South west Scotland has been a no-fly zone for the last five weeks or so because folks in the Caribbean keep sending us their used hurricanes - their latest offering was called Abigail, but there are others queued up behind her like city buses to somewhere you don't want to go.

So, "cabin fever" is taking hold here and I've been wondering (thinking is too grand a word) about back-country landing site evaluation, especially for steeply-sloping strips.

The first question is obviously "can I take off from there?", but I'm going to dodge that issue for the time being.

The next question is "can I land there in one piece?", which boils down to two considerations:
- Is the landing surface suitable?
- Is the landing site long enough for my airplane in the current conditions?

We can inspect the surface and in some cases feel or "drag" it, which is usually satisfactory - although some granular materials can surprise after we're stopped (how to detect quicksand, for instance?).

The traditional (FAA/CAA) way to estimate ground roll required is to look in the Airplane Flight Manual for data complied on a flat lowland concrete runway by some long-dead test pilot and extrapolate from that by piling on a heap of approximate "fudge factors" for aircraft mass, density altitude, tail-wind, runway slope, runway surface (grass, gravel, wet or dry), and... have I forgotten something, apart from the fact that I would search in vain for any kind of performance data in my AFM...

Armed with this fine approximation of an approximation of the approximate landing roll required by an approximately similar airplane we can do what? I think most of us just look at a back-country landing site and use our experience to judge "GO" or "NO-GO".

That kind of gut-feeling served the world's engineers pretty well until Galileo and Newton came along to analyse how a pendulum works and why an apple drops from a tree, but now that we learn that good stuff in school, why not put it to use for flying in the back-country?

Jacko's thesis, somewhat inspired by the aforementioned cabin fever, is that we can make a decent stab at landing site evaluation by carrying out a simple in-flight stall test and then applying the principle of conservation of energy:

Kinetic energy at touch-down (KE) = potential energy gained running up the slope (PE) + work done against the brakes (Work)

(Note that if KE is less than PE, then instead of braking we will use some engine power to get up the slope).

KE = 1/2 m V^2 (0.5 times MASS times TOUCH-DOWN SPEED squared)
PE = m g H (MASS times ACCELERATION DUE TO GRAVITY times HEIGHT GAINED)
Work = F D (BRAKING FORCE times GROUND ROLL DISTANCE)
But maximum braking force is = m g µ, where µ is the coefficient of friction between tire and runway surface

So we can write our energy equation as:

1/2 m V^2 = (m g H) – (m g Cf D)

Dividing by m and re-arranging we have:

µ = (((V^2)/(2 g))-H)/D

We can estimate touch-down speed V directly from GPS ground speed with the airplane stalled at a safe height above the runway. This allows for density altitude, but we may need to allow a little more or less for wind speed at ground level, and of course stall in ground effect is a few knots slower than in free air.

The formula tells us (in theory) whether we can stop in the available runway length with the main wheel braking friction estimated to be available/usable according to the "runway" surface and aircraft configuration.

In case anyone has got this far, I've put a spreadsheet calculation of the above formula at:

https://docs.google.com/spreadsheets/d/ ... cslist_api

...and I'd be interested to know if anyone else finds it (a) reasonably accurate or (b) any better than just eyeballing and voting yes or no.

I'm afraid this is in metric units, but I can easily do a version in US units (feet instead of meters).

N-Jacko wrote:So we can write our energy equation as:

1/2 m V^2 = (m g H) – (m g Cf D)

Dividing by m and re-arranging we have:

µ = (((V^2)/(2 g))-H)/D

That looks like airliner speak... If I had to try to work that stuff out I think I would never pull the bird out of the hanger.

I'm a bigger fan of, "I guess that looks ok? What's the worst that could happen?"

I like your thinking - math and science is my go-to approach also.

Trouble is, all the implicit assumptions necessary to use such simple math, mean the result is all but useless in a real world situation. You could set up a computer to calculate an answer for minimum distance, but then you have to add a large "fudge factor" - which is probably much bigger than the base answer, so why bother.

I think a rule of thumb would probably be the most helpful tool in those situations. e.g. for every 5 degrees up-slope, subtract 20% from required landing distance. Take-off has less advantage due to down slope, because of drag, which helps landing but hurts take-off, also the vector of the kinetic energy is going the wrong way (downhill). Naturally, you need to include your personal fudge factor.

