Mode for Climb? Descent?
I enjoyed and appreciated your articles regarding the operation and use of autopilots. In "Advanced Autopilot Use" in the October 2018 issue, you focused on altitude pre-select strategies and functional tactics in utilizing an autopilot for approach, attempted landing, and on the missed approach. You added an interesting suggestion to load a "safety precaution" altitude of 10,000 feet into the altitude preselect. Should work well over Kansas. I understand your recommendations and think they can help us all.
What I thought missing was any mention of how to best deal with altitude changes as necessary. Obviously the many different types and models of autopilots make it difficult, or most likely impossible, to summarize autopilot vertical ascend/descend control inputs and appropriate monitoring of the desired outcome. I realize that many of your readers do not have autopilots that can maintain an indicated airspeed.
I've found that the easiest way to fly an approach on autopilot so enabled is to get the plane established in approach mode, on speed, and engage the autopilot in IAS hold mode. For most of my flights it will be near 140 KTS for Class D (can be 170 KTS for Class B). Want to go down? Decrease power. Want to go up? Increase power. The autopilot will pitch to remain at 140 KTS (or the selected IAS) throughout. The altitude preselect functions remain operative. Get to your pre-selected altitude of 1500 feet at 140 knots the altitude selector will level the aircraft. Will it slow? Yes, as expected. The altitude pre-select took over from the IAS. Reset power as appropriate.
Maybe in your article comparing the Garmin GFC 600 and the Genesys S-TEC 3100 you'll address how each can be best utilized to manage ascents and descents.
Just thought it might be helpful for us all to get advice on all aspects of autopilot use.
Wellesley Hills, MA
That 10,000-foot cruise safety setting on altitude preselect in pressurized aircraft is a reasonable compromise. Yes, there is terrain above 10,000 feet in North America, but not much. If you happen to pass out due to hypoxia, the altitude to which you descend partially determines how fast you recover. If you select, say, 15,000feet to clear anything in the contiguous U.S. you're wasting a lot of altitude you could almost always use safely.
You make a valid observation that we could have further explored the modes available in new autopilots. So, we did just that in the final article that appeared last month. You might note that we also choose primarily to use IAS mode as our default mode for climbs and descents.