Judging take off distance (touch'n'go)

[Mak]

Hi guys,

In a touch and go I always find it hard to judge whether I have enough tarmac remaining, given speed and position at time of decision. The same problem would apply on take off in difficult conditions (heavy, grass, etc.). Fortunately I tend to err on the side of caution. but I'm curious about techniques and rules of thumb used by more experienced pilots. Thanks.

by the way I'm happy to report I've recently passed my PPL skills test!!

Mak

[AliB]

Hi Mak,

Well - I'm not going to be a lot of use to you I am afraid, but I thought I would add some solidarity.

I hated T&G's during training (my landings before a T&G were invariably worse as I was dreading that decision moment in the roll). Now (still sub 100 hours) I have often thrown away T&Gs and turned them into full stops. I have done this several times with an instructor on board (and I recall doing it during my skills test too) and have never been criticised for it - it is definately the best side of caution to err on!

Sorry if this is a bit of a waste of space - hope you get some more constructive replies ;)

Congrats on the skills test pass :D

AliB

[Pilot16]

Im still a student so I dont fit in the 'experienced' catagory...
you dont have to take my words :)

The secret is (ahem) ;) :

1) Make sure you dont flare too long
2) to do this you need to be careful on your airspeed (keep it low but not below minimum) and maintain an accurate glide path, so you touch just after your aiming point (without eating a lot of runway), that way you have plenty of room for your take off roll for t&g, therefore there is no need to be erring ;)

What i do if I run short of runway and have decided to carry on with touch and go, is make a take off much like a short field take off (a low climb angle untill airspeed has reached sufficiently high) before I commence best rate of climb.

I hope i made sense there :rolleyes:

and congratulations on passing your skill test!! :)

P16

[Speedbird252]

Its a tough one doing touch`n`goes. Being used to the Airfield and runway makes the difference, having an idea of what the remaining amount of usable runway looks like will help. But, at the end of the day it boils down to one thing, and one thing only -

- if in doubt, there is no doubt.

Get on the brakes. taxi off the active, go thru the departure checks again and get back in the circuit.

Taking off in difficult conditions, heavy, grass etc should be part of the basic weight and balance calcs that you have already learnt. It is either a go, or it isnt. No half way allowed!

Enjoy your flying, but make sure you are comfortable with the above!

Regards, Speedy



;)

[andrewc]

Since PPL-ing I've given up touch and go's on grass.

They are a reasonable way of getting the maximum
number of landings into an hour while training on
a nice long tarmac strip - or getting back in the
groove after a time away.

However they are a very unnatural phase in normal
flight and that combined with a bumpy short grass
strip makes them a poor and possibly expensive
experience.

If I'm flying into a short grass strip I'm either going to
land or go around.

-- Andrew

[Flyin'Dutch']

Mak

Congrats on passing and have fun. I think it is excellent that you are obviously keen to continue the learning experience!

As Andrewc remarked most PPLs dont do a fat lot of T&Gs.

If you want to do some make sure you have an abundance of RWY to your disposition and make sure that you are happy to do them on your own.

After all they are a great way of circuitbashing but are something which is never (?hardly ever) needed in the real flying life. (Unlike low approaches and go-arounds)

You will need a thorough knowledge of the performance of your mount both from the POH and how that translates into real life both for the aircraft and your own performance.

After touch down you need some time to change configuration with flaps/carb heat/ etc before you get going again. During this time you will be using up RWY at near flying speed.

Let us assume that is 60 KTS (Just as it is an easy number) which means that every 5 seconds you do 90 meters.

So no time for messing or revert the go-nogo decision a few times.

If you consider that the braking ability of most machines equals the acceleration power it is not a bad starting point to consult the POH for a distance groundrol. Double that and I think you would have a safe margin in which you should be able to stop your mount.

Use this to set a datum by which you will abort if you are not airborne.

IMHO this would give a safe starting point to get going.

Have fun and as always if you are not sure ask or get an instructor/experienced jock to accompany you first.

FD

[eyeinthesky]

If you are worried about how much distance you have remaining, you should have done 1 of 2 things:

1) Gone around before now because you were floating for too long or not getting down close enough to the threshold.

2) Not consider them because the runway is too short.

However, consider the following:

The average training aircraft will probably touch down with an IAS of somewhere between 40 and 45 kts. The rotate speed is probably somewhere between 60 and 65 kts. 45 to zero is more than 45 to 65, so I would suggest you have a better chance of a favourable result if you apply the power, accelerate the 20 kts to rotate and try again than try and use the somewhat limited stopping abilities of some light aircraft brakes to reduce the speed by 45 kts in what you have already decided is an uncomfortable distance.

