Cruise Control — Some tips to help you determine your airplane’s actual cruise speed

A first time buyer recently asked me to clear up some confusion about significant differences in cruise performance and range between a ’75 and ’76 model of an airplane he was considering. When reviewing some of the various web sites and basic information provided about the airplanes, he noted that the ’75 model could cruise at 150 mph for a range of 735 miles, but the ’76 only cruised at 130 mph for 535 miles. What had changed about the airplane? Nothing. In fact, the ’76 model was the preferred edition.

What did change was the method for reporting the performance data, as 1976 was the first year that the type, amount, and format used for publishing pilot operating handbooks was significantly changed and standardized to the requirements of the General Aviation Manufacturers Association (GAMA). The differences occurred because the ’75 airplane had it’s cruise speed shown in miles per hour, and range was based on statute miles with no fuel reserves. In ’76, the new GAMA format meant that cruise data was shown in knots (as well as primary airspeed markings), the maximum range at 75% power was represented in nautical miles, and factored in was a 45 minute fuel reserve: much more realistic.

This encounter brought to mind just how much general confusion there seems to be about determining actual cruise performance amongst airplane owners. I see this confusion quite often on Internet forums for specific type owners, and hear it in hangar flying discussions at the airport. One owner will report a cruise speed of 150 knots, where another owner of the same type will look on in shock as he only seems to get 125 knots. Then I start asking questions: are you talking true airspeed or indicated airspeed? At what altitude are you cruising? What are you’re power and mixture settings? Are you really talking airspeed or ground speed? The apples to oranges comparisons are simply amazing.

If you’re having difficulty determining exactly what your airplane is doing in the cruise department, you can take some specific steps to nail it down exactly. The first step is to refresh yourself on some of the basic types of airspeed. The first is Indicated Airspeed. This is what’s actually displayed on the indicator. Next is Calibrated Airspeed. This is your indicated airspeed corrected for any errors inherent in your pitot/static system, known at the time of manufacturing. This speed is determined by noting your indicated speed and then referring to a conversion chart in your owner’s manual/pilot operating handbook. For example, when the airplane noted above has an indicated airspeed of 110 knots, it’s calibrated airspeed is actually 109 knots. In my airplane, an indicated speed of 120 knots results in a calibrated speed of 118 knots. Minor differences, but still important.

True airspeed is what you really want to know. True airspeed is calibrated airspeed adjusted for altitude and temperature, to determine exactly what your plane is really doing through the air. As you gain altitude, and/or temperature, the air becomes less dense. For a non-turbocharged plane, this means that you will also experience a drop in available horse power, but you also experience a drop in drag. Most marketing departments publish the speed at the maximum altitude at which the airplane is able to achieve 75% power. At 2,000 feet, our subject airplane can deliver a 120 knot true airspeed at 75% cruise power, but it’s engine can deliver 75% power all the way up to 10,000 feet, at which altitude they publish a true airspeed of 130 knots. Less drag, same percentage of power, more speed through the air. Makes sense.

What you must realize, too, is that although your true airspeed will be 130 knots at 10,000 feet, your indicated airspeed might be showing only around 115 knots, so when determining what your plane can do you must set your standard and stick to it. My standard has already been developed. It’s called the owner’s handbook. So let’s find out what we’re getting.

An important step is making sure that your pitot-static system and airspeed indicator are in proper working order. A friend of mine complained about his plane being slower than others of the same type, and that he floated a lot on landings. Turned out there was a problem with his pitot tube and the airspeed indicator was reading low. Ok, so now we have a good working airspeed system. The next step is to break out your manual. Pick a percentage of power you want to use for your test. I’d suggest 75% power, but no lower than 65%. Then find the highest altitude at which your book says you can achieve that percentage of power at normal operating rpm. Don’t go to 2,700 rpm if you normally use 2,500 rpm. Find the highest altitude you can go to using 2,500 rpm. You also need to be sure that your power indicating instruments are correct. Many tachometers are 100 rpm or more in error. The same holds true for manifold pressure gauges. These have to be correct if you’re going to get true value out of your tests.

The manufacturers develop their cruise numbers by taking a new plane, with no antennas, flown at gross weight by a factory pilot, using precise measuring equipment. My airplane is 27 years old, with more antennas than a West Virginia house trailer, and a 27 year old pitot static system that, although quite accurate, is no factory data set. You must expect some differences.

Now you need to find a smooth day and go flying. Level off at your test altitude, set your cruise power, and lean the mixture to the “best power” setting recommended in your manual’s cruise charts. Let things stabilize for a few minutes and then take down three data points: indicated airspeed, outside air temperature, and the indicated altitude with your altimeter momentarily set (unless that’s your actual setting at take-off on your test day) to 29.92 inches. This yields Pressure Altitude, which is the standard the manufacturers use for performance figures. That’s all you really need for your test. You can try different altitudes and power settings if you want, but make them close to the figures provided in your handbook so you have a comparison base.

Back on the ground, you can get out the airspeed calibration chart from your handbook, your trusty E6B computer or electronic flight calculator, crunch your true airspeed, and see how you did. Many Loran and GPS units have an E6B function with which you can crunch your TAS on the spot. Depending on the depth of data in your handbook, you may still note differences for a number of reasons. (1) The older the airplane, the more likely the numbers published were on the “optimistic” side of the curve to satisfy the marketing departments. (2) Not only are standard altimeter settings used for data points, but also standard temperatures. If your temperature at altitude was higher than the standard for that altitude, your true airspeed may be a knot or two lower. This also impacts your climb performance. In my plane, 30 degrees above standard temperature will rob me of almost 100 feet per minute of climb performance against the book charts.

Notice that nowhere have I mentioned using Ground Speed in the tests. That’s because ground speed is dependent on winds, which are too hard to quantify exactly and differ daily. All factory cruise data is about what the plane does through the air, not over the ground. Don’t fall into the ground speed trap when evaluating your plane’s performance. If you must use your GPS as part of your routine, you should do speed checks for three- to five-minute legs. I suggest that you run four legs 90 degrees apart, and average your numbers to compare with your calculated TAS. Try to do this on a day with little to light winds aloft.

Cruise speed confusion is best solved by you, in your airplane, under conditions that you control, and compared against known performance standards. Only when you satisfy yourself as to what your plane can really do, can you plan your trips with confidence — and speak the truth when hangar flying.


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