MPH or Knots? (New IFR Student)

Buck Rizvi

Pre-takeoff checklist
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Feb 10, 2017
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BoulderBuck
I'm an IFR student and I've recently updated the panel on my '73 Piper Cherokee Six.

My '73 Owners Manual and the original airspeed indicator is in MPH.

Garmin requires that the dual G5's I've installed as primary AI and HSI be set to MPH to match the owner's manual and airspeed indicator.

Is there a way for me to legally get everything switched over to knots? Wouldn't this be preferred if I'm flying IFR?

My airspeed indictor has an inner ring for knots and it's easy enough to reprogram the G5 to knots. I can also update all the cockpit placards and the owner's manual to knots easily enough. Does this require an FAA Field Approval?

What's the best practice here? Thanks!
 
I'm an IFR student and I've recently updated the panel on my '73 Piper Cherokee Six.

My '73 Owners Manual and the original airspeed indicator is in MPH.

Garmin requires that the dual G5's I've installed as primary AI and HSI be set to MPH to match the owner's manual and airspeed indicator.

Is there a way for me to legally get everything switched over to knots? Wouldn't this be preferred if I'm flying IFR?

My airspeed indictor has an inner ring for knots and it's easy enough to reprogram the G5 to knots. I can also update all the cockpit placards and the owner's manual to knots easily enough. Does this require an FAA Field Approval?

What's the best practice here? Thanks!

Not necessary. I just did my IR in my 1965 Cherokee 140. No issues. Like you my airspeed indicator had inner ring for knots and that’s what I used for timing my approaches to MAP (90 knots).
 
I can also update all the cockpit placards and the owner's manual to knots easily enough.

You can easily update the owner's manual of a '73 Piper aircraft? That doesn't sound right.

Anyway, leave it all MPH. It's faster that way. And use the knts only for flight planning and looking at your GPS ground speed.
 
It’s somewhat dumb that Garmin can’t or doesn’t do the same thing the split ASI with the window does. Just display both and make the font bigger on MPH if the aircraft is stuck certified in MPH.

We have the window ASI in the 182. I do everything in knots. Well I did. Once I started learning to fly it and teach in it from the right seat I had to memorize ANGLES since the panel cover blocks the entire right side of the instrument. Ha.

“If the needle is pointing at me we’re ten knots faster than Vy at this altitude... 45 degrees is right about rotation and hold off speed for a Robertson STOL short field takeoff...” and I can JUST see the top of the white arc for keeping an eyeball for someone extending the flaps too soon...

LOL LOL. Luckily there’s plenty of other cues for being too slow or too fast and not being able to see half the ASI just makes you learn to use those. Which I already did, but it magnifies it.

Our stall fences ring about ten knots above a stall, normal or accelerated, so that’s a plus. Robertson didn’t officially change the manufacturer’s stall speeds, but did publish new calibrated airspeed numbers, and stall indicated is incredibly low. Way below that 45 degree point you look for on a short field takeoff. It’ll hang there in a massive sink rate with the power screaming and the needle bouncing off the bottom peg and just mush straight ahead and down rapidly. So it’s not a very good airplane to teach anyone stalls in anyway.

My DPE and I had a discussion about this behavior on the ground before the CFI checkride so I could ask him what he wanted to see. There’s no real break. He said “first indication of a stall, recover, will be fine”.

I then made sure to tell him about the stall fences ringing. Technically they’re the first indication of a stall, but I would fly it to the horn. The fences sound a lot like the suction type of Cessna horn getting ready to blow, so that was also mentioned. The horn in ours is electric and piezo buzzer and sounds significantly different than the already whining stall fences that are already making a racket by the time you get to the horn.

He said all that was fine and off we went.

My airplane has to be described to people unfamiliar with it or they can be fooled into thinking that a) it’s stalling when its not, and b) it’s going to do much at the stall. It just hangs there power on, and drops the nose and starts to porpoise without power.

So yeah. Anyway. ASI in MPH is dumb. But if we ever do a G5, and we say we will if we toss a gyro, showing it only in MPH will be annoying.

Dear Garmin. Show both. Emphasis on the one the aircraft needs for legal reasons. I’ll happily read the smaller font of the knots scale. Just offer a mode to show both. :)

They won’t. But I can dream that they’ll just pop that feature into a firmware update. If FAA says I’m safer with MPH because my airplane is certified in MPH, they’re stupid.

I fly it in knots because I fly OTHER 182s occasionally and they’re in knots. Do it the same way every time is way smarter than flipping back and forth in identical types.

Sorry. Kinda ranty. But if some dumb jack wagon has told Garmin not to display both, they need to be keel hauled. If Garmin would fight that those of us who fly the same types of different vintages would be quite grateful. And write the code to do it, of course.

The GTN in the center stack is going to be in knots too, of course. Mixing and matching is dumb dumb dumb.
 
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