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Autopilot

posted Aug 28, 2013, 8:13 AM by George Finlay   [ updated Aug 29, 2013, 6:59 AM by Nathaniel Cauldwell ]
Been thinking about teaching granddaughter Naomi how to ride a bike. An autopilot would be handy. But we would want to turn it off more and more so Naomi would get more and more comfortable balancing and steering on her own.

Long trips in fast airplanes are mostly flown on autopilot, of course. Relieved of the need to closely monitor attitude, the crew can focus more attention on the big picture. It is easy for crews to become overly reliant on autopilots though, and lax about checking on them. Those errors may have contributed to the July 2013 crash in San Francisco. 


I and the owner of Columbia 400 N1134N often fly as a crew. The pilot flying is responsible for setting and checking the Garmin GFC 700 while the pilot not flying backs him up. We have made our share of mistakes. But fortunately we have caught and corrected them before we generated headlines. Here is a partial list:

Once we have our clearance, we set our initial altitude, load the departure procedure and set the flight director with the convenient "go around" (GA) button for takeoff. This sets the pitch command bar at 7.5 degrees. At full power, which we always use for take off as well as go around, this is a safe and useful initial pitch, to be used until we switch to flight level change (FLC).

Cleared for take off, we will set the heading bug on initial climb heading. On the MMU6.MMU departure procedure for runway 23 at our home airport, that is runway heading to 600 ft, then 210 to 2000 ft, then 160 degrees. Unless we remember to put the flight director in heading mode though, it will stay in its default roll mode. Should we lose the visual horizon early in the climb, that is somewhat useful from a safety standpoint. But it not safe when it puts us immediately off course as it has several times before we realized our mistake and switched to heading mode.

Clear of immediate obstacles and flaps confirmed up, we will switch to navigate mode so the flight director can give us guidance on the departure procedure, and the pilot flying will normally plan to turn on the autopilot and relax his grip on the stick. More than once, we have been distracted by weather or ATC, and forgotten to switch the autopilot on. Unless we double check for the annunciation on the PFD indicating the autopilot has indeed taken control, the PFD can initially look reassuringly normal, until you realize that the commanded climbs and turns are not happening. 

On that 160 heading we are in NY Class B before long, so TRACON issues vectors for the climb to cruise, and often requests best rate, which means FLC with 124 KIAS selected, and back to heading mode. More opportunity for error: we have sometimes forgotten to switch back to heading mode. And sometimes we have remembered that, but then absent-mindedly toggled the autopilot off, intending to simple confirm it is indeed on. 

Once TRACON has us clear of their traffic they typically issue a vector toward our first waypoint and then turn us loose on our own navigation direct to that point. We have sometimes forgotten to select direct before switching back to navigation mode, resulting in big heading changes that may have led TRACON to wonder where we might be off to.

Overall, the GFC 700 excels, keeping the ship on course through the roughest air. There is only one in 34N, and it is has occasionally quit at inconvenient moments. You would think that would motivate us to constantly practice hand flying. But like many pilots, we practice too little. The rust shows especially during the critical descent and approach phases when we find ourselves having to think about what we are doing more than in the early days of our careers, when our skills were sharp.

Status information is prominently displayed at the top of the PFD. Red X's cover over data locations when their sources quit. Loose the AHRS, and the attitude indicator in the top center is replaced by such an X, reminding us to refer to the backup unit in the center of the panel. In such a scenario, the HSI on the PFD remains partially functional, allowing us to follow the CDI.

We have lost the autopilot and found the only missing data to be outside air temperature, which you would not expect to be required information for an autopilot. When this happened, the PFD was otherwise unimpaired, with the flight director still functioning.

On descent we are in the habit of using VS. While FLC is preferred in the climb, where the throttle is typically left wide open, using it on descent leads to delays in starting or changing the rate of descent, since it takes some time for the plane to loose momentum when power is reduced. 

Control wheel steering (CWS) temporarily gives the pilot control without disconnecting the autopilot and it is a convenient way to set vertical speed for the descent, as well as for handling traffic diversions. When it is released with a selected mode other than ROL, the autopilot will steer the plane back to the selected path.

The other method of setting VS and selecting airspeeds for FLC is via the UP/DOWN button on the A/P panel. For large changes in airspeed that can be tedious, especially in turbulence, since the UP/DOWN button is not available on the RediPad.

Finally, back to that Go Around (GA) button mentioned at the beginning: it is a nice labor-saver at that critical moment when DA or MDA is reached with no runway in sight and any help getting safely up and away is appreciated. The pilot flying presses GA which turns the autopilot off, gives him a 7.5 degree pitch up command bar on the FD, and sequences the GPS to the next waypoint in the missed approach procedure. All that is left is to turn the backup fuel pump on, and smoothly apply full power, stowing flaps when clear of immediate obstacles. Very helpful indeed.




Vertical Navigation (VNAV) is handy when we are getting close to our busy home airspace around New York, cruising high in the teens as we usually are, and TRACON gives us a typical descent clearance: "cross 10 miles west of HUO and maintain 7000". Or when cleared for a full RNAV approach with no procedure turns, since the VNV will pick up and adhere to all the step downs.

The one requirement of VNV that took us the longest time to remember is that any manually selected altitude takes precedence. So typically we would be cruising along at 15,000, with 15,000 as our selected altitude, when the voice behind the panel announces "vertical track" and then dutifully flies right through the interception point on that vertical track. Why? Because we have 15,000 ft still selected, and that altitude will always be the limit to our descent, as it should be, when you think about it.
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