Aborted take-off. Puzzled.

[Neil MacG]

I'm happy to post this as me rather than anon. While I'm a bit embarrassed I don't think I really did anything wrong (but I'm sure you'll let me know if I did), more than anything I'm really puzzled.

I created a bit of an embarrasing situation yesterday for myself and my passenger (a fellow aviator who hasn't flown for a while)

I was flying the Cessna 172. With 2 up, and not full fuel (I'd done W*B calcs earlier in the day which showed we'd be well within MAUW even with full fuel) I had few qualms about the airfields we were likely to be landing and taking off from.

Not unsurprisingly the take-off run at Draycot on recently cut but damp grass and with my passenger was somewhat longer than normal - but well within the 800m.

We planned to do a bit of a circular tour stopping at Popham for fuel and then Sandown before returning via the New Forest path under the Solent CTA.

After a bite to eat and refuelling (where I dipped the tanks which showed I was just short of 15Imp gallons on Right and 10Imp gallons on Left). We went to depart from Popham's 21. All the windsock's were limp.

I had 10degrees flap for short field/grass technique and started applying power as I turned into the line-up. I was closely watching the ASI and it seemed to be ages before it started registering, at about half way down the runway I still hadn't got to 40mph and I was grossly unhappy given I needed 60mph to unstick and there were lots of trees ahead. So I aborted the take-off.

We returned to the airfield parking area, shutdown and went for another drink while I sat down and went through the calculations again. With an unfactored calculation Take-Off run required at sea-level should have been around 460m, given the 900m and even allowing guestimated factors for upslope and grass this was a real puzzle. One of the reasons I had aborted was that on the taxy down to the threshold the aircraft kep wandering off to the left - but I had put this down to the slope in the off-runway area. I hadn't noticed any binding or turning when at full power, but at the back of my mind I just wondered if there was a brake problem. When we taxied back to clubhouse area over the same sloped area the aircraft kept wandering to the right - confirming in my mind that there was actually no brake problem.

So, I was in a quandry. I couldn't see what I could do differently in take-off technique to improve the take-off. I called another member of the group who has quite a bit more experience in the aircraft than me and he agreed with my actions so far and couldn't offer any alternative suggestions apart from possibly requesting to use 08 which allows for a reasonably gentle turn onto the path of 03. But by this time the airfield was beginning to shut so I decided there was no time to mess about and to improve my chances with the only definitive method I knew.

I asked my passenger if he wouldn't mind staying at the airfield while I departed back to Draycott on my own and I would fetch him in the car.

Fortunately, being a fellow aviator he understood the situation and as it would hopefully be a short flight and drive for me back to pick him up he readily agreed.

Without his weight in the aircraft and possibly due to other (unknown) differences compared to the original departure I was well in the air by half way down the same runway. The flight back was relatively quick and uneventful. The return by car to Popham was less so, in that while I'd had no problems navigating to it earlier in the air, I got lost several times on the ground trying to find the airfield. Finally when I got there, I found the entrance gates closed. So my poor passenger needed to walk all the way back to the gates and then climb over the wall!

This morning I sat down again and even using all the factors according to POH

Slightly greater than stated weights for pilot and passenger.
Wet Grass (it was actually dry and hard) - POH says add 7% for Grass - I added 20%
2 degree upslope
5 knot tailwind

30% Safety margin

This still comes out at TODR of 875m for a 900m runway. Maybe I baulked too quickly, maybe there was some other factor I had not taken into account. Would others have ploughed on? Either way, I'm still here and so is my passenger so I'm happy to live with some embarrassment but I'm still rather puzzled.

[Irv Lee]

Obviously i don't know much about the first 21 attempt but if i had been there i would have suggested it was worth a trial run on the flatter 26 or 08 depending on whether any wind would be SW or SE even if light, then even a solo circuit off same 26 or 08 if the run proved ok, then 2-up departure if that suggested it, but usual rules apply which are if you are not happy, you don't go

[Flintstone]

Fellow aviator? Wasn't covering the controls by any chance? Unintentional brake pressure?

[Neil MacG]

Obviously i don't know much about the first 21 attempt but if i had been there i would have suggested it was worth a trial run on the flatter 26 or 08 depending on whether any wind would be SW or SE even if light, then even a solo circuit off same 26 or 08 if the run proved ok, then 2-up departure if that suggested it, but usual rules apply which are if you are not happy, you don't go

Thanks Irv,

I think these were options I discussed with the other group member, but with the airfield shutting I was unwilling to rush trying something. Had it been earlier in the day I may have requested to use one of the other runways. In fact - 03 might have been an option given that it apears to slope downhill. There's no mention of the slope on 03 in the AFE guide though.

