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Twin Jet Takeoff

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Twin Jet Takeoff

Old 26th Mar 2009, 02:36
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Twin Jet Takeoff

In a twin engine jet OEI just after takeoff, can it still fly with the gear down and go around? Think MD80s, RJs, 737, 757, 767, 777, Airbus etc. What about 1 gear down? I'm primarily interested in RJs.

I'm going to guess that at light weights the answer may universally be yes but at gross you might say "Houston we have problem".

I'm looking at a Functional Hazard Assessment that calls this Catastrophic. It may be that for design considerations but not so clear cut in practice.


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Old 26th Mar 2009, 02:45
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While there is a decrement in climb capability with the gear hanging, it is not so great that disaster is imminent, except in the case of terrain in the departure path. The gear down decrement for the C-5 (the only plane I have seen drag indices published) was 1.3%, I wouldn't be surprised it the gear down penalty for most planes is in the range of 1% to 1.5%. You can see, if climb gradient is limited by terrain, the gear down case might have one "scratching gravel", as my glider friends say.

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Old 26th Mar 2009, 04:13
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You should find that a fair few planes have provision for dispatch with the gear locked down, usually for a ferry to a maintenance base in the event of a problem at an outstation. The flight manual should provide the guidance in such cases.

Looking at one such supplement for a small, twin-engined a/c, I see a WAT penalty of about 15% (so you look up your WAT limited weight and then have to knock off 15%) and also climb gradient penalties of from 2.4% to 3.3%. I'd expect the penalties to be highest for a small a/c, since the gear probably is a larger component of drag in such cases (but of course, not all).

I'd interpret that as roughly that any TO within 15% or so of your limiting weight is a potential issue, depending on the actual obstacles.
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Old 26th Mar 2009, 12:05
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Given that 1st segment requires at least a positive climb gradient I'm going to make a stab and say they will all fly with the gear down at V2 to V2 + 10. If not some one made a boo boo!!

As for "go around" I don't know what that means in this context.
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Old 26th Mar 2009, 16:14
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As Hoppy says, the rules for transport category planes require that it be able to lose an engine during the TO roll and if this happened above the speed at which it could be stopped on the runway, it will have a positive rate of climb. There is no standard for what that rate of climb is with the gear down, just that it be positive. The first thing that happens after the TO is gear retraction and that gets you into the second and third segment climbs which each have some requirements. You're putting multiple failures together which go beyond the certification of the aircraft. If you're takeing off out over the ocean I'd said you should make it, if the airport is surrounded by mountains maybe not.
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Old 2nd Apr 2009, 09:28
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You're talking multiple failures and/or conditions, to wit -

Gear Down + Engine Failure + Critical obstacles.

Take away any one of those three components and you're still stuck with a multiple failure/condition situation, something that basic statistical analysis and regulations do not call for, and is probably surviveable.

The "positive rate of climb" requirement for a 2 engined aircraft with OEI in the first segment may at first sound alarming, but in reality is not. Most aircraft that I do performance work for have a 'gear down' climb decrement of the order of 0.9 to 1.0%. Galaxy flyer speaks of 1.3% for one type (I'll bet a dollar it's a type like the DC9 where the Gear doors must open during retraction, thus increasing drag).

As a 2 engined aircraft must make a MINIMUM Gross gradient in the 2nd segment of 2.4%, subtracting 1.0% still leaves a fairly healthy 1.4% gradient, not fantastic, but if there are no obstacles, you'll live to fly another day. Most pilots observedly achieve close to Gross gradients during training.

Even if you wish to stick to the Triple 'failure' scenario, remember that probably at least 90% of Takeoffs are at less than performance limiting weight, which will significantly improve climb gradient. From a 1.0% degradation, you only have to pick up an additional 0.2% gradient from the lighter weight to achieve the Net flight path, which will (just) ensure obstacle clearance.

The "odds" are very much in your favour.


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Old 2nd Apr 2009, 10:11
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While it might be in the 1% range on a big jet, it's much more on smaller or medium jets. It's all very type specific. You strongly feel the drag on your performance, if you, say, should forget to retract the gear.

Some aircraft have very "aerodynamic" gears, while others nearly fall out of the skies. The type I'm flying e.g., has stronger performance penalties on medium flap setting than on the gear. What I gathered, the medium jets with their high leg gear struts have the biggest drag, like the A310, A320, B757. Smaller aircraft have smaller gears, and bigger aircraft have the same gear size with much bigger overall size.

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Old 2nd Apr 2009, 11:14
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Smokey, just a thought on the first segment where you say:

"type like the DC9 where the Gear doors must open during retraction, thus increasing drag"

While I understand the profile drag increase with gear doors open, as mentioned in various texts and by instructors, I'm wondering if there isn't a further drag contribution due to the decrease in the surface area providing lift. Seems to me the area on the underside of the aircraft wing has decreased by a ratio of the door areas to the wing areas, plus there are the effects of the disturbed airflow under the wing around these doors. Since lift must remain near constant, and S has effectively decreased, now the aircraft has to fly at at a higher angle of attack thus producing a higher component of drag due to the aircraft's increased aoa profile drag, as well as increased induced drag.

Am I talking nonsense?
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Old 2nd Apr 2009, 11:39
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No hawk37, you're not talking nonsense, what you say is perfectly valid.

Dani, you state that "it might be in the 1% range on a big jet, it's much more on smaller or medium jets". Our airline and it's subsidiaries operate aircraft from the A380 down to the Learjet 45, that's got to be some kind of record of weight range in one company. I have access to all of the AFMs for all of these aircraft, and for the Learjet 45, the Delta gradient between V2 Gear UP and V2 Gear DOWN is 0.89%. The Learjet is definately in the SMALL jet bracket, but with similar figures in this area to the "heavy metal".

At the end of the day it's type specific, but consider this - The maximum Delta tolerable for any twin jet would be 1.59%, as 1.59% subtracted from the minimum required 1.6% Net gradient leaves a mere 0.1% to satisfy the "positive climb in the first segment" requirement. Any aircraft with a greater than 1.59% Delta would require more than the regulatory requirement in the 2nd segment, and would not be economically viable!


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