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Old 19th Feb 2003, 10:45   #1 (permalink)
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SID - Climb gradient

Basic question I know, but can someone post the formula for caclulating climb gradients, e.g.

Gradient of 5.7% required to 3600ft, thereafter 3.3%. ROC 1200ft/min and GS of 200kts, Can aircraft comply?

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Old 19th Feb 2003, 11:58   #2 (permalink)
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Check out DAP 2-2 which is a chart of climb rates/gradients/ground speeds. As a rule of thumb you can find a climb gradient (%) by dividing the rate of climb (ft/min) by the ground speed (knots).

If you apply the formula to your question 1200 ft/min divided by 200 kt gives 6%, and the DAP chart quotes a required ROC of 1155 ft/min for a 5.7% gradient at 200 kt.

Good luck.
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Old 19th Feb 2003, 12:14   #3 (permalink)
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Old 21st Feb 2003, 10:00   #4 (permalink)
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Another way to calculate a very close ball park figure for day to day operations, use this

Climb gradient required X airspeed = rate of climb required

eg Gradient reqd 3.3% ; airspeed 120 kts

3.3 X 120 = 396 fpm

Not sure how this compares with the tables though.
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Old 21st Feb 2003, 17:13   #5 (permalink)

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Airframe Driver... I'd be a bit more comfortable with that one if you're talking TAS, rather than generically "airspeed", which most folks will more usually equate to IAS. However, your example is really only close to the right value at 1000 FT (ISA +15) and gets behind the eightball from 3,000 FT on up. Lower ISA deviation values will give a few bonus points, of course.
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Old 22nd Feb 2003, 05:31   #6 (permalink)
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The value you need to use is groundspeed not TAS or IAS as climb gradient is relative to distance covered over the ground.

3 percent required at 150kts ground speed = approx 450 fpm req'd.
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Old 23rd Feb 2003, 09:37   #7 (permalink)
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Point taken guys, but at lower altitudes where you need to know the climb gradient info, doesn't tas almost equal ias almonst equal gs.
I mean, in my examples above, 3.3 % required at 120 kts, requires 396 fpm. The books quote 401 fpm. I aint ever flown an aircraft with gauges that accurate, nor am I able to fly an aircraft that accurately with one donk out in IMC and me shitting my pants all at the same time.
Chill out guys, it was only a helpful 'ball park' figure, because I know some guys would blast off in the smuck with no idea of required performance requirements and just pray like hell that they don't need to know or find out !!

This is just a convenient way to have SOME IDEA of what we should be hoping to get in the event of a EFATO.
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Old 23rd Feb 2003, 22:48   #8 (permalink)
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Sure IAS and TAS are pretty close low down, but it's got to be a good idea to consider GS rather than these two given the track of the SID and what the wind's doing especially if you're in something with less than startling climb performance.
The extension of that is also having a hopefully cunning plan of what to do if you don't happen to be making the magic number of fpm, given the ludicrously low rate of climb on one engine that may or may not be available on the day!
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Old 23rd Feb 2003, 23:29   #9 (permalink)
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Bear in mind also that in most twin engine aeroplanes that you won't get 3.3% on one engine.

The CAO 20.7.1B aeroplanes will give you 1.6% nett (97ft/nm) 2.4% gross.

Any aeroplane that gives you more than that is giving away payload unless it is grossly overpowered - the B1900D might be in this category if what people tell me is true.

It surprises people how "badly" the aeroplane performs on one engine compared to on two - the 737 typically has 2 engine climb gradients of around 14% - dropping back to 2.4% is a huge reduction.

I guess the point is that you won't get to fly the SID on one engineunless you take a huge performance penalty in terms of uplift.
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