A touch and go is after all really just a go around with the runway putting in a momentary appearance, and therefore it seems sensible to use the momentum you have to get you back where you can make a better shot of it than fight it whilst trying to prevent your propeller becoming an improvised hedge trimmer.

I think you'll find you need less runway to accelerate back to rotate speed than you will to stop the aircraft.

[Lowtimer]

Bear in mind also that the point at which it's better to open up and go around rather than stop varies according to the usual performance factors like runway slope and surface, and wind: e.g. (For a given touchdown point, runway length etc)
- if you're going up the hill (07) at Old Buckenham, heading for the haystacks, in a flat calm, on a hot day, in still air, the balance moves strongly in favour of stopping, and
- if you're going down the hill (25), into a cheerful breeze, and the runway is wet or you're landing on a slippery surface like short wet grass, those factors all move the balance in favour of opening up for the touch and go.

[bookworm]

I think you'll find you need less runway to accelerate back to rotate speed than you will to stop the aircraft.

That's a potentially dangerous generalisation for a number of reasons.

Aircraft have very different acceleration vs braking characteristics. Some may stop more readily than others.

Though slim_slag and I disagreed on a few details in a previous thread, his point that the speed vs distance relationship is non-linear is applicable here. Even if acceleration and deceleration were equal and constant, 45 to 0 takes less distance than 45 to 65.

If you add a headwind, your 45 knots may in fact be much less as a groundspeed.

If you have left it too late to do the right thing, it's better to hit the fence at low speed under braking than at higher speeds close to flying speeds.

[eyeinthesky]

Bookworm: I missed the thread to which you refer. Can you explain the following comment:
"Even if acceleration and deceleration were equal and constant, 45 to 0 takes less distance than 45 to 65."

The trouble is, these type of issues invite generalisations, and of course the only correct answer is to consult your specific aircraft's POH, add a large pinch of salt for the degradation of performance from those figures, and then see whether T&Gs are a good bet.

The issue of headwind applies of course to both the stopping and the going scenarios.

I woud also agree that a low speed hedge incursion is better than a high speed snag of the gear as you just fail to clear it, but my point was that in many cases (not ALL), you might find that applying the power and following the go-around drills will make the issue of a visit to the hedge a non-issue.

Another way to settle it for yourself might be to go to somewhere with plenty of runway and try doing both: Stopping in the shortest possible distance from landing, and comparing that with the distance used from touchdown to getting airborne again. Perhaps it would be a good idea to have someone with you to note the relevant points while you concentrate on flying the aircraft! ;)

[Mak]

The overall feel I get from the answers is that it is down to personal judgement. However professional pilots have well defined procedures to handle go/nogo decisions of this type.

Are any of these techniques applicable to a small a/c on a small strip? Or is our best judgement really the only answer?

Mak

[Flyin'Dutch']

Hi MAK

I think you are referring to the ops for AOC/Public Transport rules.

They will not have figures for circuits and bumps.

FD

[bookworm]

The trouble is, these type of issues invite generalisations

Indeed they do -- you can be sure that if you make a generalisation someone here will point out a specific counterexample, and if you say something about a specific case someone will point out that it doesn't work in general. :) It's all fun debate.

The issue of headwind applies of course to both the stopping and the going scenarios.

I don't think it does. To go (in your example) you have to get to a airspeed of 65. To stop you have to get to a groundspeed of 0. Put another way, to stop from 45 KIAS with a 20 knot headwind, you only need to brake from a groundspeed of 25 knots to zero.

Bookworm: I missed the thread to which you refer. Can you explain the following comment:
"Even if acceleration and deceleration were equal and constant, 45 to 0 takes less distance than 45 to 65."

Yes I was thinking of the Newton, Runways and Rules (http://www.pprune.org/forums/showthread.php?s=&threadid=79321&highlight=Newton) thread.

With constant acceleration, the distance travelled between two speeds is proportional to the difference between the square of the speeds. So 0 to 45 takes 45*45 units of distance, while 45 to 65 takes 65*65 - 45*45 units of distance, with the same magnitude of acceleration. The latter is greater. Since generally light aircraft brake better than they accelerate, 45 to 65 is likely to take much more distance that 45 to 0. But that's a generalisation... :)