The implicit mathematical assumptions I refer to are ones like:
- 100% of energy is transferred to brakes or potential energy.
- perfect braking action
- instant application of 100% brakes on touchdown and during entire roll-out
- you can perfectly judge at least two of: length, angle, height gained
- the basic formula for static friction holds true in a real world situation (it doesn't)
- you don't lock the wheels by applying full brakes, else you need a different friction coefficient
- no bounces etc
- no mistakes on approach speed and hitting touchdown point

Yes a computer could be used to model all those things, but the mistakes make all the difference.

Example:
I need 50m to land a dead-empty Bearhawk with no mistakes on my part, and >100m with any kind of meaningful mistake, more if I really balls up the touchdown. On a one-way uphill airstrip, by that point I am committed to landing or crashing. So why bother calculating that I require 48.9m minimum, when I know I need a safety factor of 210% at times.

I had no idea there was still places left which allowed for off-airport flying in Scotland?? Private farm land only?
Videos please!

I don't know the math, but I have found the gravity thrust help taking off down hill more dramatic than the slow you down help landing up hill. But having poor brakes on crop dusters, I never used them much. No need up hill, right? Or if you actually land slow where you wish?

I'll leave the math to others, but will say from time to time I get all focused/tunnel visioned on the first landing of a new site, only to realize that I had somehow not given sufficient thought TO THE TAKEOFF Obvious, sure, but I'll be so wrapped up in eyeballing things ( no math involved though) for the set down that getting off again takes a backseat. No choice of course, short of hiking out , but to make the best of it, and resolve again to not forget to give the TO some thought also. The wind of course, usually combined with a one way up hill landing, is the major factor.

I prefer to see tracks already there.

gbflyer wrote:I prefer to see tracks already there.

Especially on skis! Better to drag, evaluate it from the air, before a full stop!

gbflyer wrote:I prefer to see tracks already there.

"Never be number one to land" is good advice.
You should already have a pretty good idea of your airplane's capability based on airport performance.
The unknowns are the surface (which is gonna stay an unknown until you walk it drive it or land on it), and the available length. However, you can come pretty close to determining the length -- at 70 mph you are travelling 100' per second. At 87 mph it's 125 FPS, at 105 mph it's 150 FPS. So do a flyby and use the wing strut or whatever as a bombsight, click your stop watch at the beginning & end of the site and do the math. Slower is better as far as accuracy goes.

Battson wrote:
Trouble is, all the implicit assumptions necessary to use such simple math, mean the result is all but useless in a real world situation. You could set up a computer to calculate an answer for minimum distance, but then you have to add a large "fudge factor" - which is probably much bigger than the base answer, so why bother.

(snip)

I had no idea there was still places left which allowed for off-airport flying in Scotland?? Private farm land only?
Videos please!

Yes, most land in Scotland is privately owned, but permission is given more often than not and if the landowner is offered a ride he often asks to go visit a neighbour. This can produce a nice snowball effect. There's a choice running from immaculate silage fields to the roughest of rough grazing for deer or sheep, rock-strewn hillsides, tidal flats and river banks. One of my favourites is a local WW1 seaplane hangar floor slab which is submerged most of the year until we get a dry spell when the loch water level drops to expose 400 ft of 100 year old concrete...

It's very rare to see tracks off airport because there are so few bushplanes in Scotland. Anyway if someone has already landed in a place it's legally an "aerodrome", so it no longer counts as an off-airport landing.

I agree that however we go about it we have to make assumptions but whenever going somewhere new, I like to look on Google earth, from which available runway length and ground heights can be measured quite accurately. Then, as Jim says, if there's enough height gain up the runway and the touch down is slow enough, we don't need much braking so any assumptions relating to that hardly matter.

I also prefer thinking in terms of "how much braking might I need" rather than "how much spare runway length do I have", and checking actual stall speed above the field allows automatically for density altitude and aircraft weight. I think that's an old RAF trick, just modified to use GPS ground speed instead of indicated air speed.

ATB, Peter.

N-Jacko wrote:I also prefer thinking in terms of "how much braking might I need" rather than "how much spare runway length do I have",

ATB, Peter.

Fair enough too.

