I've looked a bit at the CFRs trying to determine this for sure, but haven't found a clear answer..

One of the Cessna 150s that I rent from the local FBO had it's airspeed indicator swapped out four or five months ago.. the new indicator, in addition to having the low end of the speed range super clumped together so it's difficult to see the accurate reading near the stall speed, is clearly MARKED incorrectly for the airplane that it's in. The most obvious example is the white arc -- on the ASI that they installed, the top of the white arc is at 80mph, when this Cessna 150M should have the top of the white arc up closer to 93mph.

I know it's not a super huge difference, and I feel that I can fly the airplane safely, but it made me wonder whether it's airworthy with such an obviously incorrectly marked ASI in it.


  • $\begingroup$ At least it's on the safe side for your flaps... I have no idea about the regulation but it is definitely weird. $\endgroup$
    – MaximEck
    Sep 21, 2020 at 21:06
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    $\begingroup$ 80 knots is 92mph. Perhaps the units have been confused? $\endgroup$
    – Pondlife
    Sep 21, 2020 at 21:17
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    $\begingroup$ @Pondlife That was my first thought when I first hopped in the aircraft and saw it, but nope, it's mph on the outside and knots on the inside, and the top of the white arc corresponds to 80mph.. I unfortunately don't have a picture of the instrument handy, but I did verify units. $\endgroup$
    – Joe
    Sep 21, 2020 at 21:39

1 Answer 1


Take a look at the equipment list of the Cessna 150. There are part numbers for every piece of equipment installed. If the part numbers do not match, then the airplane does not comply with its type certificate data sheet and is not airworthy. Take a look in the maintenance manuals and find the part numbers. If the rental FBO refuses to let you see those manuals find a different FBO to rent from.

Unfortunately, that is not the whole story. There are also other ways for this to be an airworthy airplane:

  • PMA part. If the new airspeed indicator has been approved as a direct replacement parts manufactured approved part, it can go in along with the equipment necessary to make it work. I doubt this is the case as the part would have to be FAA approved for that airplane. The most common application of a PMA approved part is the installation of LED bulbs for incandescent bulbs.

  • STC. The airplane has a supplemental type certificate that modifies the airplane in such a way the new airspeed indicator markings are required for the STC.


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