Take-offs from short runways

[Nicholas49]

Hello

I've thought about how best to phrase this question, so I hope it makes sense:

When you perform a take-off from a 'short' runway (I'm thinking of those European airports where the runway is 'just' long enough for a B737 / A320, for example Rome Ciampino, Bari or Lamezia in Italy), is there ever a situation where you: (a) need a higher N1% than you would using a longer runway to make sure you get airborne before the end of the runway; (b) then need to reduce the thrust level during the climb-out so as not to accelerate too quickly towards (and potentially pass) 250 kts before FL10? Or can you 'compensate' for the higher thrust needed for the take-off by using a higher than normal rate of climb?

I know there are lots of variables (weight, weather etc.) that affect the answer, but I hope you see what I'm getting at.

Nick

[foxmoth]

Most aircraft these days use a reduced power setting for TO anyway, this will depend on a variety of factors including - weight, temperature, airfield elevation and of course runway length, the shorter the runway the less "Flex" (term used for reduced power on Airbus) you get until you are doing a full power t.o. Some of the Airbus also have a setting called "Bump" which is in effect overboosting the engines, there if needed but reduces engine life (and using Flex extends it). There may also be other factors come into play that mean you could be using full power for T.O. even on a long runway.
After T.O. power is normally reduced from T.O to a climb setting anyway (in my company at 1500' AGL) and runway length will not affect this.

[Nicholas49]

foxmoth - thanks for the answer.

What would be a 'typical' N1 power setting to take-off when fully loaded from a short runway (such as Bari or Lamezia - not sure if you have flown there)?

[barit1]

For a given thrust rating, N1 will vary day to day; temperature affects the air density, so to get constant thrust, N1 must be higher on a hot day. So for a "typical" N1, you must specify the air temperature. It's all a part of normal performance calculation for every takeoff (but the computer does it all anyway).

[Denti]

There are a lot of variables in present day conditions and a lot of variables we can influence. For example we can use different flap settings, different thrust ratings, different airplanes. For example a 737-700 vs an -800 if passenger load permits, or one with winglets instead of those without, those with the short field performance kit instead of one without, and of course we can always change the weight either by offloading cargo or offloading both cargo/baggage and passengers or re-adjusting our fuel load, if need be plan for a refuel on the way.

And all that influences the N1 used since we usually reduce the thrust if we do not need it. There is a thrust bump feature available on 737s as well, available only for take off and as emergency thrust (by firewalling), however i have no clue how much extra that is as we do not really use it and have only one plane with it.

[foxmoth]

Airbus normally uses EPR rather than N1 - we can fly N1 but this is a degraded mode - you can get an idea of settings for this and a bunch of other stuff here http://www.satavirtual.org/fleet/A320PERFORMANCE.PDF

Flex is worked out by taking the temperature you could T.O. with using the runway and conditions you have at the weight you are, then entering the tables at that temperature.

[Piltdown Man]

I think the first part has been answered quite well but nobody has touched on (b) then need to reduce the thrust level during the climb-out so as not to accelerate too quickly towards (and potentially pass) 250 kts before FL10? Climb thrust is another compromise and most of us will use as much thrust as makes economic sense. A higher thrust means that a greater climb attitude can be held and remember, pitch controls speed, not thrust. For my aircraft (E190) it always makes economic sense to use the highest possible "Climb" thrust.

PM

[Denti]

Fox, i guess that depends on engine choice, doesn't it? Our A320 fleet uses N1 on all but 2 aircraft, those two have V2500, the rest CFM.

Piltdown man, that is true. We know that the highest possible climb thrust, given a climb clearance that can accomodate the usual rate, is economically the best. However from our usual very low take off thrust (nearly always more than 25% reduction) even the lowest climb setting is quite a nice climb thrust increase instead of a reduction. We have three climb thrust settings on our 737 fleet and usually the FMC selects automatically the fitting one depending on TO thrust reduction, we can manually override that and i usually do to the next higher one. Apparently there is a change of SOP in the works to always change it to the highest possible once we receive a climb clearance to at least FL100 if not higher to accomodate the high climb rate.

