by Ken Ibold
Tucked into a pocket in nearly every airplane is a book claimed to hold the secrets of what that airplane can do – and whats beyond its capabilities. From engine start to shutdown, the pilots operating handbook (or aircraft flight manual) promises to give true and accurate indications of the airplanes speed, fuel burn, takeoff/landing performance, load-handling ability and operating procedures.
Most pilots, however, take the book numbers with a grain of salt. Cruise speeds, for example, are notorious for being optimistically presented in the book – if theyre even presented at all. Fuel burn figures may apply only to a well-instrumented engine being flown at mixture settings that may produce a rough-running engine. Takeoff and landing, well, the airplane was certified using expert pilots who had lots of time to get the best numbers possible.
In planning for flight, pilots must consider that, while the book is the Bible according to the FAA for performance information, it necessarily puts the airplane in the best light possible. The figures do not all come from test flights; some values are calculated on the basis of test flights. The pilots are pros. The airplane, prop and engine are new. And sometimes, it seems, all possible rounding errors are made in the most beneficial direction.
For all of those reasons, it makes sense to come to some conclusions about the airplanes performance for yourself – to double-check just how close you can come in your airplane to reproducing those numbers. Youre not a test pilot and your airplane is probably a few years old. The question becomes not, What does the book say? but rather, What can I get out of my airplane today?
We recognize that there is huge variation in POH/AFMs. Some are little more than a collection of placards while others are more involved. We have the manual for a 1974 Cessna 172 that spans merely 80 pages. A 1982 Cessna 182, however, carries a book thats an inch thick.
Among the inaccuracies of the airplanes paperwork is likely to be the weight and balance form. Airplanes tend to gain weight as they age, if for no other reason than the dirt that collects inside the wings and fuselage. However, most that have been conscientiously maintained are accurate enough that the weight gain is lost in the noise level of the loading chart.
More important for most operators is takeoff distance, airspeed, fuel burn and landing distance. Those are the most marketable commodities the airplane has and they are also the areas where pilot technique can cause huge variations. Here, then, are some methods of determining how close you can come to your airplanes book numbers.
Since physically measuring the takeoff roll is difficult under most circumstances, the most convenient way to measure takeoff performance is to compare the roll to the known dimensions of the runway.
For this test, pick a runway served by an instrument approach. The runway markings are the key to making the measurement.
There are two sets of stripes of interest for this test, the runway centerline and the aiming point marking. Threshold markings and touchdown zone markings can also be used, but theyre not necessary.
The key measurements here are that the runway centerline stripes are 120 feet long and there is an 80-foot space in between them. So the start on one stripe to the start of the next is 200 feet. Simply position yourself on the beginning of a stripe, execute the type of takeoff you wish to measure, and count the number of stripes that go by.
If the runway youre using is longer than 4,000 feet and is used by jets, it probably also has aiming point markings. These are 150-foot long stripes on either side of the centerline. On a 150-foot wide runway, the stripes will be 30 feet wide and will go to within nine feet of the runway edge. On narrower runways, they will not be as wide and may be closer together.
These stripes start 1,020 feet from the beginning of the runway, which make them a good measuring point for airplanes with ground rolls of 1200 feet or more, because you dont have to count centerline stripes during the initial roll when you should be checking engine gauges and maintaining directional control. Just make sure you have a good feel for how far from the runway threshold you start the roll.
Note that the measurements from the runway threshold are from the start of the runway, not from the threshold bar or threshold markings.
Airspeed that seems to vary from book figures can be the result of various errors.
Your tach or manifold pressure gauge may be inaccurate, causing you to be running at higher or lower power settings than you intend.
You can check your tach by making a full power runup (taking care to avoid debris-strewn areas that can damage the prop) and noting whether the airplane makes full rated power. But the failure to get maximum doesnt necessarily mean youre maintenance bay-bound. The allowable range may be from redline to nearly 200 rpm less than redline. Note also that static rpm will be higher when its hot out than when its cold.
Your manifold pressure gauge can also fail. We once owned a Mooney that wouldnt meet book speeds but consistently burned less fuel than expected. Turns out the manifold pressure gauge was reading nearly an inch high, causing us to run with about 3 percent less power than we thought. Fixing the gauge bought us nearly five knots.
Airspeed is also dependent on how precisely the airplane is leaned. Performance data comes from best power mixture. Running richer or leaner will necessarily mean a slight reduction in cruise speed. Depending on the instrumentation involved in your airplane, mixture control may be a guess. If no instrumentation is available, lean a fixed-pitch prop to peak rpm and a constant speed prop to the point it runs roughly, then enrich just enough to smooth out the engine. Better yet, use an exhaust gas temperature gauge. Best of all is an engine monitor.
Even with correct instrumentation and operation, there are still places for speed-robbing gremlins to live.
Dings in the wings and nicks in the prop reduce the efficiency of the airplane. Improper rigging of flaps, ailerons, elevators and gear doors can add huge amounts of drag. If your airplane wont trim to straight and level flight, you may have a rigging problem.
