Convert Mechanical Airspeed Instrument MPH to KTs

WDD

WDD

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Is there a way for a shop to convert an existing analog / mechanical airspeed indicator from MPH to KTs? Seems like it should be a matter of replacing the dial face.
 
WDD said:
Is there a way for a shop to convert an existing analog / mechanical airspeed indicator from MPH to KTs? Seems like it should be a matter of replacing the dial face.
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If it’s a certified plane, I believe you must keep it matching the POH
 
Only a CRS with instrument repair/overhaul for that particular instrument can reface it. That said, Salty is correct for a certificated plane the instruments are supposed to match the POH. Even if installing glass, the installer is supposed to set it to match the manual. What the owner does after return to service.................
 
Ah - didn’t know that. Yeah, the POH is all MPH.
 
WDD said:
Is there a way for a shop to convert an existing analog / mechanical airspeed indicator from MPH to KTs?
Click to expand...
It depends on what route you need to take, but sure. The easiest way is to see if your aircraft OEM offers an alternate airspeed indicator in knots either by a listing in the parts manual or possibly a bulletin. Another easy option provided its available for the specific airspeed you have installed is to replace the mph scale with a dual mph/knots scale. Another more involved option falls under an aircraft alteration which depends on the model year whether it would be a major alteration or simply changing some decals, etc. as a minor, but your mechanic would be the one to advise on that route.
 
Van Johnston said:
There are ASIs out there with both units on the face. Shop around.
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So just buy a new unit?

Or…. Can it be refaced with both mph and kts, just that the mph is small and the kts is prominent/ large font?
 
Why not add a ring around the circumference showing knots? That way you keep the original as-is for regulatory reasons and convenience, but you also have the knots display that you want. If you can't find one ready-made you can design one yourself on a computer, print it full size on sticky label paper, cut it out and BANG you're done.
 
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FAR 23, under which an awful lot of airplanes were built and certified, had this to say:

Sec. 23.1545 — Airspeed indicator.​

(a) Each airspeed indicator must be marked as specified in paragraph (b) of this section, with the marks located at the corresponding indicated airspeeds.


(b) The following markings must be made:

(1) For the never-exceed speed V NE,a radial red line.

(2) For the caution range, a yellow arc extending from the red line specified in paragraph (b)(1) of this section to the upper limit of the green arc specified in paragraph (b)(3) of this section.

(3) For the normal operating range, a green arc with the lower limit at V S1with maximum weight and with landing gear and wing flaps retracted, and the upper limit at the maximum structural cruising speed V NOestablished under §23.1505(b).

(4) For the flap operating range, a white arc with the lower limit at V S0at the maximum weight, and the upper limit at the flaps-extended speed V FEestablished under §23.1511.


For this reason, the POH will usually have something like this in it:

1751061973474.png

Any ASI used to replace the original must also have these marks.
 
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Right after I got my plane it became apparent that the ASI was off. So I sent it in to Midwest Avionics and while it was there I had them silk screen the dial to knots. But even though my plane came with MPH, the POH had both, so it was okay for me to change.

If your POH only has MPH, then you won't be able to do it.
 
WDD said:
So just buy a new unit?

Or…. Can it be refaced with both mph and kts, just that the mph is small and the kts is prominent/ large font?
Click to expand...

I was thinking of our specific example, where we replaced our analog ASI w mph on outside and knots on inside w a GI 275, and the old ASI is sitting in the hangar waiting for an omniscient buyer to come along and ask how much we want for it.

i assume there are lots of used analog ASIs out there for sale, but have not verified.
 
Van Johnston said:
I was thinking of our specific example, where we replaced our analog ASI w mph on outside and knots on inside w a GI 275, and the old ASI is sitting in the hangar waiting for an omniscient buyer to come along and ask how much we want for it.

i assume there are lots of used analog ASIs out there for sale, but have not verified.
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I've been very satisfied with work from instrument shops such as Century. Turnaround is fast, usually a week or so. I think they need the business.

Another option is replace your AI with a digital AHI that displays airspeed, and set it to kts.
 
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I do have a HI 275 AHRs next to the mechanical one. I wasn’t aware I could reset it myself - I’ll look I RJ how to do that.
 
WDD said:
Is there a way for a shop to convert an existing analog / mechanical airspeed indicator from MPH to KTs? Seems like it should be a matter of replacing the dial face.
Click to expand...

Is there a particular reason why you want knots?
 
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