent rate depends entirely on the orientation. E.G., is it gliding, diving or stalled?

Also, its not uncommon that even passenger jets, which are normally supposed to operate with their engines on at all times, can be glided to a (more or less) safe landing in case of engine outage.
And on the other side of the coin is Air France 447, which stalled, fell out of the sky and crashed with little forward speed and the engines at full throttle.

Air France 447 was an Airbus A330, a plane in about the same class as the 737. It fell in a powered stall from 38,000 feet (11.6 km) in about 4 minutes. That's an average vertical speed of 108 mph or about the terminal velocity of a skydiver.
 
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  • #7
A.T.
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I would not surprised to find that the airframe breaks up before that terminal speed is reached.
Especially the 737.
 
  • #8
DrStupid
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And the oddly specific height?

Its just 41000 feet converted to km with improper truncation. That fits quite well to the overall quality of the question.
 
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  • #9
boneh3ad
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41,000 feet happens to be the ceiling for late-model 737 aircraft, so I suspect that is where the oddly specific height came from. The rest... who knows.
 
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  • #10
fresh_42
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Say that all the engines of a boeing 737 failed while it was 12.496km in the air
This is inevitable. However, I would be interested in the rocket that put the 36 tons up there. That's 50% above what a space shuttle could launch.
 
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  • #11
pbuk
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This is inevitable.
I think you have messed up a conversion somewhere. As @boneh3ad says, 12,497m = 41,000 feet which is the service ceiling for a (modern) 737.
 
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  • #12
fresh_42
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I think you have messed up a conversion somewhere. As @boneh3ad says, 12,497m = 41,000 feet which is the service ceiling for a (modern) 737.
Yes, I got lost in translation, sorry. We use points to separate the three digit packages and commas as decimal separator.
 
  • #13
pbuk
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Here's what happened when both engines failed on a 737-3T0 at about 16,500 feet during descent towards New Orleans in 1998. TL;DR it landed safely, no injuries.
 
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  • #14
pbuk
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Yes, I got lost in translation, sorry. We use points to separate the three digit packages and commas as decimal separator.
Not in aviation we don't, such errors would be... unfortunate :biggrin:. Even a Lufthansa flight at that altitude would report flight level (FL) 410.
 
  • #15
fresh_42
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Not in aviation we don't, such errors would be... unfortunate :biggrin:. Even a Lufthansa flight at that altitude would report flight level (FL) 410.
Sure. I wouldn't have made the mistake if it was 41,000 ft. But it was 12.946 km, which isn't used in aviation. I normally recognize it, but once in a while I slip, seduced by the imagination of a B737 in space.
 
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  • #16
boneh3ad
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Sure. I wouldn't have made the mistake if it was 41,000 ft. But it was 12.946 km, which isn't used in aviation. I normally recognize it, but once in a while I slip, seduced by the imagination of a B737 in space.

Incidentally, that's not far off of what L. Ron Hubbard imagined.
 
  • #17
cjl
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Air France 447 was an Airbus A330, a plane in about the same class as the 737.
The main points relevant to this have already been addressed by many other people, but as an airplane buff, I'd just like to point out that the 737 has a max takeoff weight of just under 200,000lb in its largest configuration, and can carry around 175 people 3000-3500 nautical miles or so. The A330 in its largest configuration is well over 500,000lb, with a max seating capacity over 400 people and a range of over 6000 nmi. I wouldn't really consider the two in the same class. Here's a picture showing both for comparison.

That having been said, both would behave pretty similarly in the case of a full stall, and both would also behave similarly in an engine out glide. As everyone has already said, engine failures have occurred more than once in aviation history, and generally it results in a relatively uneventful glide, though this does of course depend on there being a suitable landing location within gliding range.
 
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  • #18
gmax137
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I was on a flight once with an engine failure during take-off. The plane had just left the ground and was accelerating upwards pretty hard when we heard a loud bang and everyone sort of swayed forward as the acceleration reduced sharply. The flight was headed to West Palm Beach Florida, out of Hartford, Connecticut and half the passengers were Pratt & Whitney engineers: I could hear the diagnostic mumbling up and down the aisle, "... second stage framowitz..."

