IAP airspeed - IAS or TAS??

[wrxpilot]

well, to address the question in the context of that approach: the conditions quoted in the newspaper for the time of the accident would have resulted in an TAS about 5% greater than the IAS, or .3 nm over the 6 nm timing segment. , or an error of a little over half the runway length, or more relevantly, .3 nm past the MAP would still be 1 nm before the runway threshold.


Something a lot more serious than a 5% error in groundspeed happened to put him into the mountain on the far side of the airport.

A Squared,

I am not in any way insinuating anything like that happened in that accident. I have no idea what did and wouldn't insult everyone hear with my opinion if I even had one.

That particular appch plate layout (with mtns just beyone MAP) just made me think about the subject I posted.

 

[A Squared]

A Squared,

I am not in any way insinuating anything like that happened in that accident. I have no idea what did and wouldn't insult everyone hear with my opinion if I even had one.

That particular appch plate layout (with mtns just beyone MAP) just made me think about the subject I posted.

Yeah, I know you weren't suggesting that, I was just trying to put it into a real context, a "how much difference does it actually make" frame of reference.

 

[CalifDan]

Seems like a pretty obvious answer...the timing charts are based on groundspeed. How could it be anything else and be accurate? You calculate your Category as indicated above (1.3 times your stall speed), that's your category. Fly your approaches at whatever speed seems comfortable. I do mine at 100 knots and though I'm just in the edge of Category A, I use Category B minimums.

Yes, at higher altitudes your TAS wil be higher than your IAS, perhaps significantly. At the same time, your GS may be lower than the IAS based on winds. Ultimately, you need to make a good estimate of your GS if you don;t have a direct readout of it and time the approach accordingly.

I have dual 430's so the MAP is clearly shown and I time as a back-up. If you are using time only, you should know the GS or have a pretty good idea of it. I fly out of KAPC and the VOR approach could have a large tailwind during some IFR operations. The hills east of the airport will come into play quick if you are timing with a 90 knot IAS (rather than 125 knot GS) and have a 35 knot tailwind all the way down.

 

[nosehair]

trying to put it into a real context, a "how much difference does it actually make" frame of reference.

Right - that's the answer to your question. Real world estimates. If you don't have much wind, it won't make enough difference to matter - maybe 1/2 a runway length. If you do have significant head or tail wind, you still can roughly estimate your groundspeed within 10 kts to keep the distance reasonably within parameters. Work out a few with a computer and you will see there has to be quite a lot of miscalculating to go far enough beyound MAP to be outside of protected space.