DA42 Wet take off
 

A student of mine had an oral question for their CPL skills test that has us all a little confused.
The question was “why don’t we use a factor for wet runway on take off”
The afm states the following:
“The figures in the following NOTE are typical values. On wet ground or wet soft grass covered runways the take-off roll may become significantly longer than stated below. In any case the pilot must allow for the condition of the runway to ensure a safe take-off.”
Now to me that sounds like we should anticipate an increase in distance but we’re given no factor for a paved runway.
I would have answered saying an increase in distance is expected however the afm provides no factor.
Anyone got any more input?

Was it a grooved wet runway that was being discussed? Some a/c and jurisdictions allow grooved wet to be treated as dry. Not sure where you are and it’s been too long since I flew a DA42 to recall the specifics for the aircraft.

The take-off distance on a paved surface is not influenced to a very great degree by water on the runway (not until you get to standing water of several cms deep). Landing distance is another matter of course as you are looking at decreased braking performance in that situation. The Note in the AFM covers the other situations where you do get an increase in rolling friction during take off: grass of various lengths for example, but does not cover the situations where a grass runway or an unpaved runway is wet. I suspect that Diamond did not test these situations and therefore will not be able to provide any factors for those situations. As for the why? Your answer is similar to what I would have given but I suspect that you could also state that Diamond did not consider the wet pavement situation worthy of a significant factor. Note that the caution only mentions wet grass or wet ground, not wet paved surfaces.
You could also try this question in the flight instructors forum or the private flying one.

We don’t because it’s not required for the rules under which we fly and the rules under which the aircraft is certified.
Its that simple.
Air Transport Category certified aircraft are required to have all that information in their performance information.
Keep in mind that a 10-15% change in an aircraft that light doesn’t really amount to much.

Jhieminga is right. The specific question is "why don’t we use a factor for wet runway on take off." Unless there is something more to the question that we don't know, the answer is that if the surface is unpaved, grass (esp. long wet grass) or flooded, we should use a factor (easily researched and in the POH). If it's paved we don't need to worry too much unless there is standing water. Even flying off packed snow made little discernible difference to the take-off distance [unless there was a crosswind :} ] but landing would be a whole different thing.

Other then the note on wet grass the DA42 manual has NO wet runway performance numbers.



Care to share what this nugget of wisdom is based on?
The manufacturers are not required to provide the performance numbers.

This way out of my league, but as a thought:-
The wet takeoff 'adjustment' in large aircraft is generally in the RTO data - just use wet charts. Opposed to wet 'factoring' of dry landing data.
Does the aircraft have required takeoff distance charts with RTO dry/wet adjustment; if so no factoring required, just use the wet chart.
Wet take off may not be a problem, but wet stopping, RTO could be.

Basically, the DA42 is a light twin certified to and operating under a different set of regulations to transport category aircraft. You don’t need to worry about V1, screen heights or second segment climb performance.

A wet paved runway makes little difference to take off distance unless there is significant standing water, it’s the braking performance if you need to stop that is important. Wet grass would probably be the equivalent of a contaminated runway for a large aircraft.

Paved runways are generally longer than grass ones, and other factors such as weight and density altitude are usually more prominent.

Examiner: “why don’t we use a factor for wet runway on take off?”

Applicant:..because it’s not required.

Next question please.

kris, #8, it is hard to believe that a light twin-engine trainer, 'under different rules', can be operated without vmcg, V1, rto, etc.
How are pilots trained re these concepts, and practice the procedures which will be required in larger aircraft.

It would be reassuring if an instructor on type will answer the conceptual and practical training aspects; then the astute question #1 might only be an issue of wording, terminology.

A further concern is the current lack of answer, and simplistic 'not required' statements without any 'because' explanation - no better than admitting I don't know.
I don't know, but would consider that a landing factor of 1.15 should be applied as airmanship for the take-off rto case:-

GM1 CAT.POL.A.335 Landing – wet and contaminated runways
LANDING ON WET GRASS RUNWAYS
…
(b) As it may not be possible for a pilot to determine accurately the degree of wetness of the grass, particularly when airborne, in cases of doubt, the use of the wet factor (1.15) is recommended.

------
P.S.
Assuming the relevant AFM is: http://support.diamond-air.at/filead...8-complete.pdf
The text on page 5-11 (doc page 243) adds context to the question.

