Saturday, December 18, 2010

Why Fly An Approach Coupled To The Autopilot?

The other day I was having lunch with a pilot friend. We were discussing my recent blog “How To Execute An Actual Instrument Approach”. Apparently he got the idea that this was a coupled approach, one that uses the autopilot (AP) to follow the commands of the ILS signals. In fact, I hadn’t written that this was the case, rather describing a “hand” flown approach, without partial or complete use of the autopilot. Anyway, he questioned its use and so this blog.

As I believe that there are many advantages to making the approach, particularly the ILS, coupled to the autopilot, I will explain how to do it. Before going into the nitty-gritty however let me give my reasoning for doing it.

As you can see from my former article, or if you have done an ILS approach, you realize how busy things can get at certain times, especially in single pilot operations. The autopilot frees one up a bit to allow you to fully be aware of what and where you are in the approach. Meaning that you have some time to look at the all important approach chart without fighting a wing drop or altitude change. The latter assuming you have both heading and altitude coupling capabilities. That said, here is how it would work.

Basically, before the approach you would have been using the “nav and alt” features of the 2-axis AP. Different models of AP have different presentations. The picture above is just one, albeit quite popular. As the controller turns you to the inbound localizer course you should have your Nav 1 tuned to the ILS frequency. Set the inbound heading on the HSI and engage the Approach button on the AP control head. The AP will guide the plane deftly onto the localizer course while you, the PIC, verify that all is well, The altitude will be held until the glide slope is intercepted, usually at the outer marker (OM). There will be audible and usually visual indication as the altitude hold grabs onto the glide slope. Again, just monitor the approach as you manage the flight taking care to slow, add flaps, check fuel selector, mixture full rich, change the props to high RPM and drop the gear. A lot to do in a short space of time.

Ok, so that’s that. Sure I practiced flying non-autopilot approaches to make sure I stayed sharp, but also used the AP as needed, respecting its limitations (just like those of its pilot).

I just ran across an excellent article discussing all of this as a further reading: