Thrust
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Why 4 Engine Jets Have Longer Takeoff Rolls?

Sat Jun 29, 2013 11:05 pm

Is it because of drag? Thrust to weight ratios? Why?
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Thrust
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Sat Jun 29, 2013 11:09 pm

And also, why are the climb rates so poor for quads?
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timz
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Sat Jun 29, 2013 11:26 pm

You have to decide what you're comparing. How about a 747-400 at MTOW against a 767-300 at its MTOW?

The 767 is only allowed to take off with payload/fuel that it can lug out of the airport on one engine if the other one fails too late for them to stop on the runway. It has to be able to fly with half its thrust gone.

The 747 is required to be able to climb with 1/4 of its thrust gone. So it can be loaded heavier than a twin that has the same total thrust.
 
Thrust
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Sun Jun 30, 2013 12:26 am

Then I guess my question would be why we have gone down on the amounts of people and cargo that can be transported across the Atlantic? At one time, it was largely 747s flying people overseas...now we've downgraded to mostly twin jets that can't carry as many people/cargo.
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Polot
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Sun Jun 30, 2013 12:38 am

Quoting THrust (Reply 3):
Then I guess my question would be why we have gone down on the amounts of people and cargo that can be transported across the Atlantic? At one time, it was largely 747s flying people overseas...now we've downgraded to mostly twin jets that can't carry as many people/cargo.

You ignoring the fact that the number of transatlantic airlines, the number of transatlantic routes, and the frequency of many transatlantic routes have all increased significantly since then.

Pan Am and TWA, the previous main transatlantic US airlines, were at their peak much much smaller (in both size and operations) than airlines such as UA, DL, and AA today.
 
rfields5421
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Sun Jun 30, 2013 12:48 am

Quoting THrust (Reply 3):
Then I guess my question would be why we have gone down on the amounts of people and cargo that can be transported across the Atlantic?

Frequency and multiple airports. Yes, there is a place for VLA flights between major hubs, but those are not the predominante aircraft in use.

Airlines find the TATL routes work better with more flights of smaller capacity aircraft than fewer flights of larger aircraft.

That also allows the airlines to have multiple gateway airports in Europe and in the US/ Canda for that traffic. Passengers can avoid having to connect at major hubs for many destinations/ routes.
 
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Starlionblue
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Sun Jun 30, 2013 12:52 am

Quoting THrust (Reply 1):

And also, why are the climb rates so poor for quads?

Depends what you're comparing. As timz says on all engines twins, all other things being equal, have more power since they have to be able to climb on one engine (half its installed power) while a quad in the same situation has three engines and thus three quarters of its installed power. However In an engine out scenario, certification requires quads to have a higher climb gradient than twins.

Another factor is that quads tend to be used on longer routes, and are thus heavier with fuel when they take off. Thus longer take-off rolls.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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Jetlagged
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Sun Jun 30, 2013 3:44 pm

All other things being equal, the essential difference is that a twin needs additional thrust on takeoff to allow for the engine failure case. Both must be able to climb out with one engine failed. A twin must climb on one engine while a quad need only be able to climb on three. Therefore the twin has a greater installed thrust than a quad (other things being the same). This will lead to comparatively shorter takeoff roll and higher climb rate if full rated thrust is used.
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zeke
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Sun Jun 30, 2013 4:00 pm

Quoting THrust (Thread starter):

The takeoff rolls and climb rate we look at from a performance point of view are mainly engine out, in these cases quads normally have shorter distances, and better gradients (and certified as such).

If you are spotting at an airport and think a quad is taking longer to get off the ground, all engines operating they normally do as they have excess thrust which is not required for the engine out situation, so the takeoffs are derated. This lower takeoff thrust also means lower climb performance.

Generally speaking, a twin will have all engines operating a high climb gradient, as their performances is based upon one engine inoperative (50% of the thrust), so it needs a thrust to weight ratio one engine out to achieve its regulatory performance. A quad needs to achieve its regulatory climb performance with 75% of its thrust (loss of 25% with OEI).

So when you are spotting next, have a think about what the performance would be on a twin if it lost 50% of its thrust, it would need more runway to accelerate from V1 to rotation than a quad, as the quad has only lost 25% of the thrust, vs the 50% on the twin.

Another factor is that the majority of twins operate shorter distances, and may have the takeoff weight of the fuel load on a quad. It is very difficult as an observer to judge exactly what is going on from the outside.
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ImperialEagle
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Sun Jun 30, 2013 7:56 pm

Too bad the 720-B's are gone. They could climb-out like a rocket!  Wow!

Worst I ever saw were the evening departures LH flew out of ATL with 342's or 343's. Oh, the early water-wagon 8's and 707's were pretty bad as well. I recall a few of the company -8's blowing the leaves off the trees over Mountain View on a sultry summers day at ATL.   
Also, the DC-8-61's and -63's on MGTO on a hot day. FLL comes to mind with the -61's. JFK on a hot summer's day with the -63's.  Wow!
"If everything seems under control, you're just not going fast enough!"
 
prebennorholm
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Mon Jul 01, 2013 7:24 pm

Quoting THrust (Thread starter):
Why 4 Engine Jets Have Longer Takeoff Rolls?

