I didn’t say you can “rely” on it. I said you can use it as a guideline to start from.
boscomantico wrote:
Most probably, you “calculations” didn’t bear this out at all (probably more like 400m). Hence, it was useless.
I was neither the PIC on that flight nor did I do the calcs. The owner of that particular C told us prior departure that this was the normal condition here and he was flying in and out regularly. Ok, that is a 1000m runway so approx 600 m is not a problem per se, he was right. Basically he did it with your method: experience.
boscomantico wrote:
Flying (and in particular, soft field takeoffs) is not such about mathmatics as you want to make it. It’s more about judgement. Of course, experience helps as well.
Well, having been a dispatcher and working numbers for large jets amongst other machines I would question that. The difference I see is that many GA POH’s seem to be missing vital information. And the question is, how do you deal with that.
Basically, based on what I have in the M20C POH, I have data for take off and landing on a dry, concrete runway.
The “J” POH gives a correction factor for a dry hard grass runway.
Now, the question which has popped up all the time is, what do I do if I am outside those parameters? In commercial aviation, the answer would be clear: Not authorized. Meaning, those planes would be restricted to dry, hard surface runways. Obviously, that is not what happens, but how do you deal with this?
And moreover: If something happens, how are the CAA going to deal with you?
For those figures which are there, I fully expect them to be accurate. If not, why are they there in the first place?
I disagree with Bosco, to a degree. :)
The POH/AFM must be the starting point for any pilot, not least because type-specific limitations may apply. The POH/AFM also describes the correct technique to be used (I flew in a Chieftain with a chap a couple of days ago and he decided to do all his calculations based on the short field technique but then added 10kts to rotate speed!).
Once you have done your baseline calculations, there is then a very good set of recommendations made by the UK CAA.
I’ve used these guidelines for a long time and they are always safe. Note they also recommend application of the Commercial Air Transport 1.33/1.43 ‘fudge’ factors.
Would I take a Mooney not a 500m grass strip? No, because I’m reasonably confident that the POH and recommended allowances would give a number significantly greater than 500m. Of course, it gets a little more interesting when the book says that 501m will work. Then, in Dirty Harry fashion, you need to ask yourself “Do you feel lucky, punk?” 
Fly Safely
Dave P
Hello Dave,
this does look reasonable indeed. will look at it for further reference.
Dave_Phillips wrote:
Would I take a Mooney not a 500m grass strip? No, because I’m reasonably confident that the POH and recommended allowances would give a number significantly greater than 500m.
Again, “a” Mooney per se does not exist. There are models such as the C and E which can in most cases go to such places and get out again, there are others like the “K” or most probably the Ovations and Acclaims which definitly can not.
Actually, do I get this right? You take the factors and multiply them? So say for wet grass you take 1.3 and to that add again 1.33 for the commercial limits? So you end up with 1.73? or 1.6 for dry? Then indeed it will get very tight and probably would limit even STOL airplanes severely. In which case I’d have to agree that no Mooney (and not a lot of other planes as well) can get out of 500m grass…
You apply each factor. In other words, if your starting number is 100 and you have a 1.40 and a 1.33 you have 100 × 1.40 × 1.33 = 182 Yes, the numbers get big, quickly.
My personal experience is that it is rare for anyone to accurately achieve the POH/AFM figures; none of us are test pilots and most of us are flying aircraft that are not picture perfect. A good analogy would be achieving the manufacturers fuel consumption figures when driving your car. In other words, things are almost always going to be worse than the book, hence the 1.33/1.43 factors which are mandatory for commercial air transport. I think it could be argued that there might be some reasonable middle ground but this is all about risk management and ultimately that is a very personal decision.
PS. Yep, acknowledged that there are numerous varieties of Mooney and associated capability, hence my initial comment about using POH/AFM as the starting point.
Dave_Phillips wrote:
My personal experience is that it is rare for anyone to accurately achieve the POH/AFM figures; none of us are test pilots and most of us are flying aircraft that are not picture perfect.
Having figured that out I have repeatedly measured takeoff ground roll using Foreflight/GPS and Google Earth, and can use factors to extrapolate from that real data, for example to high altitude. I can check the accuracy of those factors in the same way, and now have a general idea of what I can reasonably achieve in my plane.
Dave,
ok, thanks, that is a starting point. I’ll have to play a bit with Excel to get a hold of it.
From experience, I’d say the figures in the Mooney book are quite accurate for the operation they are indicated for. I am in contact with them now to find out why they never published proper factors for other conditions.
What I heard from other Mooney users so far is, that it is not the prop clearance which most people are concerned about when operating on grass, particularly on wet grass, but the main gear doors and the gear well getting dirty. Well, my own M20C had been based for almost 40 years on gras, and not one I’d classify as particularly nice, and it has not taken any damage.
Grass grows. Underlying earth softens/hardens. You need a very recent report.
RobertL18C wrote:
The old rule of thumb to reject take off if you haven’t achieved 75% of lift off speed by half way may apply.
But if it’s wet and short, can you then stop on the remaining runway? My experience with grass strips is extremely limited, but I have had a couple of, ahem, surprises, so would treat them with respect.
The rule of thumb might work better for a Super Cub than a Lear Jet – 75% of around 50 MPH being only 38 MPH indicated.
