Flying on autopilot ain't automatic!

Today I flew N97PD, a Cessna 172SP in BEFA's fleet, for the first time. I had originally started my IFR rating on N435SP, 97PD's sister ship in 2004. This time around I have flown most of my IFR sorties on the Cessna 182RG - but decided to go back to using the 172 SPs. This was mostly because of scheduling reasons - the 182RG is going to get a new set of avionics including a Garmin 480, which is significantly different from the KLN 94 I had been flying it with. I would have used the Cessna 182 straight leg for the rest of my training, but there is only one of those in the club. I needed something that had the proper avionics and of which there were at least two in the club - and that narrowed the choices down to the Cessna 172SP or the Piper Warriors. The 172SPs have a 180 hp engine and have better climb performance compared to the Warriors. Besides I was already checked out on the 172SPs. I had flown and checked out on the Piper Arrow, but not the Piper Warriors.

However 97PD has the King KAP 140 autopilot with heading and altitude hold. So, I needed to get familiar with it in IFR conditions. 435 SP has the same autopilot but without altitude hold. The KAP 140 is a very capable autopilot but has its' idiosyncracies. In an IFR environment an autopilot can reduce workload significantly, but if the pilot doesn't know how exactly the autopilot works it can increase workload at critical junctures.

I have used the KAP 140 before, in the Garmin 1000 suite equipped Cessna 182T for the Civil Air Patrol. However I haven't flown the 182T for a while and it was good to get a refresher. Howard has a presentation that he had prepared for the KAP 140 when 97PD was first acquired. It was also a refresher for the heading only autopilot in 435SP. I won't go into too much detail about the autopilot operation, but I will point you to a very nice succint description of it. This description is from the Flightgear wiki. Flightgear is an excellent open source flight simulator.

This is what I gathered from the briefing

1. The trim and autopilot need to be tested thoroughly - this is part of the procedure whether or not one intends to use the autopilot. There is a special trim switch and disconnect button. The trim switch is split and for the trim to move both switches have to moved. This is so that a short in one switch will not cause trim runaway.

2. There were a number of audio alerts for the autopilot. These included the following

1. TRIM IN MOTION, TRIM IN MOTION… (pitch trim running for more than 5 sec)


2. CHECK PITCH TRIM (out of trim condition for more than 15 seconds)


3. ALTITUDE (1,000’ before)


4. LEAVING ALTITUDE (200’ away)


5. AUTOPILOT (AP has disengaged)

3. Both the HDG (heading) and the ALT (altitude) buttons are toggles. The HDG buttons toggles between ROLL (maintain zero rate of turn) and HDG (maintain the heading dialed on DG with the heading bug). The ALT button toggles between Altitude Hold (ALT) and Vertical Speed (VS) mode.

4. The ALT mode is confusing. Even if the altitude is armed (shown by an ARM indication) if there is no vertical speed up or down towards the altitude the autopilot will not hold that altitude. Once it reaches the altitude (after going up or down towards it) it will hold that altitude but the initial VS component has to be present.

5. The APR mode has a "standard entry" mode, which is a 45 degree intercept of any published leg that is dialed in on the DG and the CDI for NAV1. In order to do a standard entry one has to go from HDG mode to APR mode. An "all aspect entry" goes from your current position to intercepting any published leg that is dialed in on the DG and the CDI for NAV1 (at any angle). In order to do this kind of entry one has to go from ROL mode to APR mode. The procedure to do any kind of approach is to dial in the heading on the DG and then dial in the course on the CDI for NAV1. Then go from HDG to APR mode (if standard 45 intercept) or go to ROL mode then to APR mode (for any other type of intercept). The autopilot will not follow a procedure (as there is no GPSS steering) but will wait for you to dial in the heading and course in the DG and the CDI respectively for the next leg.

6. For any barometric pressure change the process is to change the pressure in 3 different places - altimeter, GPS and autopilot. The BARO key allows one to input the current altimeter setting into the autopilot

7. The autopilot holds altitude by using elevator and elevator trim. The power and yaw of the airplane need to be managed by the pilot. So if you are at idle power and have armed an altitude hold at an altitude, the autopilot will hold it for you - into the stall!

8. VS is the same way, power and yaw have to be managed by the pilot. When the autopilot is first put on, typically in the climb on takeoff, the default mode for it is ROL and VS. The VS is to whatever rate the aircraft is climbing when the autopilot is put on. So if you have a 1000 ft / min rate of climb, that rate will try to be held to whatever holding altitude you set on the ground. I don't know about you but my Cessna 172 doesn't hold that kind of rate of climb for very long - and it is possible that the autopilot will take it into the stall, unless rate of climb is adjusted using the up or down keys.

9. The big knob on the autopilot changes altitudes by a 1000 ft up or down and the small knob by a 100 ft increment. Just changing that value has no effect as explained in 4 above. The UP and DOWN buttons when in ALT hold will increase/decrease the altitude held by 20 ft increments and is useful when temporary pressure altitude changes need to be fixed.

10. On the ILS, once in APR mode, on capturing the glideslope a GS indication will come on and the autopilot will track the GS down to whatever altitude. The approach speed has to be maintained using power by the human pilot!

We carried out the entire flight from Renton to Paine Field and back on the autopilot. It certainly flys the airplane a lot better than me! We did the VOR/DME 16R into KPAE, a missed approach and then the ILS 16R into KPAE another missed and onto KRNT for the RNAV/GPS 15 for a circle to land 33.

One thing I learned is that the pilot has to be scanning continuosly and maintaining situtational awareness. The autopilot has so many modes and transitions that it is easy to get confused. One has to be able to take over control at any time and fly safely and correctly immediately. Fixation on the autopilot can be dangerous.