The latest AOPA Air Safety Institute in-person seminar, “Peaks-to-Pavement… Applying Lessons from the Backcountry,” was a big hit. The crux of the seminar was emphasizing that if certain tactics, techniques, and procedures (TTPs) kept you safe while operating in challenging backcountry conditions, then they’d probably also serve you well in more benign frontcountry flying.
I’d like to share some of my observations; a sort of PIREP summary of what I encountered during my presentations in six states/13 cities. Some caveats: This was not a Bush Pilot 101 course. We made it clear that this wasn’t a comprehensive how-to on backcountry flying, and stressed that pilots should seek qualified instruction before flying in the backcountry.
We also pointed out that the concept of “backcountry” has more to do with the condition of an airstrip, than its actual geographic location. They don’t have to be isolated, one-way in/out, dirt strips, at 9,000 ft MSL in the Rocky Mountains. You can have an airstrip that exhibits a lot of backcountry traits (short/soft/rough, sloped, obstructed takeoff and departure paths, no weather info or services available, etc.), located within earshot of the drive-thru window at your local, sea-level, Burger King.
I always enjoy poking, good-naturedly, at pilots who own their airplanes. It’s my contention that pilots ought to know their planes intimately, and not just from a seat-of-the-pants, stick-and-rudder proficiency standpoint. They should know everything about their plane that is, in fact, critical to their safe, proper, and efficient operation. The most important information is readily available; it’s right there in the Pilot Operating Handbook, or Owner’s/Aircraft Flight Manual.
It was pretty obvious that some folks hadn’t cracked open their respective book(s) in a long time. Those who had studied their documents, tended to be familiar with the BIG PRINT stuff, like their Normal Procedures sections and Emergency checklists, but were not so well-versed when it came to the various Notes, Warnings, and Cautions found throughout. There’s a lot of free, but hard-earned, wisdom in that fine print, all intended to protect life and limb.
When it came to aircraft performance-related issues, lots of folks either never, or at least rarely, applied the adjustments/corrections, as directed by the manufacturer.
It was also clear some pilots had been operating (successfully, since they’re still alive and kicking), mostly by using various combinations of ROTs (Rules of Thumb), WOM (Word of Mouth), TLAR (That Looks About Right), etc., and in some cases, the good ol’ WAG (Wild A** Guess)! Since that stuff worked for them so far, and nothing bad had happened yet, they saw no reason to change their behavior.
One of the key areas we focused on was short and/or soft field operations, and all the associated factors that can ambush you. I’d start this discussion with a trick question: “What’s the FAA definition of a short field?”
The answer, of course, is there isn’t one; the pilot decides what is short, and therefore it’s subjective. What may be short for someone flying an ancient Stinson 108 at max gross weight, may not be short for a guy in a featherweight Carbon Cub. Equally as important, what is short for you today, may not be short for you tomorrow (OK, maybe next week), based on your proficiency in short field ops.
Deciding what constitutes a soft field was a bit easier; we generally agreed that anything not paved was soft, although there are certainly some very hard, but unpaved, surfaces.
My next question: “What’s the effect of grass on takeoff and landing distances?”
Most would answer correctly that grass increases takeoff distances, including both ground run and distance to clear the proverbial 50 ft. obstacle. But, by how much made them pause: “It depends; is the grass tall or short, wet or dry?” was a typical, but correct, response.
The answer may, or may not, be found in the performance chart “Notes” section in your plane’s POH. Some aircraft manufacturers direct you to make very specific adjustments to the info you derive from their charts or tables. Others, not so much.
Cessna’s guidance varies from “…on a dry, grass runway, increase takeoff distances [both “ground run” and “total to clear 50 ft. obstacle”] by 7% of the ‘total to clear 50 ft. obstacle’ figure” (1973 Cessna 150 Owner’s Manual) to “…on a dry, grass runway, increase distances by 15% of the ‘ground roll’ figure“ (1976 Cessna Skyhawk Model 172M Pilot’s Operating Handbook.)
Having different correction factors for different airplanes makes sense, because they are different. The danger comes when you arbitrarily apply an adjustment from a previously-owned plane to the one you’re flying right now… and it doesn’t end well.
Beechcraft must not trust pilots to do math; they publish separate charts just for grass surfaces: e.g., “Normal Takeoff Distance-Grass Surface.” However, the really fine print in their Notes section says, “For each 100 pounds below 2750 lbs., reduce tabulated distance by 7% and takeoff speed by 1 mph” (Beechcraft Sierra 200 B24R Pilot’s Operating Handbook). Now that’s some precision flight planning guidance!
By comparison, the Piper Cherokee 140B Owner’s Handbook only provides a vanilla “Take-off Distance vs. Density Altitude” chart, with no guidance on adjustments.
At the extreme end of the manufacturer’s takeoff guidance spectrum, the one POH for all Piper Super Cubs simply states, “Don’t worry about it.” (I’m kidding.)
When it came to landing, a fair number of pilots were adamant that grass decreases landing distances.
No, it doesn’t. At least according to the manufacturers. I got into several heated discussions with folks who swore (while demonstrating it with their hands) that they could drop it in, over a 50 ft. obstacle, plant it firmly, dump the flaps, slam on the brakes, and stop less than 100 feet past the threshold.
I’d like to think I could too, but, for example, the Notes section on the “Short Field Landing Distance at 2550 Pounds” chart for a 180 hp Cessna 172S unequivocally states: “For operation on dry grass, INCREASE distances by 45% of the ‘ground roll’ figure.”
