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Depart uphill or downhill?

This accident highlights the situation.

What is the best way to make the calculation?

Some planes come with takeoff performance charts which include a gradient, but some don’t. And on grass you have a huge unknown: the wheel drag of the grass.

Administrator
Shoreham EGKA, United Kingdom

The best way I’d think would be to try to get some data yourself for your aircraft at the weights you’re likely to be at. I’d almost go as far to say it’s the only way to get useful figures for your own performance.

For small tyres I’d think grass / ground conditions could be a major additional factor.

I have done the maths once for grass/tarmac, including decision points where to reject takeoff & failure after takeoff, it will be into wind for me everytime

Load of consederations on friction, wheel size, L/D, wing load, stall/wind speed and thrust to weight vs terrain slope

On decision making it is easy to notice early you will not make takeoff going uphill into wind (even way before half the runway) than when going downhill with tail wind (you may not even notice untill you are at the runway hedge), the other configurations should be easy to compare IMO (question when do you know you will make it or not rather than if you will make it)

Btw “conservative 70/50 rule” has more tendency to fail you with upslope due to loss of thrust force to accelerate at high airspeeds but then if engine fails or you cut it at 50kts you should be ok stopping uphill into wind even with 100m remaining runway (got a loss of power once in Cub in that configuration, carb heat?, almost non-event to reject the takeoff and land/stop but had an exciting NFW moment and it could have been really worse 5 seconds later)

Last Edited by Ibra at 20 Jun 12:02
Paris/Essex, France/UK, United Kingdom

The point about aborting uphill is a valid one. Particularly if the runway is wet! Also, you will abort at a lower groundspeed.

Maybe in the Frydlant case, he did actually abort at some point, and didn‘t make it. Actually, most takeoff runway overruns actually are aborted takeoffs, at least in the last few seconds…

Last Edited by boscomantico at 20 Jun 12:20
Mainz (EDFZ) & Egelsbach (EDFE), Germany

The rule of thumb I was taught was every 1% of slope changes the effective runway length by 10%.

EGTK Oxford

There is a Safety Sense leaflet 7, last page summarises these rules of thumb.
https://publicapps.caa.co.uk/docs/33/20130121SSL07.pdf local copy

I’ve been taught to use those.

EGTR

Personal experience :
We were taking turn to fly legs. Dave flew Inverness to Stronsay, taking off at near max AUW. He landed uphill, into wind, with turbulence on short final.
I intended a downhill, downwind departure, but agreed to try uphill into wind.
The Jodel DR1050 lifted off with plenty of runway left, but did not climb. Thinking I was only airborne in ground effect, I put the wheels down, then lifted off again. I was still not climbing, could see poles with wires on the hill ahead, and aborted knowing I didn’t have enough runway left to land.
I should have pulled mixture, and might have stopped short of fence. The ground fell away to the left of the runway, and a left turn on lift-off would have allowed a safe departure.
Damage took one day to repair, and I flew her solo back to Inverness.
Inadequete planning for the take-off, but I believe the correct and safest decision in the situation.

Last Edited by Maoraigh at 20 Jun 19:16
Maoraigh
EGPE, United Kingdom

The rule of thumb I was taught was every 1% of slope changes the effective runway length by 10%.

Excluding wheel drag (which is highly surface type and wheel diameter dependent) it should be possible to derive the relationship purely by calculation, but I have no idea how to do that. Maybe @davids knows?

Administrator
Shoreham EGKA, United Kingdom

It can’t stay linear 1% slope for 10% runway that long? I think at 20% slope you should just assume infinite tarmac runway on a typical GA aircraft (it will getvstuck at 30kts) and for those with small wheels they would not even make 0kts with 10% slope on soft wet grass

At Dunstable gliding they do aerotows on 25% slope gradient from a big bowl that sits in the middle of the runway, usualy the tug & glider go down 25% and up 25%, there is no way to accelerate from the buttom of bowl without accelerating downhill first, if you lose ground roll on downhill you have to restart from scratch, if you taxi from the buttom on bowl you barely make 20kts on max power and almost nothing when grass is wet so you have to go left/right in zigzag to take less slope (while grass everywhere, I plan a taxi itenerary to avoid getting stuck on tug aeroplane or motor-glider in winter, also never cut power or put too much while taxying)

Good thing with landing in that big deep bowl is that you always stop at the tope of the bowl on landing irrepective of approach speed and flare comes naturally, the bad thing is that you get used that when landing on the flat

Paris/Essex, France/UK, United Kingdom

Ibra wrote:

It can’t stay linear 1% slope for 10% runway that long? I think at 20% slope you should just assume infinite tarmac runway on a typical GA aircraft (it will getvstuck at 30kts) and for those with small wheels they would not even make 0kts with 10% slope on soft wet grass

Of course not but where would we be seriously talking 20%? It is a rule of thumb for a normal runway where you derive it by the drag component of the gravity vector.

EGTK Oxford
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