September 1, 2009
By Bruce Landsberg
Don’t believe everything you read. Section 5 in the General Aviation Manufacturers (GAMA)-format pilot operating handbook (POH) is where the takeoff and landing distances, time to climb, fuel burn, and most other performance numbers reside. Most pilots, after spending a few hours with an aircraft, invest little time with the POH because they’ve learned the machine’s ways. But there’s a nasty little secret that most pilots can recite but perhaps don’t fully understand the ramifications.
The highly precise performance numbers derived from the charts or graphs in the POH are an engineering truth but mostly an operational fiction. If you fly an airplane built before the mid 1970s, and many of us do, it’s even worse since the handbook information is sparse—and that’s being charitable.
Here’s the disclaimer we can all recite: That on the best day of testing under standard conditions, by the manufacturer’s test pilot with a brand-new aircraft tweaked to top performance (engine, airframe, prop, and brakes)—the performance numbers were recorded and memorialized in the manual. Those conditional statements almost rival a politician’s explanation of the national deficit! Everyone knows that this isn’t reality for most of us and, yet, pilots are tested on them, and periodically an accident occurs because someone forgot the caveats.
Let’s start with the FAA’s knowledge test. There are questions which, quite rightly, are intended to measure our ability to determine if the aircraft is capable of performing a particular action. But, as sometimes measured, the questions can be misguided for two reasons. The questions are based on the results in the POH, and pilots may have to interpolate between values that are only a few hundred feet or a few gallons apart. In the world of professional flight, when the estimated value falls in between, the guidance is to use the worst case. There’s currently a question in the private pilot test bank that asks pilots to discriminate between 3.1 gallons total fuel burn on a 1,000-nm trip. That’s asking a bit much, in my view, since it implies a level of accuracy that normally doesn’t exist with the vagaries of winds aloft and temperature.
The airlines add 67 percent to flight manual landing distances, and to takeoff distances the airlines factor in stopping distance. If the runway is wet or there is a gradient that also gets consideration, but these factors typically aren’t in our POHs. GA numbers are based on level, paved, dry runways, but if it’s wet and slopes in an unfavorable direction, y’all be careful!
Most of the time, this isn’t a problem because we usually don’t fly close to the edge of the performance envelope. Time to climb usually isn’t critical, and stall speeds will be close to published. Likewise, the runway margins for takeoff and landing don’t matter on a 4,000-foot runway at sea level since there’s plenty of room under just about any condition. But start getting to the edges of the envelope and that’s a different story.
A disastrous accident happened a few years ago in Burnsville, North Carolina. A Columbia 350, flown by a 1,700-hour pilot with only 11 hours in make and model, attempted a landing on the 2,875-foot runway. The density altitude was about 5,800 feet. According to the POH, the aircraft should have been able to clear an obstacle and land, using the engineering numbers, with about 300 feet to spare. The approach and landing was, according to witnesses, a case study in how not to go into a short field with a terribly botched go-around that resulted in three fatalities and the destruction of several aircraft.
The pilot’s low experience in the aircraft is a solid predictor that he wasn’t proficient even though he had completed the factory school. The POH numbers showed that, had the approach been perfectly flown with maximum braking after touchdown, the required landing distance was around 2,600 feet. Pilots willing to fly with only 10-percent margins in critical areas, such as mountain runways, are periodically going to exercise the contractual aspects of their life insurance policy and give GA a black eye in the process.
Here the responsibility rests predominantly with the pilot, but I think the system played at least a partial role. As mentioned earlier, the FAA’s knowledge test forces us to predict with high accuracy how well a flight test pilot might do with a new aircraft. The sanctioned implication is that we can fly this well too. Demo pilots are also very good, possibly approaching flight-test levels. When I flew for Cessna our job was to make a prospective buyer look really good in the airplane and highlight how well the aircraft could perform using the engineering numbers. The customer’s reality was quite different from the fantasy that we were promoting. Yes, the aircraft could do that, but unless the pilot were up to it and willing to stress the machine to the max, perhaps not so much.
I’d like to see both the FAA and the manufacturers get together and come up with a more realistic way of determining operational reality. That applies to the knowledge test, as well. Personally, I prefer tables to multivariate graphs with five or six parameters squeezed into 5-inch by 8-inch handbooks. This information is too important, in a critical situation, to be fussing with pencil widths that might amount to several hundred feet.
As an aside, the takeoff and landing distances over the obstacle leave several rather important details to our imagination. The obvious one is that the obstacle might be a wee bit taller than 50 feet and secondly, clearing a 50-foot tree by as little as a 100 feet, will have most pilots lifting their feet and wishing for another 50 feet. The performance numbers assume no clearance above the obstacle. To be sure, most airports have something of a clear zone but it’s not often published so we’re left guessing. What might a 10-inch tree trunk do to a wing or landing gear at 75 knots?
Some numbers from a light jet manufacturer might cause some reflection: For every knot above VREF, landing distance increases by 120 feet, for every 10 feet above specified threshold crossing height add 200 feet, for every second beyond three seconds in the flare add 230 feet, and for every 1-second delay in applying braking, add 220 feet. It doesn’t take much variance—a few seconds here, a knot or two there and you’ve added 1,000 feet to landing distance.
Until the system addresses these issues—and that may not be any time soon—here’s how the pros would handle it. Dig into the POH and figure out the engineering number and then add 40 to 50 percent to it. In the North Carolina accident cited previously, the pilot should have started with a runway somewhere around 3,600 to 3,900 feet long (2,600 feet plus 1,300 feet). After getting to know his aircraft, it might be OK to drop that somewhat, but 10 percent really isn’t much of a margin—do you feel lucky?
Just as the EPA requires car makers to be conservative on fuel economy promises, your actual mileage and aircraft performance may vary significantly.
Bruce Landsberg was manager of Cessna’s education department prior to joining AOPA.
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