To test airspeed, pick a calm day and make a test flight. Pick a direction and fly at your desired altitude and power setting. Fly the compass heading, not the ground direction, and let the wind do whatever it wants with you. Note your ground speed via GPS or by timing your flight between two objects a known distance apart. Then pick the reciprocal heading and do it again at the same altitude and power setting.
By not correcting for wind drift, the two runs will cancel the wind components out. A GPS-generated ground speed of 120 knots in one direction and 140 knots in the other means your airplane was flying 130 knots true airspeed.
The amount of fuel youll burn per hour – or on a given trip – is closely related to the vagaries of power setting mentioned earlier. Only by precisely setting power can you home in on what your airplanes fuel burn actually is.
Some airplanes are equipped with fuel flow meters that are of questionable value. They are good enough to approximate a power setting that is then refined with the EGT gauge, but theyre not accurate enough to rely on burning 14.5 gph just because thats what the gauge says.
Regardless, experience is the best teacher when it comes to determining fuel burn. The clearest way to do that is to dedicate several flights to measuring fuel burn – provided you have a more than one fuel tank and can select individual tanks.
First, top off all tanks. Dont ask the line guys to do it unless youre watching. Top them off, then check them for crossfeeding and top off again if you must.
Take off and climb to altitude, then switch to a different tank upon leveling off. Cruise for one hour. At the end of an hour, switch to a third tank for descent and landing. Then top off those three tanks again and note how much fuel was burned from each. The first tank is fuel burned for climb to altitude, the second is cruise fuel flow in gallons per hour, and the third is for descent and landing.
If you only have two tanks, you must make multiple flights, or you can combine the climb and descent on one tank. If you have only one tank or cannot select individual tanks, make the climb and immediate descent, then top off. Then do it again, this time cruising for an hour.
Crucial to good fuel management is the need to precisely measure how much fuel is in your tanks. Some airplanes are particularly vulnerable to fueling errors when the wings are not level. Others may have bladders that are wrinkled or partially collapsed.
Ensure your tanks are in fact full when the fueler thinks hes through. At the same time, dont be fooled by fuel receipts from operators that go by the notion that a gallon spilled is a gallon sold. In some ways, self-serve fueling is the way to go, particularly during these test flights.
If you ever find yourself flying with less than full fuel, due to payload or logistical reasons, it pays to use a calibrated dip stick to determine precisely how much fuel is aboard. Doing the actual calibration is a pain, and once again it pays to do this either at a self-serve pump or at a quiet airport where the fueler doesnt have nine other airplanes waiting for service.
Starting with a dry tank, add a gallon at a time and stick the tank between each gallon. Mark off the level for each gallon. You can use a simple wooden dowel for this job, or you can buy glass tubes from pilot supply shops.
Knowing precisely how much fuel you have aboard – and just how fast youll burn it – will pay dividends when you try to make maximum endurance flights. Use the book numbers and youll either make unnecessary fuel stops or will stop prematurely when the tanks run dry.
Landing distance can be calculated in much the same way as takeoff distances, with the exception that its a bit tougher to consistently hit a spot from which you start counting. In addition, the landing flare is not the time you should be trying to count runway stripes.
Use the airport measurements to your advantage to assess landing performance, but please do it with an observer who can count while the pilot simply flies.
One of the primary factors in landing distance variations comes from poor airspeed control on final or else an inaccurate airspeed indicator. In most handbooks, the landing distances come from a pilot who is well practiced in short-field technique.
Another source of error comes when pilots neglect to make the conversion from calibrated airspeed to indicated airspeed. For example, a Piper Arrow handbook lists an approach speed of 90 mph. With a stalling speed of about 62 mph, most pilots carry about 65 to 70 mph into the flare.
However, flying 65 mph indicated means your calibrated airspeed is more than 70. Indicate 60 and your calibrated airspeed is 67 – which allows for a much better touchdown without float.
When using the results of your test flying in the real world, keep in mind that obstructions can dramatically challenge both your approach path and your sight picture when landing. Also, keep in mind that these landing distances apply to the point at which you touch down. Float a quarter of the way down the runway and all your test flights are garbage.
Note also that approach speeds and stall speeds are weight dependent. A landing distance figure thats based on an approach by a max gross weight airplane loses validity unless you adjust it for weight reductions. If youre flying at light weights, adjust the calibrated airspeed by making a ratio of actual weight divided by max gross weight. Take the square root of that. Multiply by the calibrated airspeed. The result is a calibrated airspeed for your actual weight, which must then be converted to indicated airspeed using the conversion table in the POH.
In undertaking any of the test flying required to verify your POH numbers or expand beyond them, make sure your first responsibility is toward flying the airplane – especially around the runway. Use an observer if at all possible. If not, immediately abandon any efforts at collecting data at the slightest indication that youre getting behind the airplane.
After all, youre in this to rewrite your airplanes Bible, not to meet your maker.