I have a 1975 plane with ASI (airspeed indicators) calibrated in mph on the outer scale and knots on the inner one. Additionally, I carry the 1976 POH in the plane as it is of the newer format that includes considerably more detail than the POH offered in 1975 and earlier.  


  • Is it possible to replace the ASI with one calibrated in knots without a lot of red tape (and then change any interior placards referring to speeds to ones in knots, for instance the placard on the flap control)?  
  • Is it legal to carry the 1976 POH only in the plane for the operator’s reference?  

Also, years ago when the issue of wrinkled bladder tanks caused an AD, we decided to only replace the tanks with ones correctly dimensioned. As a result, our total capacity is 42 gallons per side for 84 total. The POH says the capacity is 79 gallons, with 75 usable. I surmise that we now are at a useable of 80 gallons. I use 79 on my totalizer for a little margin.  

However, after installing the new tanks many years ago, the fuel selector placard has never been changed to read the new capacities. I know of no FAA direction to do so, but it is now confusing to explain to other pilots who fly the plane about the differences. We certainly know the capacities, as we fueled the plane from dead dry after changing them — 84 gallons. So, does it take an act of Congress to change that placard?  

— John 


Regarding the ASI, you can contact an instrument shop and ask them, but usually you can go either way on ASI markings as long as they are correct. Modern glass has the option to show either, in most instances. The most important thing is that you have a POH/AFM (Pilot’s Operating Handbook or Airplane Flight Manual) that shows speeds in the relevant unit of measurement. I think calculating the speeds and noting them in your POH/AFM is acceptable, but that’s a good question for your FSDO. 

You will need to talk to a shop to have an ASI built up anyway, so you may want to ask them about overhauling yours and re-marking it in knots. Oftentimes, they can do a dual scale to show both knots and mph, which you have; so really you just want the ranges swapped, which I think would be no big deal. I think changing the placards would be a minor alteration.  

The short answer on the POH is that your Make and Model (and serial number range, if applicable) need to match to be legal.  

Did the fuel tanks come with any paperwork that talked about placards? Check that.  

Are the tanks STCed? Or are they direct replacement by PN (Part Number)? If they are not STCed and are the same PN as what you took out, they should be the same capacity. If they aren’t, you need to account for why they are bigger (or smaller). This affects the POH/AFM, weight and balance, and yes, the placards and markings.  

Call the bladder manufacturer and explain your situation and see what they have to say. You may only need a log entry and to make the relevant changes to the above, or you may need to file a 337 and get the STC paperwork, or you may need to contact your FSDO for a field approval for “extended range fuel,” which might be sticky.  

You don’t want to be basing any flight planning calculations on “they’re marked 75 gallons, but I know they really hold 84.” Your IA should be happy to help iron this out because it would reflect poorly on you both if there’s an accident and it’s found the tanks are marked improperly. Currently, I’d say the airplane is not legal to fly because there are questions that need to be resolved around the fuel capacity. If that’s determined to be a contributing factor in an accident, you and your mechanic may have some liability, and insurance may deny any claim.  

Good questions. Get the paperwork in order. It may be a couple simple hoops, but you don’t want loose ends like these out there.  

— Erich Rempert, A&P IA Consultant