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"My new autopilot has an IAS mode, which the manual says is 'indicated airspeed mode' for climbs and descents. My old autopilot just had VS (vertical speed mode), which this new one has as well. VS gets the job done. Why would I ever use the IAS mode?"
In general, we recommend using IAS for climbs and VS for descents. This technique is especially helpful if you don’t fly with a turbocharged engine.
Indicated airspeed mode maintains a selected airspeed and delivers whatever rate of climb the engine can produce at that speed, depending on the aircraft’s current weight and ambient conditions- that is, the density altitude. IAS mode has been a common function in jet autopilots for years, but it’s usually called ‘flight level change mode.’ That’s what indicated airspeed mode is called on older G1000 systems and some autopilots, and the button label says ‘FLC,’ not ‘IAS.’
The power of IAS hold in a climb is that the autopilot holds the selected cruise climb or Vy airspeed by adjusting pitch, just as you would during a climb, with power at full throttle or at the cruise climb setting. As the power available from the engine decreases with altitude, so does the climb rate, but the autopilot will lower the pitch on its own to hold the airspeed you selected.
If you select VS mode for a climb, the autopilot dutifully attempts to maintain that rate of climb as you ascend. As available power decreases, the autopilot gradually increases pitch, and the airspeed slowly decays. Unless you manually reduce the rate of climb to hold the best airspeed for the climb, the airplane struggles upward at an inefficient airspeed. Depending on your autopilot, it will eventually reach a minimum airspeed and chime at you, or it will pitch up so steeply that you risk a stall. For the same reason, using VS mode can lead to trouble when departing an airport at a high density altitude.
When starting down from cruise in a typical piston airplane, VS usually works best. The autopilot quickly begins a stabilized descent. You can adjust power to keep the airspeed from increasing into the yellow arc. VS mode also lets you set a comfortable descent rate in an unpressurized cabin.
If you select IAS mode at cruise speed and then reduce power to begin a descent, it can take a few minutes for the descent to stabilize, and establishing at least a 500 fpm descent may require a large power reduction unless you dial in a high indicated airspeed.
On the other hand, if you encounter turbulence, the better choice may be setting an IAS below your current turbulence penetration or maneuvering speed and reducing power to achieve a specific rate of descent.
IAS mode can also be useful in the terminal environment, when you’re already operating at low cruise speed, preparing for an instrument approach or to enter the VFR traffic pattern. The autopilot maintains a constant airspeed while you control level flight, descents, or even short climbs, by adjusting power. This technique takes a bit of practice with your airplane’s specific power settings, but it can be a handy tool in your bag of tricks.