Photo: C-141, official USAF photo, from Wikimedia Commons.

Eddie Sez:

One of the most valuable resources I've ever had in a cockpit was a Military Airlift Command flight engineer. Not only were they invaluable when it came to fixing things that were broken, they tended to be excellent cockpit procedure stewards. Their manual, AFM 51-9, is filled with techniques still applicable today.

I learned the minimum ground speed technique for dealing with wind shear early on. Flight Lessons / KC-135A Windshear. Our flight engineers could zero in on the aircraft's ground speed as if our lives depended on it. More about this: Abnormal Procedures / Windshear / Minimum Ground Speed.

What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.


Energy Management During Landing

[Air Force Manual 51-9, ¶11-5.] The energy management concept is a proven procedure for coping with wind shear during final approach. It resulted from an intense analysis of adverse weather airflows, motivated by the loss of several aircraft, and made possible by current weather technology. As you may know, an aircraft's ability to maintain lift is dependent on aerodynamic flow, airspeed, and movement within an air mass. Jet streams, independent air mass movement, and airflow in and around thunderstorms provide an environment where an aircraft can almost instantaneously transition from one air mass to another. This is wind shear. The effect of wind shear is similar to the effect of wind gusts, except it can be much more severe. It can increase or decrease airspeed until engine thrust has no opportunity to reestablish the proper airspeed within the new air mass. Further, it can increase or decrease airspeed by the difference in velocity between the two air masses. At high speeds, the aircraft could exceed its maximum design airspeed limit, or at low speeds it could stall.

Caution

This is a valid technique for dealing with a shear caused by one air mass sitting atop another. It should not be employed when dealing with convective activity, especially a microburst, where the magnitude of the speed loss or gain can be unpredictable.

a. Let's assume that an aircraft is on final approach at a safe margin above stall speed. Further assume that we have a 50-knot headwind on this approach and that the aircraft is flying within this air mass at 125 knots indicated airspeed. If this aircraft transits a wind shear into another air mass that suddenly gives up the 50-knot headwind, the indicated airspeed instantly would drop from 125 knots to 75 knots and the aircraft will stall. In preparation for transiting this wind shear, we increase approach speed by the amount of the predicted loss. After penetrating the shear, airspeed will immediately reduce to the approach speed. We predict this airspeed loss by making a comparison between the reference groundspeed and the actual approach groundspeed. We compute reference groundspeed by applying 100 percent of the reported runway winds to the approach true airspeed. We compute an approach groundspeed by applying 100 percent of the actual winds (at approach altitude) to the approach true airspeed. Any significant difference between these two groundspeeds is reason to expect a wind shear. For example: assume you have a 10-knot headwind on the runway and a 50 knot-headwind at approach altitude. Obviously, the headwind must decrease by 40 knots by the time the aircraft reaches the runway. This can happen gradually or in a matter of seconds. If the pilot maintains approach speed plus the groundspeed difference (40 knots in this case), transiting the wind shear will reduce airspeed by 40 knots to the desired approach speed.

The first time I saw this technique was in the 1-C-141B-1 and it didn't last long there. The Air Force became concerned pilots would believe windshear can be beat and instead preached a "When in doubt, go around" philosophy. AFM 51-9 makes a rather clumsy attempt at explaining it, perhaps we can do better.

Figure: Minimum groundspeed technique, from Eddie's notes.

The technique:

  1. Convert tower winds to a headwind or tailwind component

  2. Determine aircraft approach speed (VAPP) for configuration crossing the runway threshold

  3. Compute Minimum Ground Speed (VMIN-GS) by subtracting the headwind from the approach speed or adding the tailwind

  4. Once the airplane is in approach configuration, monitor actual ground speed

  5. If actual ground speed goes below VMIN-GS, add the difference to your target approach speed

  6. If actual ground speed goes above VMIN-GS, be aware of a possible loss of wind which will cause a sudden sink rate, or if the wind continues a long landing

References

Air Force Manual (AFM) 51-9, Aircraft Performance, 7 September 1990

Wikimedia Commons, Public Domain Artwork