On a related note.
I always try to stop as fast as I can, each foot of ground I cover is another chance for either wheel to hit an unseen insurance claim. Of course on a nice paddock that hardly applied, but river bed or beach etc make me nervous every single time.

Landing on old track requires knowledge of previous pilot skill, aircraft, and if helicopter was used for aircraft removal!!!!
DENNY

That sounds like voice of experience from NZ and AK...

Changing the subject to take-off (the question which has to be answered first), how do you go about assessing that? Is actual rate of climb a good starting point?

I rejected a landing this summer at Tignes (http://www.panoramio.com/m/photo/24344927#) when my Maule was struggling at 11,000 ft DA and there was a slight tailwind for take off, but it wasn't a reasoned or calculated decision. I'd be interested to know how more experienced pilots make that kind of call.

Well, I can't help you there.

I think there is probably something to be said for learning in a lower hp plane, but I always had a fear of getting myself trapped some place remote hahaha

How do we determine landing and takeoff distance? By practicing and measuring, is the simple answer. That gives you a good starting point, but then as you say, some assumptions have to be made. In general, those assumptions are fairly conservative, especially if I bought the airplane .

My off airport experience began in aircraft types that provided precisely zero landing/takeoff distance data in the AFM. Couple of them had no AFM, one or two had a one page AFM with no reference to that kind of data.

So, you go out with your airplane and work it short. And after doing that, measure the results. you can do that with help from a friend or? Google Earth is a cool tool in this regard.

Then, you have to learn how conditions affect that performance....you noted DA, and that's a big one, but one of the big ones you didn't mention was wind. Learning to READ the environment for wind direction and velocity is an essential skill set as well. Flying seaplane really helps develop that skill, but any time you're out and about, pay attention to the wind, and try to find clues that you could read from the air.

And then, add a little bit of extra room for Mom and the insurance company and go for it.

When I first started landing off airport, I took forever to evaluate a site and commit to land there. The last time I took an off airport checkride, I think I spooked the check airman....but I was really comfortable with a quick recon. I had a lot more time in that type than he did.

As they say practice makes perfect....or at least as close as we'll likely get.

MTV

My experience has been similar to MTV's. It was helpful in Ag work to be able to make a takeoff and landing every thirty minutes or so. Also, the liquid load was easy to measure and manipulate. This meant that very accurate data could be compiled very quickly, even for a young pilot just starting out. No mathematical calculations, based on theory, ever gave me as much peace of mind.

MTV made a good forum post a few years back that I turned into an article for the Knowledge Base:

https://www.backcountrypilot.org/knowle ... t-landings

MTV's method obviously works, but I would liken it to making a ballistic table for long-range shooting by measuring actual bullet trajectory (windage and elevation) at various ranges.

That can be done, and it works after a fashion, but to an engineer it seems like the hard way when we can plug wind, range, atmospheric pressure, temperature, muzzle velocity and ballistic coefficient into a simple equation and kill or convert our V-bull sighters to finish our string before an unreadable squall catches out our competitors. Of course, we need experience (and more than a little luck) to estimate wind for long range shooting, particularly in the mountains.

An airplane is just another machine which obeys the laws of physics, so if we're happy to trust engineering calculations as we drive across a new bridge, we should be able to do the same for take-off and landing.

I'm not for a moment suggesting that experience is unimportant, but if we ignore the science and just treat off-airport flying as an art, we're just not using the tools developed over centuries by the human race.

I've found that the above formula correlates pretty well with actual landings, in spite of the seemingly broad assumptions involved (many of which tend to cancel each other). Doing the same for take-off shouldn't be beyond the wit of man. Our VSI gives an indication of net power available at that place and time, and all other data for the energy calculation can be measured on site or beforehand.

Unfortunately, like a lot of things, STOL ops ARE an art (and a black art at that) and not just science. That's why I can get into an airplane and take my best shot at a short takeoff or landing, and someone else can get in and easily best it. Ditto shooting- you can have all the data you want, but a lot of it boils down to the shooter.

The first time I ever saw a Helio Courier, I wondered hat all the hoopla was about-- it landed just about like a Cessna. Well, the pilot was flying it like a Cessna. Later I got a chance to watch a Helio being flown by someone who knew how to fly a Helio, and it was like two different airplanes.

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Gump

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