Short Field Take Off

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dselder1962
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Short Field Take Off

Post by dselder1962 »

Had to take off from a shorter runway than usual with a cross wind, so to be sure I did the SFTO procedure and all went well with heaps to spare, however I noticed that the AFM procedure on 4A-35 doesn't actually have any reference to when to put the flaps up (or indeed have any reference to them at all after TO.)
I thought this was a bit unusual: do you think this is a mis print or is it just laissez-faire to the pilot on when to decide?
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Rich
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Re: Short Field Take Off

Post by Rich »

If you examine the POH climb procedures and performance numbers, you’ll see there is no rush to remove TO flaps in the climb.
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Re: Short Field Take Off

Post by Boatguy »

I raise them at 300' AGL. If I have an EFATO, I want to get to best glide speed as quickly as possible, and glide as far as possible; flaps are not going to help.
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Re: Short Field Take Off

Post by CFIDave »

Boatguy wrote: Fri Dec 13, 2019 4:09 am I raise them at 300' AGL. If I have an EFATO, I want to get to best glide speed as quickly as possible, and glide as far as possible; flaps are not going to help.
That's assuming you may be able to glide to a more-distant place to land from only 300 feet. Usually at only 300 feet with an engine failure you don't have much choice -- you're pretty much committed to landing straight ahead. If so, then having T/O flaps deployed will reduce your stall and touchdown speed (energy to be dissipated in a crash increases as the square of velocity, so slower is better).
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AndrewM
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Re: Short Field Take Off

Post by AndrewM »

When to remove flaps is an interesting debate. At a recent WINGS seminar I attended, the speaker (cannot remember who but was extremely experienced) was an advocate of the approach to not touch / change ANYTHING until getting up to 2000 ft. And that meant leaving T/O flaps till then. I asked the speaker about this after the seminar, specifically is it not better in the event of an engine failure after take off to have a clean aircraft for best glide and then put down flaps just before "landing"? His response was that it is better to be a little slower and be configured for landing, and hence having TO flaps already established was the better call.

I tried this for a while, and then reverted back to removing flaps around 300-500ft. The climb rate is so much better I figure it is the safer procedure.
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Re: Short Field Take Off

Post by ememic99 »

Since the OP question was related to twin engine aircraft, I would say the sooner the better, so between 200 and 300 ft. In case of EFATO in twin you want to have clean configuration and climb on blue line ASAP.
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Re: Short Field Take Off

Post by Boatguy »

I realize the OP has a DA42 for which this is sort of moot; the 42's and 62's will just go around on one engine!

But for a DA40NG this is worthy of debate because the rate of climb with TO flaps is higher without the flaps. The only advantage of the TO flaps is for angle of climb to get over obstructions. Per the AFM: Sea level, ISA temp, 1200kg, 92% power:

TO climb - TO flaps - 72KIAS: 707fpm
Cruise climb - no flaps - 88KIAS: 739fpm

As TO altitude increases the rate of climb increasingly favors the no flaps climb.

Vg is 88KIAS so getting to that speed sooner means not needing to drop the nose as much to reach Vg and the plane will always be at a slightly higher altitude.

Can't argue with Dave's assertion that slower is better when it comes time to actually land, but my CFI taught me to first know where I'm landing before I decide to drop the flaps and limit my choices. Certainly from 300' there aren't many choices, but raising the flaps at 300' doesn't mean EFATO will also occur at 300'. If the dreaded event occurs one minute later, I'll have more altitude to work with and already be at Vg, and if it occurs at < 300' then I can still lower LDG flaps in about 4s.

The only reason I don't raise the flaps even earlier (assuming no obstructions) is I want to have established a stabilized climb before changing configuration.
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Re: Short Field Take Off

Post by Rich »

Paradoxically for a DA40-180, keeping T/O flaps is beneficial. At most it suggests there is no hurry.

At 1000 Kg (a pretty representative weight for me) and SL ISA, POH climb charts show:

T/O: 60 Kt, ~1160 FPM
Flaps up: 68 Kt, ~1050 FPM

The T/O flaps setting has the added benefit of increasing angle of climb, which keep you closer to the airport for a given altitude gain. Note that you're more set up for the "impossible turn" we endlessly debate. :D
Last edited by Rich on Sat Dec 14, 2019 3:01 am, edited 2 times in total.
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ememic99
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Re: Short Field Take Off

Post by ememic99 »

These are the numbers for TD42 with CD-155 engines:
- Vr (rotate) - 72 (flaps UP), 71 (flaps APP)
- Vy (best rate of climb) - 79 (flaps UP), 76 (flaps APP)
- Vclimb (cruise climb) - 86

For DA42 Vx (best angle of climb) is same as Vy. It's worth of adding that Vyse (one engine inoperative speed for best rate of climb) is 86 KIAS.

The difference in distance for ground roll and clearing 50ft obstacle, assuming Vy after rotation, are as follows (at ISA, SL and MTOW):
- flaps UP - 428 m and 595 m
- flaps APP - 360 m and 572 m

Climbing at Vy (at ISA, SL and MTOW):
- flaps UP - 1315 FPM
- flaps APP - 1100 FPM

So, looking at these numbers and having in mind EFATO, for me the conclusion is obvious - any time after initial climb you retract the flaps, the sooner the better.

I guess that numbers for DA42-VI are similar, probably a bit higher speeds and longer ground roll but similar ROC.
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ememic99
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Re: Short Field Take Off

Post by ememic99 »

ememic99 wrote: Sat Dec 14, 2019 2:23 am I guess that numbers for DA42-VI are similar, probably a bit higher speeds and longer ground roll but similar ROC.
I checked the numbers in DA42-VI manual and I was wrong - DA42 with CD-155 engines has significantly better take-off and climb performance than DA42-VI, especially for clearing 50ft obstacle due to much higher ROC.
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