Page 20 - Volume 12 Number 2

Ask the Expert
The Autopilot’s IAS Mode
by Tom Clements
If your particular autopilot/flight director system which a GPS-derived glidepath can be displayed. When
does not have the IAS mode – the mode that adjusts
pitch attitude to hold a particular Indicated Airspeed – then this article is not for you. Turn the page; I’ll see you next month.
But for those who have this mode, let’s talk a bit about its usefulness. I would wager that the IAS mode is typically used very rarely. It is only when ATC assigns a reasonable speed while providing vectors for an approach – for example, 180 knots – that pilots sometime select the IAS mode. Realize that this is one of the vertical autopilot modes and vertical modes are mutually exclusive. For example, Altitude and IAS modes cannot be selected at the same time. You can have the autopilot hold an altitude or a speed, but never both. (It would take auto-throttles to do both, and that is a system almost never seen on King Airs.) Likewise, IAS and VS (Vertical Speed) modes are mutually exclusive; same with altitude and glideslope.
When Approach Control wants us to hold 180 KIAS, it is easy to adjust power to attain that speed in level flight and then tap the IAS button when we are assigned a descent. Now our reduction of power will cause the descent to begin, as the autopilot pitches down to keep the assigned 180 KIAS speed. The more that power is reduced, the greater the rate of descent. As the next assigned altitude is captured, the IAS mode automatically disconnects and now we add power to maintain 180 KIAS while the autopilot controls pitch to maintain the assigned altitude. Have you utilized this procedure? It’s rather easy and “cool,” no?
But IAS mode lends itself to an even better utilization in many airplanes, and that comes into play while doing a non-precision approach or a precision approach without glidepath coupling. It is widely reported that many King Airs with the Collins Pro Line II avionics suite and with early versions of an EHSI cannot be converted so to conduct coupled LPV approaches. The Pro Line II setup was a popular factory installation in the late 1980s and throughout the 1990s. Some owners of these models have elected to do a complete panel makeover, replacing the original “steam gauges” with a Garmin G600 display, perhaps receiving input from the popular Garmin GTN 750. Now autopilot coupling in the vertical axis can be easily incorporated. But coupling without installing a G600 is virtually never done due to compatibility issues.
When updating an older, non-WAAS GPS to a WAAS version – or when adding a new WAAS-enabled GPS – many installation shops include a simple mechanical display of lateral and vertical deviation. Although this display is useless most of the time, it is often the only place in
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we are conducting an RNAV (GPS) approach that has a glidepath – LPV or LNAV/VNAV or LNAV+V – this simple mechanical indicator shows glidepath deviation.
Since the autopilot cannot be wired to couple to this glidepath, the pilot is forced to do it manually. It is common to keep the autopilot engaged in the NAV or APPR mode so that it can track the approach course. Then pitch attitude is manually adjusted, by use of the autopilot’s pitch command wheel or rocker switch, to follow the glidepath in the descent. This works fine. However, I find that this is a case in which the IAS mode makes the pilot’s job a bit easier.
Here’s how it goes: Let the autopilot track the approach course, outside of the FAF, in the lateral mode you prefer: HDG/GPSS, NAV, or APPR. Engage the ALT mode to hold the proper glidepath intercept altitude. Adjust power and extend approach flaps and landing gear so that the airplane is exactly at your desired approach speed as the glidepath is intercepted.
Now tap the IAS button and set the power to what you estimate will be correct for the descent. Tapping IAS disconnects ALT and now the autopilot is adjusting pitch to hold the IAS. From past experience, you have set an appropriate torque but, of course, wind, and weight all cause the need for power adjustments. If the glidepath deviation needle shows you starting to go high, pull torque back a bit, perhaps 100 ft-lbs. Wait and observe the result. Starting to center the glidepath again? Then add 50 ft- lbs or so. Still going high? Pull off another 100 ft-lbs. By making timely and smooth, small power adjustments, you will find that it is exceedingly easy to stay centered on the glidepath.
For those systems that do indeed couple to a GPS-derived glidepath, then the LPV approach is flown identically to an ILS approach ... letting the autopilot adjust heading and pitch to remain on the proper path while we adjust power to hold the desired speed. Thus, the only difference in the procedure we are discussing now is that pitch is being used to hold airspeed and power is being used to follow the glidepath instead of the other way around.
The old “Dive and Drive” non-precision approaches are becoming less and less common, being replaced with GPS-derived precision approaches, yet there are still hundreds that exist. When executing one of these approaches, obviously the IAS mode can again be used to ease the pilot’s workload and to guarantee a nicely stabilized approach speed. As discussed before, have the
FEBRUARY 2018