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Speed for timed approaches

Asked by: 10637 views Instrument Rating

I know this is a bit of a moot point with so many approach types available but I've always been a bit perplexed how to choose the timed approach speed for my Cessna 310. The FAF to MAP chart gives times for 60 90 120 etc on non-precision approaches such as NDB and LOC approaches (ground speed). 60 and 90 are too slow for the 310 and 120 is a bit fast especially if the airport were not visible until near the MAP. So do I roar along at 120 or select 105, half way between 90 and 120, which happens to be just above blue line for the 310. That requires a bit of math or estimating to get the correct time for FAF to MAP. How do the pros do it?

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2 Answers



  1. John D Collins on Apr 10, 2015

    I don’t know how the pro’s do it, but I would choose the approach speed based on what is right for the airplane and not as a multiple of one of the times in the table. The table will only match the GS when there is no wind, otherwise you are adjusting your TAS with the estimated head or tail wind.

    I have a GPS and can set it to indicate distance and time to the MAP. If I was forced to use time to estimate the distance, I would interpolate between the two values and bias a small amount towards the shorter value. For 105 Kts, assuming 120 Kts for 5.5 NM is 2+45 and 90 Kts is 3+40, the half way point is near 3+13. I would shave that towards the lower time value by 5 seconds. The correct value for 105 Kts is 3+08 using T = D/GS x 60. Given the inaccuracies of the TAS and estimating the winds, this is good enough.

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  2. Mark Kolber on Apr 11, 2015

    John is absolutely right.

    We fly our airplanes at the proper airspeed for our airplanes. And since the timing tables are based on groundspeed, our airspeed and groundspeed are going to rarely match, and are in fact likely to change multiple times during our descent and the wind’s direction and intensity changes. Making constant recalculations and adjustments to our IAS so that we can keep a constant GS is a pretty bad idea – and the exact opposite of a stabilized approach.

    The net result is, the timing table is, at best, a ballpark to find the MAP. We fit the time to our speed, not vice versa. And probably one of the reasons, some of the earliest tech advances after VOR itself, were designed to provide distance information so timing to find the MAP becomes unnecessary.

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