I am with @boscomantico on this one. There was a very good article once in the security section of “Info Pilote” that has not only examples of planes that never got airborne from soft fields, but also a mathematical approximation for the drag calculations. The takeaway is that from a certain point of softness, the drag rises exponentially and you reach a state where it is impossible to get airborne. Even if your runway were 5 km long.
That is a relation which cannot be reflected in any percentages. And it will never be an exact science, not because of incomplete POH data, but because it is impossible to quantify and normalize all the different kinds of runways and runway conditions.
One instance where I could see these calculations even as a detriment to safety is if you think you have a good margin and you set out with the expectation to get airborne, but conditions are such that it takes you a lot more ground roll, you may be surprised and abort too late. Because you are primed incorrectly on the takeoff being possible.
Ok, point taken. You mean field conditions which by common sense imply it’s impossible to take off from, in which case you don’t even pull out the POH, because it needs either to be cut or dried out or both. I agree, in this case the POH is useless.
Yes, but it’s not black and white but rather a gradual scale of increasing drag.
Rwy20 wrote:
That is a relation which cannot be reflected in any percentages. And it will never be an exact science, not because of incomplete POH data, but because it is impossible to quantify and normalize all the different kinds of runways and runway conditions.
I’d have to look up some of my POH’s but I think some manufacturers give a lot more information than just dry asphalt. And I think it should be like that too.
For the big irons we had mostly:
- dry concrete
- wet concete with different depths of water
- compacted snow
- slush/wet snow
Now clearly, we are not going to get all that for our small planes, but it would be nice to have at least factory test flown factors for dry concrete, wet concrete, standard grass runway and soft field. Clearly, snow is a bit difficult to get by in Kerrville and some other places where airplanes are made :) . Not that we would go to extremes to prove those figures but it would be nice to know and apart, if something ever should happen, I would not like having to explain to the investigator or insurance assossor why I took off on a grass runway when I had no performance data for them.
Other than that, I would have to assume that everything not included in the POH is not allowed. In fact, that seems to be what Mooney had in mind without saying so, they were not enthusiastic about grass runways because of the gear doors and they don’t really want you to use it on a soft field because of the dirt which gets sprayed onto the stabilizer and wheel wells. Heaven knows what other manufacturers reasoning was for not including such stuff.
I put a request in at Mooney if they have something on this subject. It would be interesting to hear it from them.
This would certainly qualify for an increased drag:
Quite interesting is this one:
and this one:
And it will never be an exact science, not because of incomplete POH data, but because it is impossible to quantify and normalize all the different kinds of runways and runway conditions.
Grazie! I would never thought that anybody would not see that straight away. Well, again what learned!
There are also runways with varying degrees of slope. Impossible to calculate.
One instance where I could see these calculations even as a detriment to safety is if you think you have a good margin and you set out with the expectation to get airborne, but conditions are such that it takes you a lot more ground roll, you may be surprised and abort too late. Because you are primed incorrectly on the takeoff being possible.
Exactly. The 7% extra in that Mooney manual or those 20% extra in that CAA table for dry grass runways are highly dangerous, as it could make people think they were fine after making their “scientific” takeoff run calculations (probably happened many times).
A few years ago, I took of from Castiglione del Lago, near Perugia. The grass wasn’t exactly short, but neither tall at all. Just very “thick” and “healthy” in type. The ground wasn’t wet. It was dry. But the ground was still somewhat “soft”. Guess what? It took me 600 meters to get airborne (compared to 300 meters which it would have taken on tarmac). That was a 100% extra, on a dry and reasonably short cut grass runway in decent condition. Again, the Cirrus and some T-tail aircraft may be particularly susceptible to certain runway conditions, but surely, many other aircraft would have taken at least a 50% extra over tarmac in this situation as well. Someone who added only 7% or 20% in this case would have been caught out badly. Judgement. I can only advise pilots who have less experience with grass runway operations to be very careful with these. It’s very rewarding, but be careful.
P.S. I was obviously not talking about runways such as the ones above. Just your normal everyday European grass runways…
Mooney_Driver wrote:
Clearly, snow is a bit difficult to get by in Kerrville and some other places where airplanes are made
Likewise 9,000 ft density altitude runways in other places where aircraft were made. I may never fly off a grass runway again (I’ve only done it once) but I fly sometimes from hot high runways beyond that which the aircraft manufacturer ever tested the plane. The advantage of that scenario relative to muddy ground is that you can reasonably calculate ground roll by extrapolation from sea level to altitude, assuming you have a constant speed prop which makes the horsepower calculation simpler. In either case you don’t have the data in the POH.
Don’t let EASA know you can’t predict takeoff roll for every airfield on every day of the year… they’ll outlaw grass runways 
Some other “grass” threads here and here
What size runway? If we could make our currently “ok” grass runway “very good” for 30k, it is likely that we would.
I started a new more specific thread here
But most grass strips are poor – precisely because it is damn hard to find a group of pilots (with 50k-150k ultralights, or anything else for that matter) to stick their hands in their pockets for a few k each.
And that is the biggest problem with flying into unknown grass strips. Almost their entire purpose is to save money. Sometimes people go there to get away from airport politics (only to find other politics… they are often dominated by forceful characters). But the biggest common denominator is that most pilots don’t want to contribute to a common facility – just so long as it is just good enough to not smash up their own plane. Elstree, Spanhoe…? So you get potholes, bumps, dips, rocks… A toilet so bad you need rubber boots to use it. Transitions between grass and concrete are prime spots for prop strikes.
Numerous pilots have left Shoreham due to its £30 landing fee (I think it doubled in the last few years) only to find themselves at some waterlogged-for-3-months dump with no avgas, lugging jerrycans about in a 4×4, and then a few drift back to Shoreham and buy shares in some old wreckage so they can keep flying…
And this isn’t just my opinion. If we did a EuroGA fly-in, 1/3 will drop out if the runway turns out to be grass. Owners clearly don’t like risking it, if the place is unknown.
I take the liberty to argue the last point: a fly-in at LHSK Siofok with its 2000m grass runway would not deter many, I think. I remember rumours of even Citations operating there.
The subject of toilets at the airfield is a bit delicate, but in a way crucial: my field only has toilets in the (independently operated) bar. So when the bar is closed there’s no facilities at all, which can be annoying to a man, and must be worse for the fairer sex. Perhaps one reason we see so few of them? The operator (club with more than 100 members) could easily afford to build something, but I am sure they don’t for fear of seeing it become a shambles – pilots are indeed reluctant to pay up, but I am afraid many can’t even be trusted to keep a public toilet clean. But perhaps this topic merits a thread of its own?
Peter wrote:
Spanhoe…?
The grass runway at Spanhoe is in excellent condition. It’s well rolled, grass is kept short etc. and it dries out reasonably fast. The concrete (former peritrack) is kept clear of FOD even after (especially after :-)) Colin Furze has been on it, and has no pot holes. The toilet doesn’t need wellies to use.
Most airframes at Spanhoe are in the £20k-£40k range, it’s not an airfield full of rich people with £millions to throw around to spend the best part of a million quid on a proper hard surfaced asphalt runway. Especially because the majority of aircraft based there are Austers which were designed to fly out of unimproved airfields so would not get any benefit anyway.
Many pilots are reluctant to pay because they are already seen as easy marks for charging large amounts of money and are frankly tired of paying top dollar for very poor service, and don’t have any money left once they’ve flown their plane. There’s also the uncertainty of actually getting a return on what you pay, any airfield is just a few complaints away from very expensive legal actions to get them shut down by neighbours.
