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Approach VVI

Instrument Procedures

One of the good things about becoming an instrument pilot in the T-37 is that you learned how to do everything without a flight director and with a shotgun instrument panel. (They shot a blank piece of sheet metal with a shotgun and wherever there was a hole they put in an instrument.) You learned to fly an ILS down to minimums with an airspeed indicator, altimeter, a vertical velocity indicator, and a very basic attitude indicator. When that's all you got, each becomes critical. Knowing where that VVI should be pointing could be the difference between spotting the rabbit at minimums or having to go around and divert.

Oh yes, after the T-37 there was the T-38. Now we had a flight director but we had to fly the approach at 160 knots. Things seem to get worse.


 

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Photo: T-37B Cockpit, from Eddie's collection

The Math

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Figure: T-38 glide path, from Eddie's notes.

Height per Distance

Height = 6076 ( tan 3 ° ) = 318feet

T-38 Ground Example

We learned early in the T-38 that dividing our airspeed on final by two "kind of" gave us our VVI. We all knew that flying a final approach speed of 160 knots worked out to around 800 fpm VVI. But does the math back that up?

V = ( 160 nm hour ) X 1hour 60minutes =2.67 nm min

We need to standardized the units to make the math work. The equation for time is pretty easy:

Time to Travel 1 nm = d v = 1nm 2.67nm/min = 0.38 minutes

The equation for VVI writes itself:

Vertical Velocity = d t = 318feet 0.38minutes = 836feet/minute

So the rule of thumb is backed up by the math, but strangely it didn't always work. After a few years of practice it became clear the reason was wind. Using ground speed instead of indicated speed fixed everything:

The Rule of Thumb

A 3 degree glide path descent rate in feet/minute is equal to one-half the ground speed in nm/hour, multiplied by ten.

Down to Earth Examples

Not many airplanes fly an approach at 160 knots, granted. So here are a few examples from my past:

  • Boeing 747 — 140 knots, 10 knot headwind: [ (140 - 10) / 2 ] x 10 = 650 fpm
  • Challenger 604 — 130 knots, 10 knot headwind: [ (130 - 10) / 2 ] x 10 = 600 fpm
  • Gulfstream V — 115 knots, 5 knot headwind: [ (115 - 5) / 2 ] x 10 = 550 fpm
  • T-37B — 110 knots, 10 knot headwind: [ (110 - 10) / 2 ] x 10 = 500 fpm
  • Piper Arrow PA28 — 75 knots, 5 knot headwind: [ (75 - 5) / 2 ] x 10 = 350 fpm
Revision: 20151206
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