[Northbeach]

is there ever a situation where you: (a) need a higher N1% than you would using a longer runway to make sure you get airborne before the end of the runway

……..Yes, just about all the time. If you have a short runway then the variables one is left with are, thrust, flap setting, weight and temperature. To carry the most payload out of a short runway one would tend to select a higher power output (N1 – measure of Fan speed) and a higher flap setting. “More” flaps allow the wing to produce more lift at a slower speed allowing the jet to fly off the runway sooner, but that comes at the penalty of 2nd stage climb requirement (the ability of the aircraft to accelerate because of the additional drag created by the greater flap setting in the event of an engine failure).

For our 737-800s the maximum N1 is 104%. We have the ability to select different thrust limits for takeoff and climb. For takeoff we can select from one of the following thrust ratings: 22,000, 24,000, 26,000 or 27,000 lbs thrust.

“Derating” is the selection of a lower than maximum thrust setting for takeoff.

“Assumed Temperature Method” is another means of reducing thrust below the maximum possible. It is advantageous from engine reliability and cost standpoint to require the engine to produce as much thrust as needed for the conditions, rather than the maximum thrust possible from the engine.

Assumed Temp Reduction involves “giving the engine” an artificial temperature from which to calculate the thrust.

Here are some actual performance number from a runway we use it is a sea level runway-Hawaiian Islands heavily loaded for the U.S. mainland. With a runway 6,500’ long (1,981 meters) and a fixed temperature of 28C

Flaps 5 26,000K thrust 100.1% N1 weight =70,227Kgs
Flap 25 26,000K thrust 100.1% N1 weight = 73,772Kgs
Flap 5 27,000K thrust 102.4% N1 weight = 71,636Kgs
Flap 25 27,000K thrust 102.4% N1 weight = 75,363Kgs

For comparison here are some numbers for virtually the same conditions except having a longer runway: 11,900’, 3,627 meters.

Flap 5 22,000 thrust 93.8% N1 weight = 78,227 Kgs.

Given the same conditions, except for the length of the runway, on the long runway one could use the lowest thrust setting of 22,000 lbs of thrust and an N1 setting of 93.8% and safely meet all the performance requirements to lift 78,227 kgs off the runway.

Now reduce the length of the runway by 1,600+ meters and the “picture” changes dramatically. Using the maximum thrust rating of 27,000 lbs and the same 5 degree flap setting the N1 goes up to 102.4%.

As far as exceeding 250Kts below 10,000, at the heavier weights it really isn’t a problem the jet is a little sluggish especially with a flap 25 departure it takes time to clean up “on schedule” (as the airspeed increases). With autothrottles in use speed control should be a non event. Even without the “magic” simply controlling the speed is our job description. However high power settings with low takeoff weights, low level initial level off altitudes, turns with complex departures do increase the work load.

[Doors to Automatic]

A related question - Florence airport has a shortish runway just about long enough for A319 size aircraft. On the 23 end the threshold is displaced halfway down the runway. Does this mean the larger aircraft can only land on the 05 end?

[RTO]

For takeoff we can select from one of the following thrust ratings: 22,000, 24,000, 26,000 or 27,000 lbs thrust.
Is this a 27K or 27K-B aircraft? Was under the impression that you can only derate 2 steps i.e. 27K ->24K.

[DuneMentat]

A related question - Florence airport has a shortish runway just about long enough for A319 size aircraft. On the 23 end the threshold is displaced halfway down the runway. Does this mean the larger aircraft can only land on the 05 end?

In our company only a select few captains are qualified to land on runway 23 so the rest will have to land 05. Also landing on runway 23 is weight limited plus we would only ever consider it if we had more than 15kt tailwind on 05 (BAe AVRO RJ85).

DM