It's more relevant to turn the question 180 degrees: Why twin engine jets have shorter takeoff rolls?

And the answer is: Because twin engine jets by law must have 200% of the power needed for a safe takeoff after V1. While a quad by law must have 133%.

To this you can add the fact that most planes, which are designed to use short runways, are not the biggest planes, and therefore usually twins - A319, B737-700. There just isn't much market for A380s and B747-8s for serving regional airports.

No rule without exceptions: The BAe-146/ARJ beats them all. That was what it was designed to do, and what it still does.

Just one extra remark: When judging takeoff roll lengths, then always be sure also to check the actual derate percentage.
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Viscount724
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Mon Jul 01, 2013 9:46 pm

Quoting ImperialEagle (Reply 9):
Too bad the 720-B's are gone. They could climb-out like a rocket!

Yes, I'm glad I had the chance to fly on a few 720Bs, Boeing's hotrod of that era. At maximum takeoff weight at sea level, a 720B only required a 6,000 ft. runway, compared to 10,000 ft. or more for a 707-320B/C with the same engines but roughly 100,000 lbs. heavier.
 
timz
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Mon Jul 01, 2013 11:40 pm

Almost-- the graph on Boeing's site says about 225000 lb off a 6000 ft runway.
 
BoeingGuy
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Tue Jul 02, 2013 4:26 am

Quoting ImperialEagle (Reply 9):
Worst I ever saw were the evening departures LH flew out of ATL with 342's or 343's. Oh, the early water-wagon 8's and 707's were pretty bad as well. I recall a few of the company -8's blowing the leaves off the trees over Mountain View on a sultry summers day at ATL.
Also, the DC-8-61's and -63's on MGTO on a hot day. FLL comes to mind with the -61's. JFK on a hot summer's day with the -63's. Wow!

Yep, I flew the NW 747-200 SEA-NRT several years ago. We took off on one of the 16s over the Tye Golf Course and it was a much more shallow climb that I'm used to. It was like putt, putt, putt, putt, like the old children's train story, "I think I can, I think I can".
 
Gemuser
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Tue Jul 02, 2013 10:47 pm

Quoting BoeingGuy (Reply 13):
ep, I flew the NW 747-200 SEA-NRT several years ago. We took off on one of the 16s over the Tye Golf Course and it was a much more shallow climb that I'm used to. It was like putt, putt, putt, putt, like the old children's train story, "I think I can, I think I can".

On the other hand, the fastest, steepest climb I've ever experienced was a QF B744 out of SYD to AKL, that is less than 25% fuel. Went off like a rocket! Took off from 34L and he was making a left turn before we crossed the 07/25 cross runway. Don't know about any de rating of TO thrust, but we were 45 minutes late leaving SYD and 5 minutes early into AKL, so he was pushing it.

It's all about thrust to weight ratio!

Gemuser
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ImperialEagle
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Wed Jul 03, 2013 12:54 pm

Funny you should mention a 747 adventure like that. Back in the early 90's I believe I found myself on an NW 744 headed to MSP from DTW----evidently on the first leg of a trip to Japan.
We were late for push-back and I recall the aircraft was pretty full of pax.

We took-off like a Saturn V rocket, and we boiled across the lake and into MSP.  Wow! We arrived early. I'm thinking we must have had a pretty light fuel load on that segment. FUN!
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BoeingGuy
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Wed Jul 03, 2013 3:14 pm

Quoting gemuser (Reply 14):
On the other hand, the fastest, steepest climb I've ever experienced was a QF B744 out of SYD to AKL, that is less than 25% fuel. Went off like a rocket! Took off from 34L and he was making a left turn before we crossed the 07/25 cross runway. Don't know about any de rating of TO thrust, but we were 45 minutes late leaving SYD and 5 minutes early into AKL, so he was pushing it.

As I mentioned the 777-200LR and 777-300ER are incredible for such huge airplanes. Boeing does un-derated TO thrust and obviously low gross weights during test and ferry flights. That = rocket ship takeoffs.

I saw a 777-200LR take off from BFI . It appeared to be off the ground in about 3000 feet and you should have seen that thing climb.

Likewise, a manager was on a 777-300ER test flight. He told me, "You should have felt that acceleration during takeoff roll".

Another pilot told me that when he pushes TO/GA on the 777-300ER he gets slammed right back into his seat.

Finally, I've heard pilots say they'll be airborne in less than 17 seconds during paint ferries to PDX.

That's what 115,000 pounds of thrust will do.
 
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lightsaber
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Mon Jul 08, 2013 2:03 am

Quoting zeke (Reply 8):
Generally speaking, a twin will have all engines operating a high climb gradient, as their performances is based upon one engine inoperative (50% of the thrust), so it needs a thrust to weight ratio one engine out to achieve its regulatory performance. A quad needs to achieve its regulatory climb performance with 75% of its thrust (loss of 25% with OEI).
Quoting prebennorholm (Reply 10):
And the answer is: Because twin engine jets by law must have 200% of the power needed for a safe takeoff after V1. While a quad by law must have 133%.