The passenger next to me asked "what was that" and I replied "engine failure." That's when she started screaming. I felt bad, I should have just shrugged. We circled back and landed without further issue.
 
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  • #19
Vanadium 50
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That's when she started screaming.

Carry a sharpie. Next time, when that happens, start writing on your arms. When she asks you what you are doing say "Writing my seat number on my limbs. It makes it easier for the investigators." Then you'll really hear screaming!

Never lost an engine on takeoff, but had an aborted landing once. Coming into FRA and I am looking out the window and thinking "Hmmm...I've never seen the Waldstadion look so clear" and then we wooshed up. A 777 is big, but it can move when it has to.
 
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  • #20
Ibix
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A 777 is big, but it can move when it has to.
We got diverted to another airport due to "a problem at our destination airport". When we landed they told us that the problem with the original airport was that its runway was too short for our aircraft to land if its flaps won't deploy... I'd noticed surges of acceleration as we circled over the original airport, presumably as they throttled up to counteract the expected drag from the flaps.

The whole thing added about fifteen minutes to our flight, plus about an hour and a half on a bus to our original airport. That's when I developed a gut feel for how fast planes actually travel.
 
  • #21
fresh_42
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Never lost an engine on takeoff, but had an aborted landing once. Coming into FRA and I am looking out the window and thinking "Hmmm...I've never seen the Waldstadion look so clear" and then we wooshed up. A 777 is big, but it can move when it has to.
There was once an Indian B-747 who touched down too early and crashed the entire signal system.
 
  • #22
fresh_42
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The whole thing added about fifteen minutes to our flight, plus about an hour and a half on a bus to our original airport. That's when I developed a gut feel for how fast planes actually travel.
What a luxury. I once landed in a Russian town 300 km away from the destination. They said something about the weather, but it was a clear day. They said that we will possibly fly the remaining rest in an hour or so. The plane was full of red cross personal and other NGO stuff headed to watch what the Russians would do in Chechnya. They must have heard me talking as I had been asked in German whether I was an observer, too. No, I said, I'm here for drinking. The point is, after an hour we saw that they had packed the baggage somewhere without noticing anyone. I was accompanied by a local so we grabbed our suitcases and booked a bus tour. I have no idea what had happened to all the others, presumably not speaking Russian. The bus was funny, too. We stopped somewhere in the middle of nowhere so that people could go for the toilet, which was into the bushes, and coming back to the bus, I saw that its lights were so dim, that I wondered whether we could be seen at all in the (meanwhile) dark. Plus I could read the advertisement on the side of the bus. It was in German. And it was not new.
 
  • #23
cjl
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A 777 is big, but it can move when it has to.

It's not that often that you feel what full power at low weight is like, but yeah, when that happens, modern jetliners can really move. Especially on something long range like a 777, it can be lighter by a substantial fraction of its weight at landing just due to fuel burn during flight, so you normally don't get to feel acceleration under those conditions (and it's not uncommon to take off at below full power to decrease engine wear).
 
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  • #24
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The scenario isn't just odd, it is poorly defined and not physically realistic. Just having engines fail doesn't cause an airplane to fall in freefall, and the descent rate depends entirely on the orientation. E.G., is it gliding, diving or stalled?


And on the other side of the coin is Air France 447, which stalled, fell out of the sky and crashed with little forward speed and the engines at full throttle.

Air France 447 was an Airbus A330, a plane in about the same class as the 737. It fell in a powered stall from 38,000 feet (11.6 km) in about 4 minutes. That's an average vertical speed of 108 mph or about the terminal velocity of a skydiver.

Isn’t there a formula for terminal velocity gravity x acceleration x weight or something like that?
 
  • #25
Nugatory
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Isn’t there a formula for terminal velocity gravity x acceleration x weight or something like that?
To calculate the terminal velocity we need to know the drag force from air resistance as a function of speed. Terminal velocity is that speed at which the drag force exactly counteracts the weight.
 
  • #27
berkeman
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Perhaps the question is related to this crash?:
https://www.bbc.com/news/business-61488976
Possibly, but the plane in that crash did not stay in the vertical dive all the way from altitude. It dove fairly far down, leveled off, and then did the final fatal dive.

In any case, this thread has run its course and will now be closed. Thanks to all who have participated.
 
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