Neil

[Neil MacG]

Fellow aviator? Wasn't covering the controls by any chance? Unintentional brake pressure?

I suppose it is a possibility, but my friend was keenly watching the ASI as well (he also flies a C172 and was interested in the differences between our older model and the one he has been flying), so I don't think it's likely as it would have required fairly positive action to apply braking which is at the top of the pedals.

I do wonder if a factor I hadn't previously considered is whether grass material from Draycot - which was being mowed just before take-off may have been impacted in the spats. It's not something I checked after the aborted take-off. The landing roll on arrival at Popham was very short, but I was just putting that down to me getting the approach speed right and the upslope of the runway.

Neil

[GrahamB]

Low tyre pressures?

[Irv Lee]

[ In fact - 03 might have been an option given that it apears to slope downhill. There's no mention of the slope on 03 in the AFE guide though.
Neil

But is there a mention of how much 21 slopes uphill??
(I'd be surprised if there was any value given for the slope either way, never having been licensed.)

[Pete S]

Both mags?

[Neil MacG]

Both mags?

I don't think so.

I have been caught out by that before during power checks when I switched off the engine turning the key the two notches to the left while watching the RPM guage, so I'm normally aware of that one now!

Also I don't recall any issues with position of mags when shutting down after the aborted departure.

Neil

[Irv Lee]

You haven't been flying too many tailwheels recently have you? With the uphill & grass you have to make sure your nosewheel has no weight on it at all with the stick slightly back enough to lift the wheel a fraction out of the grass or it resists acceleration, then when entering the white arc, a little 'step up' and flat and accelerate flat in ground effect to Vx before any real rotation. If you allow weight on the nosewheel on that uphill run, it would be pretty sluggish, but the nice thing about 21 is that it's easy to abort and slow down

[Neil MacG]

You haven't been flying too many tailwheels recently have you? With the uphill & grass you have to make sure your nosewheel has no weight on it at all with the stick slightly back enough to lift the wheel a fraction out of the grass or it resists acceleration, then when entering the white arc, a little 'step up' and flat and accelerate flat in ground effect to Vx before any real rotation. If you allow weight on the nosewheel on that uphill run, it would be pretty sluggish, but the nice thing about 21 is that it's easy to abort and slow down

Haven't flown the Auster since last year (unfortunately), mainly due to being out of personal currency on it and waiting for an opportunity to refresh my skills. So it's all been Cessna 172 since. I'm normally quite conscious about keeping the load off the front mainly to protect the nose gear - particularly on grass. But I do wonder whether it may have been a combination of things. The ones in particular that might have contrived to reduce performance are:

1. As Irv suggests - not keeping the load off the front wheel
2. As my group members have suggested (as they've done it themselves) - that my feet were higher up on the pedals than desirable and creating a degree of braking action.

I didn't "feel" or notice any issue with engine performance although additional check of RPM would have been wise. Both of the issues above combined with the upslope could well result in considerably poorer performance than I expected.

My fellow group members will be flying 'NW in the next day or so and report if they find anything.

It may be a bit of a wake-up call to improve my technique. If so I'm glad it happened this way and nothing more disastrous.

[AndyR]

Carb heat left on? Unlikely I know, as you would have noticed it at some stage afterwards, but a suggestion.

[Neil MacG]

A bit of an update....

Many thanks to dublinpilot who was aware that I use PocketFMS and suggested examining the breadcrumb data using tools available.

By comparing the IAS which I know did not exceed 39mph (approx 34Kts), with the maximum recorded ground speed (40Kts) I could determine that there must have been a tailwind of 6Kts at the time I aborted the take-off.

However...

What the Breadcrumb data also shows (using this useful page here: http://www.movable-type.co.uk/scripts/latlong.html) was that I aborted the take-off at around 260m after starting the take-off run. So in all likelyhood I would have got to a rotate speed within an acceptable distance.

I suspect that the heavier than normal aircraft plus tailwind created a degree of nervousness on my part. I clearly expected a much greater IAS at the point I reached and made what I still regard as a valid choice to abort.

Neil

[Irv Lee]

People don't realise what a tailwind can do to either a landing or take off until they have even a slight one and see the effect - if they are lucky or on the ball like you, they don't end in a hedge

[KNT754G]

I still regard as a valid choice to abort

Absolutely and completely.
If in (any) doubt, chicken out.
He who gives up and runs away lives to fly another day.