(Operating outside of the manufacturer’s tested values)

by Barbara Harper


Section 91 of our FAR manual lists all the regulations we are expected to fly by. When we train, we learn the most important regulations and those regulations that will actually have a question on your “knowledge” test before going for a check ride.

Looking at Part 91, Subpart A - General, you will find several parts that are actually on the knowledge test, such as: 91.3 - Responsibility and authority of the pilot in command, and 91.17 that states the rules for alcohol consumption, and 91.13 - Careless or reckless operation. Notice that all of the rules in Subpart A are numbered with odd numbers after the 91… 1,3,5,7,9,11, etc. I have no idea why.

91.9 Civil aircraft flight manual, marking, and placard requirements

forgotten federal regulation 91.9 1

(a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards or as otherwise prescribed by the certificating authority of the country of origin.

Paragraph (d) talks about helicopters and more, so we will ignore that for now.

What does that regulation 91.9 mean? The intent of 91.9(a) is that a pilot only operate an aircraft within the limitations as stated in the AFM/POH, and that 91.9(b)(1) further indicates that (if a manual is required by 21.5) the approved manual should be available to the pilot (i.e., on board and available to the pilot). This means that the entire manual, including the limitation section, performance charts, emergency procedures, abnormal procedures, etc. are FAR 91.9. Anything in that Flight Manual is “fair game” for an examiner to ask during a check ride for a rating. You may not remember all of the numbers for any performance maneuver, but you should at least know where to look for that information.

On these hot summer days here in Arizona, the temperatures can be well over 100 degrees F. The ATIS or AWOS may report the temperature in Celsius, and your performance charts in the Manual may be in Celsius.

forgotten federal regulation 91.9 2

There may be a conversion chart for you too. In any case, when you hear the temperature stated as 40, 41, or 42 degrees C, you already know it is hot outside.

Most single engine aircraft have performance charts that END at 40 deg C. That means that the aircraft has not been tested for anything above that temperature. If you do decide to take off with an outside temperature above those ranges, you are operating outside of the manufacturer’s tested values, and per Cessna, you are on your own. If an accident or incident were to happen, it is possible that the insurance company may not pay for damages or injuries incurred, because the aircraft was operating outside of its parameters.

On really hot summer days in Phoenix, airlines may delay a departure or arrival due to high temperatures being off the charts of their operating manuals. That will likely make it to the media, too. If a general aviation aircraft has an accident, you can bet that will make the news, too. If the airlines are paying attention to their flight manuals, maybe we should as well.

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