CAUTION
The figures in the following NOTE are typical values. On wet ground or wet soft grass covered runways the take-off roll may become significantly longer than stated below. In any case the pilot must allow for the condition of the runway to ensure a safe take-off.
NOTE
For take-off from dry, short-cut grass covered runways, the following corrections must be taken into account, compared to paved runways (typical values, see CAUTION above):
- Grass up to 5 cm (2 in) long: 10 % increase in take-off roll.
- Grass 5 to 10 cm (2 to 4 in) long: 15 % increase in take-off roll.
- Grass longer than 10 cm (4 in): at least 25 % increase in take-off roll.
- On grass longer than 25 cm (10 in), a take-off should not be attempted.

Although no wet 'factor' is given, which is surprising when discussed under a caution, the cautionary text might be read as covering the grass factors given as a note below. Thus there should be an increased distance on wet grass, but no factor is given.

P.P.S. Reading page 244
NOTE
For wet grass, an additional 10 % increase in take-off roll must be expected.
Back to #1, with EASA 1.15 factor or AFM 10% expectation.

First, can I commend safetypee's observations and thoughts - he has a lot of very appropriate runs on the board in this sort of area over many years and his counsel is well worthy of our attention.

My thoughts, as a sideline -

(a) why don’t we use a factor for wet runway on take off The standards for scheduled performance differ significantly between lighties and big iron and it is worthwhile having a read through both Part 23 and 25 (along with the relevant flight test guides) to get a basic overview of the differences. A rather decent read, though, and you can anticipate going through a large jar of coffee in the execution. One needs to note that, should we be talking about a particular aircraft, it is necessary to do some homework to identify which revisions of the design standards might be the most appropriate to review.

Basically, it comes down to using AFM/POH guidance, should it be available, or standard factors, such as safetypee has suggested or, if you have some mathematics in your background, doing some sensibly conservative sums to give yourself a bit of a pad. Blindly launching in blissful ignorance of anything to do with runway contamination probably is not a good idea.

I had a quick look at the DA42NG book from the OEM site and it seems to provide a better than average set of guidance compared to what we might see in many other light aircraft POHs.

(b) In any case the pilot must allow for the condition of the runway to ensure a safe take-off. Be wary of recommendations for surfaces such as grass. The data is based on presumptions as to coefficients of friction (rather than grass lengths, etc., although reasonable if from a respectable source). Please do be both careful and conservative. I note that the OEM has provided both ground and air distance data so we have the basis for some sums to target a go/no go lift off point decision for strips which might be approaching limiting lengths; better to call the reject and hit the fence at low speed rather than continue and hit it at high speed ?

(c) The take-off distance on a paved surface is not influenced to a very great degree by water on the runway That might be a reasonable call, depending on what kit the aircraft may have. I'd rather give myself a bit of fat to keep the excitement level down to a sensibly calm level, suitable for old chaps. If it's wet seal, and I don't have anything else to work with, I'd still want to give myself a pad, even for the continued takeoff case. Just because there might not be a rule saying I should doesn't preclude my doing so.

(d) I suspect that Diamond did not test these situations and therefore will not be able to provide any factors for those situations Refer to (b). The best we can do for grass surfaces is make some calculated guesses as to what might be good coefficients of friction for the sums and then come up with fudge factors based on the sums. Just what you might get for a given surface may be somewhat different to what the OEM opines. There are techniques whereby you can run highspeed taxy tests to get an idea but that is probably not useful for the typical light aircraft pilot. I recall reading a rather old report which looked at this for a single engined military jet.

(e) We don’t because it’s not required for the rules under which we fly and the rules under which the aircraft is certified. Its that simple. As I observed earlier, just because the rules don't require a pilot to think, sometimes, perhaps it might be a good idea anyway ? If it all comes unstuck, you're dead, not the Regulator who decided that a rule wasn't appropriate.

(f) You don’t need to worry about V1, screen heights or second segment climb performance. The POH might not have data if it's not required by the design standards and we all have a handle on why that is so. That doesn't mean, though, that we ought not worry about such things or, at the very least, have a bit of a think about them and inject some conservatism into the operation.

We need to define the parameters of what we are trying to argue here:

Light GA private pilot owner flown, none of this is required.
Not saying it’s not recommended but that’s something different.
T/O distance ground roll + landing distance ground roll + 20% is a safe runway length in my book.