Adding a few more tidbits.

Because a twin loses so much thrust with the loss of an engine, it must accelerate quicker so that when an engine is lost at the go/no-go decision point and the pilot decides to go that under very reduced acceleration takeoff and climb rate is acheived. For a quad, the acceleration doesn't drop as much and thus the velocity at the go/no-go decision point can be a bit lower.

The other bit is that since a quad has less surplus thrust, the turbine is higher loaded. Thus there is more climb wear. So there is less surplus climb thrust.

Quoting gemuser (Reply 14):
It's all about thrust to weight ratio!

Get > 1:1 and then we'll talk.  

Lightsaber
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Ferroviarius
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Wed Jul 10, 2013 2:53 pm

Quoting ImperialEagle (Reply 9):
Worst I ever saw were the evening departures LH flew out of ATL with 342's or 343's. Oh, the early water-wagon 8's and 707's were pretty bad as well. I recall a few of the company -8's blowing the leaves off the trees over Mountain View on a sultry

Well, this might have been intentional. I recall once departing from ORD on a mildly busy flight to CPH. The captain told us during his welcome announcement that we would be able to start at low thrust since we were not in a rush and thus be friendly to the environment. I do not know how much of thrust they really used, but I fealt it quite comfortable to have low acceleration values.

In other words: It might happen that there are intentionally slow starts.

Best,

Ferroviarius
 
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Starlionblue
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Thu Jul 11, 2013 12:11 am

Quoting Ferroviarius (Reply 18):
The captain told us during his welcome announcement that we would be able to start at low thrust since we were not in a rush and thus be friendly to the environment. I do not know how much of thrust they really used, but I fealt it quite comfortable to have low acceleration values.

That sounds completely off and it almost sounds as if your Captain doesn't understand how derate works. Derate decreases engine wear, but it actually uses more fuel than full power.

If you want to be good to the environment in a jet you want to get to altitude as quickly as possible. That's what gives lower fuel burn. The best profile is a long runway so you can use a lower flap setting, meaning higher climb speed, then zoom to altitude where the fuel burn goes way down.
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triple7man
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Thu Jul 11, 2013 12:44 pm

Several years ago I made my first flight in a UA 747-400 SFO-HKG. The captain told me we would see all of 28R and we did, with a long slow climbout.
My next time in a 747-400 was 3 years later in a LH 747-400 simulator. I told my instructor I wanted to make some max gross weight takeoffs and land at HKG Kai Tak. The first time we took off we were light, just so I could get the feel of the airplane. The next takeoff from FRA we topped off the scales at 875,000 pounds. We used up much more runway, and I could feel the difference; the airplane was much heavier on the controls and more sluggish. Still we climbed out normally.
The 747-400 is good at what it does, and I still love watching them take off from LAX and HKG, especially a heavy cargo one.
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rfields5421
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Thu Jul 11, 2013 2:20 pm

Quoting Starlionblue (Reply 19):
That sounds completely off and it almost sounds as if your Captain doesn't understand how derate works.

It does sound odd - but it might be a Captain trying to make a complex subject simplistic and easy to understand by passengers. Since passengers might be concerned about a lack of 'full power' engine sounds and a long takeoff run.

And doing poor job with his explaination.
 
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jetblastdubai
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Mon Aug 19, 2013 11:28 am

Quoting Ferroviarius (Reply 18):


As an ex-ORD controller, these slow-climbing aircraft were a nightmare. The A340-200/300s that SAS, LH, RJA and Turkish flew caused major problems since they rarely made the mandatory altitude-crossing restrictions. These restrictions are established to avoid traffic operating at other satellite airports as well as the downtown buildings that are 15 miles from the airport. The number of times we had to scatter traffic to accommodate these 'lead sleds' or turn them away from buildings would surprise you.
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Fabo
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RE: Why 4 Engine Jets Have Longer Takeoff Rolls?

Mon Aug 19, 2013 12:35 pm

Quoting Starlionblue (Reply 19):
That sounds completely off and it almost sounds as if your Captain doesn't understand how derate works. Derate decreases engine wear, but it actually uses more fuel than full power.
Quoting rfields5421 (Reply 21):
It does sound odd - but it might be a Captain trying to make a complex subject simplistic and easy to understand by passengers. Since passengers might be concerned about a lack of 'full power' engine sounds and a long takeoff run.

And doing poor job with his explaination.

There is an overemphasis in fuel burn as the single "environmental factor" Derates could lower the noise footprint for one of the other concerns.

Also, Starlionblue - I have also believed that derates use more fuel in general - but apparently this is (at least) airframe dependent. I have seen some study of A320 derate and it turns out fuel is saved as well. The report came from some technical university in Germany (IIRC), so a reputable source as well.
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