Here’s a trick question: How do you how much runway you’ll need to take off today?
The gotcha answer: You don’t know. The best you can do is estimate performance based on the available data and then add a conservative safety factor.
Pilot Workshops recently posted a Tip of the Week about aircraft performance. You can find it here. (Full disclosure, I participate in the IFR Mastery series at Pilot Workshops, and I occasionally contribute tips of the week).
During stage checks at the flight school where I instruct, we ask pilots to calculate and explain takeoff and landing performance. When I review their numbers, I always ask about the assumptions behind the numbers. The discussion that follows typically goes something like this:
First, note that for most light aircraft, the POH/AFM includes takeoff and landing numbers only for short-field operations. You don’t typically find “normal” takeoff and landing data.
So the values you calculate based on the tables or charts assume setting the flaps as specified, running the engine up to takeoff power while you hold brakes, raising the nose at the specified airspeed, etc.
Is that how you typically take off?
The performance data also assume a new engine that produces rated horsepower, but none of us knows if our engine, today, is achieving that number.
Now, let’s do some basic arithmetic. The book for a C172S says that at sea level on a 20 C day, the ground run at 2550 lbs. takeoff weight is 995 ft. Call it 1000 ft. Distance to clear a standard 50 ft. FAA tree is 1690 ft. Again, assuming precise short-field technique.
Knowing the above, most pilots tell me they’d add 10 or 15 percent to those numbers as a buffer. But 10 percent of that 1000-ground roll is 100 ft. Ten percent of 1690 (call it 1700) is 170 ft.
The C172 takeoff table assumes liftoff at 51 KIAS (86 ft/sec) and 56 KIAS (95 ft/sec) at 50 ft. In other words, as you lift off and climb out, you’re covering about 100 ft/sec. In three seconds (“one-potato, two-potato…”), you fly the length of a football field (or soccer pitch). Have you watched runway stripes go by as you try to urge the airplane aloft or float a bit on landing? Runway stripes are typically 120 ft long, with 80 ft gaps between them (minimum stripe length is 80 ft).
Similar considerations apply when you estimate how much distance you’ll need to land.
These numbers are one reason that I recommend viewing REALITY CHECK: TAKEOFF AND LANDING PERFORMANCE from the AOPA Air Safety Institute and then following the long-standing AOPA ASI advice to multiply your takeoff/landing calculations by at least 1.5.
And for a more detailed discussion of aircraft performance, see Performance: What Matters Most by Catherine Cavagnaro in the July 2021 issue of AOPA Pilot magazine.