When landing our Cessna 150, the POH guidance is: “For operation on a dry, grass runway, INCREASE distances (both ‘ground roll’ and ‘total to clear 50 ft. obstacle’) by 20% of the ‘total to clear 50 ft. obstacle’ figure.”
Piper doesn’t even address landing on anything other than a “Paved Level Dry Runway, No Wind, Maximum Braking, Short Field Effort,” with power “Off” and 40 degrees of flaps, so it’s up to you to figure out your own correction calculus for any other combination.
Other aircraft’s POHs direct different adjustments; but in all cases, if they address grass landings, they’ll tell you to increase the distances – regardless of your technique.
Of course, the manufacturer is taking into account all the science behind the landing kinetics; boring stuff like mass, inertia, friction coefficients, braking effectiveness on different surfaces, etc. We all believe that with our finely-honed pilot skills, we can do way better than what their data says. But when it comes down to Art vs. Science, the latter usually wins.
My next question: “What are the effects of slope on takeoff and landing distances?” Now, it got really interesting!
Since not all manufacturers address the impact of slope on their specific airplanes, you really must contemplate the guidance that is out there and evaluate the TTPs you’re comfortable with.
Some commonly accepted, generic ROTs for the effects of slope, include the following relationships: a 1% upslope will increase effective takeoff distance by 5%; a 5% upslope will increase effective takeoff distance by 25%. (Note: Don’t ask me to quote a single source; there’s lots of reputable ones out there, including Wolfgang Langewiesche.)
If you further adjust/pad these numbers, or interpolate for a slope somewhere between 1% and 5 %, or greater (yikes!), the math overwhelmingly supports the commonly accepted notion that you “always” takeoff downhill and “always” land uphill.
But “always” isn’t the same as “always, always, always…” because there are times when it may be prudent to do the opposite: takeoff uphill and land downhill. Two of the key variables being wind direction and velocity.
Understanding the impact wind has on takeoff and landing distances was another area folks seemed content to guess at, instead of applying specific POH guidance.
The Cessna 172S “Short Field Takeoff Distance at 2550 Pounds” chart Notes state: “Decrease distances 10% for each 9 knots headwind.” You could be confronted with a situation where the 5% increase in takeoff distance caused by a 1% upslope, is more than offset by the 10% decrease in distance afforded by a headwind.
Even more interesting, the Notes also add: “For operations with TAIL WINDS up to 10 knots, increase distances by 10% for each 2 knots.”
Although it is absolutely desirable that you always take off and land into the wind, you might not have that option. The manufacturer may provide the data you need to make a pragmatic decision regarding tailwind operations. The real question is: are you comfortable and competent enough to do it? Or the more appropriate question: should you? The best answer might be, “Wait for conditions to improve.”
All these discussions led nicely into the next topic that people tended to guess about, possibly at their peril: can I clear that obstacle off the end of the runway?
Obstacles aren’t always the natural kind (high terrain, tall trees, Bigfoot, etc.); some of the most dangerous ones are man-made (powerlines, towers, bridges, buildings, wind turbines) because we’ve gotten so used to seeing them, they’ve become innocuous. For our discussion, “Spiraling up over the airfield” wasn’t an option; we’ll talk about the impact of density altitude on aircraft performance in another segment.
The concept of flying an Obstacle Departure Procedure (ODP) doesn’t just apply to IFR operations.
The challenge is that ODPs require understanding climb gradient requirements versus rate of climb requirements, and how the two are related. The idea of flying a self-engineered ODP, if required, to escape from an obstructed environment, shouldn’t intimidate anyone – if they know how to do the math.
There are two key ingredients needed to design your own ODP. First, you must calculate your ground speed. Second, you must know how tall and how far away the obstacle is, so you can calculate the climb gradient required; i.e., how many feet per mile you’ve got to climb to clear it.
Since we don’t have climb gradient “Feet per Mile” gauges in our planes, we’ve got to come up with a performance metric we can use: our rate of climb in Feet per Minute.
The math is simple: multiply your groundspeed (we’ll use 70 mph), by the climb gradient required (e.g., a mountain ridge, 800 ft. above field elevation, 2 miles away = 400 ft. per mile): 70 X 400 =28,000.
Take the “28,000” number, divide it by 60, and you’ve just come up with 467 feet per minute – the rate of climb required to clear the ridge. Having a vertical speed indicator is a bonus, but if you’re familiar enough with your plane’s performance, you shouldn’t need one.
The bottom line: don’t tempt fate. If you determine that you’ll need to generate a 467 foot-per-minute rate of climb, and on your best day, at sea level, your 65 hp Champ can barely wheeze at 200 feet per minute… don’t even bother starting the engine.
Note: if you want to err on the conservative/safe side, overestimate your groundspeed – that means you’ll be closing on the obstacle quicker and will require a higher rate of climb.
Of course, you can also just cheat and carry the descent/climb tables from a government approach plate book. They’ve done the math for you.
At the End of the Day
Depending on the make, model, and vintage of your airplane, there may not be any useful manufacturer guidance on performance computations. But understanding the basic concepts, like distances on grass increase, not decrease, is important. The exact adjustments required are going to depend on a lot of factors unique to your plane, your techniques, and the conditions. Don’t pick a random rule of thumb and apply it blindly to your circumstances. To paraphrase astronaut Matt Damon in The Martian, you need to “science the poop out of it,” before you’re past the point of no return.
Unfortunately, hope is not a strategy when it comes to flying safe, especially in backcountry conditions, where the margins for error are miniscule, and the consequences of small lapses in judgment, or errors in planning or execution, are usually catastrophic. So, please, always read the fine print.