Friday, April 24, 2009

Your Pal George

Research into human multi-tasking, especially an often-cited study by Carnegie Mellon University, has shown that there are significant neurological costs associated with switching between multiple tasks, something that happens a lot in single-pilot instrument flying. For example, when subjects in the CMU study were told to imagine rotating an object while listening to complex sentences, their performance on both tasks dropped significantly. Guess that helps explain why pilots flying single-pilot IFR miss so many radio calls from ATC and why long periods of hand-flying can be so tiring. Trying to multi-task does not make us stupid per se, but it does reduce our performance.

Most newer GA aircraft are equipped with autopilots and this is a good trend. I can't think of one instrument instructor I've spoken to who doesn't believe a functioning autopilot to be a critical safety factor in single-pilot flying. Effective use of an autopilot might not make you a smarter, but it can improve your performance, reduce fatigue, and keep you from doing something stupid.

It's common for me to encounter pilots who have never been trained in the use of autopilots and lack system-level knowledge. Some pilots may even have been taught, or they subscribe to the notion, that using the autopilot is cheating. I believe that autopilot knowledge is important, so even student pilots who train with me in autopilot-equipped aircraft must to demonstrate competence in using George (a common monicker for the autopilot). Of course they also need to demonstrate stick-and-rudder hand-flying skills, too.

Consider the recent events in Florida where a low-time private pilot had to take control of a King Air B200 after the pilot became incapacitated and then died. One of the first issues facing the pilot was how to wrest control of the aircraft from the autopilot. You never know when an understanding of autopilot flying could come in handy!

Before George can be your buddy, you must understand him and a good way to start is to read the documentation for your particular aircraft. A/P information is usually found in your aircraft's Approved Flight Manual in the Supplements section. Since this section is toward the back of the AFM, pilots seem to never get to reading this important stuff. Once you've found the supplement, you'll see that it follows the same GAMA format that is used in most AFM - Limitations, Emergency Procedures, Normal Procedures, System Description, Performance, and so on. Reading the supplement can help you avoid these common A/P mistakes.
  • Failing to know or respect the A/P limitations
  • Lack of system-level knowledge
  • Not understanding the modes of operation
  • Trial-and-error button pushing
  • Failing to recognize undesired or uncommanded mode changes
Know George's Limits

The Limitations section of the autopilot supplement usually contains the following limits. Your autopilot may list other limits, too.
  • Minimum altitude for engaging after takeoff
  • Minimum and maximum airspeeds with the A/P engaged
  • Maximum flap configuration with the A/P engaged
  • Minimum altitude for flying precision and non-precision approaches
  • A/P use in airframe icing conditions
I frequently encounter pilots who don't know or respect the altitude limitations for their autopilot. These altitude limitation are based, in part, on the maximum predicted altitude loss that could occur should the autopilot malfunction. Knowing these numbers are critical and could save you should your autopilot malfunction at the end of an instrument approach down to minima.

Trust, but Verify

Most manufacturer's documentation will state that the A/P may not be used in flight unless it was first tested on the ground prior to flight. Don't confuse this preflight functional test with the system self-test that many units will perform when they are powered on or that you initiate by pressing a TEST button. Many manufacturers' checklists don't include the numerous steps for the autopilot functional test in their normal checklist, so use the procedure found in the supplement.

The preflight test usually includes engaging the autopilot, verifying resistance on the pitch and roll axes, engaging the heading and navigation functions and verifying that the roll inputs are correct. The last step is to disengage George, so you may as well test the A/P prior to verifying the flight controls are free and correct. The actual test procedure varies by model of autopilot, so always use the manufacturer's procedure for your particular unit. For convenience, some pilots create their own aircraft checklist that includes the manufacturer's A/P preflight test.

If the autopilot fails the pre-flight test, don't use it in flight! What's more, you should disable the autopilot (usually by pulling and wire-wrapping the circuit breaker), placard the autopilot as inoperative, and make a maintenance log entry per 14 CFR 91.213. You should also consider the added risk of a single-pilot IFR flight with an inoperative autopilot. A long flight in IMC without an autopilot may entail risks you're not willing to take.

Ain't Misbehavin'

To use an A/P effectively, you must understand what equipment provides pitch and roll inputs to your particular autopilot. For example, the King KAP140, popular in Cessna aircraft, receives roll input from an electrically-driven turn coordinator. Two-axis KAP 140 autopilots sense changes in pitch with an accelerometer and through static pressure changes via a dedicated pitot system. The autopilot/flight director in the Caravans I used to fly receive pitch and roll inputs from a special attitude indicator (called an attitude deviation indicator) that was often vacuum-driven. Once you understand how your A/P senses pitch and roll, you'll be better prepared to know when George is sick and not to be trusted.

If George malfunctions, and malfunctions are quite possible, you'll need to know all the ways to disengage him. A disconnect button (often red in color) is usually provided on the left yoke and many A/Ps will provide an aural alarm when the autopilot disengages. Pilots not used to autopilots often mistake the disconnect button for the communication radio's push-to-talk button. Some A/P systems (like the KAP 140) will disconnect if the pilot uses the electric trim switch. Pulling the A/P circuit breaker may be your last resort if George is being disagreeable and won't disengage in the normal fashion. Again, let the manufacturer's documentation for your particular autopilot be your guide.

The State You're In

Autopilots usually have a variety modes of operation for roll and pitch control. Lateral (roll) modes include simply holding the wings level, following the heading bug on the HSI or heading indicator, or intercepting and tracking a navigation source.

