Do you know which autopilot modes are best for climbs vs. descents? Selecting the wrong mode could leave you in a precarious spot.
Case Study: Inadvertent Citation Jet Stall
In an article published by the Flight Safety Foundation, an incident is described where a Citation Jet (CJ2+) pilot experienced an inadvertent stall after becoming distracted flying in V/S Mode...
The autopilot had been engaged in the vertical speed mode for the climb, and the pilot was using a portable electronic device (PED) as the Cessna Citation CJ2+ neared the assigned cruise altitude. As a result, he did not notice that the aircraft's airspeed had decreased and that its pitch attitude had increased significantly. The stall warning system did not provide a warning before the autopilot automatically disengaged and the Citation departed from controlled flight over central England the morning of Dec. 31, 2013.
After stalling, the aircraft rolled several times before the pilot was able to regain control, according to the accident report by the U.K. Air Accidents Investigation Branch (AAIB). Neither the pilot nor his passenger was hurt, but the Citation's wings were substantially damaged by overload forces during the upset.
When To Use "FLC" Mode
"Flight Level Change" Mode, or "Speed" Mode, adjusts aircraft pitch to maintain a constant indicated airspeed. When you engage FLC during a climb or descent, the autopilot will hold the aircraft in the climb or descent at the airspeed you've selected. With FLC engaged, when you increase power, your vertical speed (fpm) also increases. Conversely, when you reduce power, vertical speed decreases.
This mode is extremely helpful when ATC requests you to maintain a specified airspeed in a climb or descent. It also protects you from getting slow in a climb (more on that in a bit). Generally speaking, FLC is the best mode to use in a climb.
Once you've set climb power, the aircraft will change pitch to maintain the speed you've selected. For example, in a GA aircraft, if you'd like to climb at your best rate of climb (Vy), simply set the speed, add power, and your aircraft will adjust vertical speed to match Vy airspeed.
When To Use "V/S" Mode
According to the FAA's Advanced Avionics Handbook, when you engage "Vertical Speed" Mode (V/S), the autopilot will attempt to maintain the specified Foot-Per-Minute vertical speed until you choose a different setting in autopilot, the aircraft reaches an assigned altitude set into the assigned altitude selector/alerter,
or the autopilot is disconnected. While V/S Mode works for both climbs and descents, it is best used for descents.
You need to be extremely careful when flying a V/S Mode climb, because the aircraft will try to fly the specified climb rate, regardless of airspeed. In a worst-case scenario, your autopilot could fly itself into a stall, if you command it to climb at a greater FPM rate than your aircraft can perform. As airspeed decays, the autopilot will continually pitch the aircraft back in an attempt to maintain the vertical speed selected. This could happen all the way to the point of stalling.
V/S Mode is best used for descents, because your power setting will control your airspeed and descent track. However, you still need to monitor descent airspeeds in V/S Mode, to ensure you don't exceed Vne/Vmo and Va, or turbulence penetration speeds.
Stall Protection Systems (SPS)
Many new new TAA aircraft have a variety of stability protection features. When approaching a stall, SPS autopilots can automatically adjust your vertical rate to prevent a stall. They're a great backup to protect you from becoming distracted, and can prevent incidents like what happened to the Citation jet.
What's Your Strategy?
When you fly using an autopilot, what's your strategy when selecting climb and descent modes? Are there any exceptions to advice above? Tell us in the comments below.
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