Vertical (pitch) modes are provided in two-axis A/Ps and include maintaining altitude, climbing or descending at a particular rate or airspeed, and tracking a vertical navigation source (ILS glideslope or GPS glidepath or vertical path). Some A/Ps will allow you to specify an altitude to capture when climbing or descending. You may also be able to specify the vertical speed (feet per minute) or airspeed (in knots) you want the autopilot to maintain in the climb or descent. Light GA aircraft A/Ps don't (yet) control engine power, so you'll need to manage the throttle yourself.

The adage "Garbage in, garbage out" applies to autopilots: You can command George to enter a climb or a descent that may exceed the capabilities of your aircraft or cause you to enter a stall. Remember that while George may be controlling the plane, you are pilot in command. Never let George take you in any direction or into any flight condition you don't want to go.

Most autopilots will display the current lateral and vertical modes as well as any modes that are armed. For example, many autopilots allow you to engage heading mode, set the heading bug to an intercept for a VOR or GPS course, and then arm NAV (navigation) mode. As the selected navigation course comes alive, the A/P will switch to NAV mode and track the desired navigational course (VOR, localizer, or GPS).

Many autopilots (but not all) will not only track a localizer course, they will also descend on a glideslope for an ILS. The recommended procedure for engaging this approach mode is usually to start out in NAV mode, with the A/P tracking the localizer. Command the A/P to descend to the published glideslope intercept altitude at the appropriate time and capture that altitude. Then arm the approach mode and you should see an indication that the glideslope is armed. As the glideslope is intercepted from below, the autopilot will pitch down to track the glideslope. You'll need to manage the power and be ready to disconnect the A/P at or slightly before the decision height.

It's critical that you read the display before you start impulsively pushing buttons. This is by far the most common error I see pilots make. Trial and error is not a productive activity to be engaged in during a high-workload phase of flight close to the ground and it usually belies a lack of understanding of basic autopilot functions and modes.

The KAP 140 display, for example, shows the selected modes in the top half of the A/P display and any modes that are armed are shown in the lower half of the display. Here's the KAP 140 set to maintain altitude (ALT), track the heading bug (HDG), and intercept the current navigation course (NAV ARM).

The G1000 with the GFC 700 autopilot displays its mode information right above the attitude indicator on the primary flight display. Here's the GFC 700 set to intercept the a localizer and descend to 3,300 feet. The active modes are displayed in green and the armed modes in white. The lateral navigation, shown on the left side, is heading mode (HDG shown in green) with the NAV mode armed to capture the localizer (shown as LOC in white). The center field shows the A/P is active (AP in green). The vertical navigation mode is vertical speed (VS shown in green) with a 300 foot per minute descent rate to capture the selected altitude of 3,300 feet (shown as ALTS in white).

Here's the GFC 700 tracking the localizer and the glideslope.

Given all the possible combinations of autopilot modes, I recommend pilots receive training and then do some VFR practice before relying on the autopilot in instrument conditions. You'll need to regularly practice your autopilot skills to stay sharp, but you need to maintain your hand-flying skills, too. A G1000 simulator or PC-based trainer are great ways to accomplish autopilot training.

Letting George Help

Pilots often complain that using the autopilot isn't real flying, but it's really just another type of flying. To maintain proficiency, you'll need to practice both hand-flying and autopilot flying regularly. Successful A/P use depends on good systems management and a good manager knows when to delegate. You shouldn't hesitate to let George fly the plane during high-workload phases of flight, such as:
  • Flying in complex airspace with lots of traffic
  • Flying a complex departure procedure
  • While evaluating XM weather displays or radar returns
  • While copying a complicated clearance or holding instruction
  • While briefing an instrument approach or STAR (Standard Terminal ARrival)
  • When being vectored to an approach
  • Flying an approach close to minima
  • Flying an approach when you are fatigued
  • Flying the missed approach (above the minimum altitude for A/P engagement)
  • While performing an abnormal or emergency check list
It's a Wrap

Newer autopilots have brought the flight director concept to light GA aircraft. The flight director (FD) displays command bars on the attitude indicator providing visual clues to the flight control inputs you need to make. It can be harder to recognize that something is wrong when the FD alone is being used because you're busy flying the plane and following the command bars - remember that study on multi-tasking and performance? So do your best to divide your attention and make sure the pitch and roll suggestions made by the FD are what you intended. If they aren't, it's best to disengage the FD, hand-fly, and re-engage the FD or AP when you've figured out what was set incorrectly.

If you fly an autopilot-equipped aircraft, become proficient with it's operation. Read the manual, get some instruction, and be sure to practice both hand-flying and managing the autopilot. And remember that being proficient in autopilot management isn't cheating, it's often the smart thing to do.

1 comment:

Level 7,000 said...

As a Single-Pilot Part 135 pilot I cannot say how many times that I have desired to have an autopilot. Any, autopilot. When I takeoff there are two things I know for sure, I will be hand flying the entire flight, and I will be hand flying an approach, if required. Hand flying alone is hard enough. I would be exuberant as to using an autopilot. I have hand flown many approaches to minimums (200' - 1800RVR) and from experience I would have rather had an autopilot on those days. For when I do go missed, I would enjoy the fact that I would have some piece of equipment that would be able to back me up, or even, allow myself to back up that piece of equipment. I encourage everybody to maintain proficiency but flying with an autopilot is not cheating, it's just plane smart.