Why You Want an Autothrottle


It has been 50 years since Safe Flight Instrument Company developed the first practical autothrottle system but only a small minority of pilots have been able to fly this most useful and safety enhancing equipment.

But that is starting to change. Safe Flight is now offering its AutoPower system in midsize business jets, when before it had only been installed in airline and large business jets such as Gulfstreams and Global Expresses. It's not an impossible dream that autothrottles will someday be available even in single-engine airplanes.

Back in 1956 Safe Flight introduced a system it called AutoPower in a DC-3 as the first autothrottle system. The company had pioneered angle of attack sensing and stall warning equipment, and AutoPower was a logical next step. Safe Flight had already developed a device it called Speed Control that measured angle of attack and commanded the pilot, by means of a cockpit instrument, to add or reduce power to maintain optimum speed for climb or approach. AutoPower connected servos to the throttles to automatically adjust power to follow the Speed Control commands. You can imagine how easy it would be to fly an ILS approach when the airspeed was automatically and perfectly controlled, and that's what AutoPower did in 1956, and what autothrottles do today. AutoPower was a technical success, but not a big seller. The real acceptance of autothrottles came when Safe Flight linked the system to airspeed, not just angle of attack.

The modern autothrottle controls power, and thus airspeed, from takeoff to touchdown. A press of the takeoff go-around (TOGA) switches on the thrust levers sets computed takeoff thrust automatically. You dial in the climb airspeed, and the throttles adjust to maintain it. In cruise, you select indicated airspeed or Mach number, and power is continuously monitored and adjusted to maintain that exact airspeed as weight and atmospheric conditions change. Pilot workload is greatly decreased, while fuel efficiency and airplane performance improve because of the precision of power management.

The reason autothrottles have been restricted to the most capable and costly jets is that the system is complicated. Before electronic computers came along (fadec) to control jet engines, the autothrottle had the job. The system had to be smart enough not to exceed engine limits, which could easily be done if the autothrottle shoved the levers to the stops. The autothrottle also must be integrated with the autopilot for fully coupled flight, but still be capable of maintaining selected airspeed when the human pilot is hand flying.

An autothrottle system also needs servos in the power quadrant to move the levers as power changes. It would be possible to connect the autothrottle system directly to the engines and not involve movement of the actual power levers, but Safe Flight and other U.S. companies have not gone that route. Airbus, with its fly-by-wire cockpits, has chosen to not move the actual power levers in some of its airplanes because computers are manipulating every control surface in any case. People who believe-and that includes me-that the power lever position should always match the power selected, no matter if it's a human or the autothrottle changing power, call the Airbus method "limp sticks." Most of us want to see and even feel the power levers moving, just like we want to see the autopilot moving the controls and wiggling the pedals.

However, the biggest reason I'm hopeful that autothrottles will become available in more and more airplanes is the coming of computerized engine control. With fadec managing all aspects of engine operation, the autothrottles mission is reduced to adjusting available power to maintain selected airspeed without worry of exceeding engine limits or calculating available power for takeoff or climb. The fadec handles that. Also, digital air data computers' prices have come way down, so the fundamental raw data for the autothrottle, precise airspeed, is now available in many airplanes.

There is, however, one fadec feature that is a problem for autothrottles, and that's throttle detents for takeoff, climb and so on. The autothrottle servo moving the power lever doesn't need the detents, but it can stumble over them as it smoothly and continuously adjusts power. A solution appears to be virtual detents that can be detected by the human pilot, but be invisible to the autothrottle when it is engaged and moving the levers. I had never dared to dream of an autothrottle in a personal airplane, turbine or even piston, but then I never imagined glass cockpits with solid state attitude and heading systems in piston singles would be here. I can't predict when autothrottles will be available in even light jets, much less piston airplanes, but I do believe they are on the way. And once you fly with it, you won't want to tackle a tight approach without it. Make Up Your Own Mind

I believe the most important element in the phenomenal safety record of jets-jets flown by the major airlines and professional corporate crews, particularly-is the comprehensive set of standard procedures pilots are required to follow. These procedures have been developed by expert pilots sitting on the ground when there is limitless time available to consider all options and complications. The results of such deliberations will always be more thorough than any crew could come up with in the cockpit in the frenzy of an actual emergency. It may not flatter our ego as pilots, but learning, practicing and then applying a standard procedure is safer than relying only on our own talent and instincts.

Because use of standard procedures is so critical, that's what we practice during training, particularly recurrent training, in jets. There are a few memory items that you apply in some critical situations-such as put on the oxygen mask immediately if the cabin depressurizes at altitude-but for most abnormal or emergency situations you and your copilot go to the checklist and follow it exactly.

But no matter how complete the standard operating procedures may be, there are still some circumstances where pilot judgment is required. And the best simulator instructors will create one or two of these situations in every recurrent training session.

For my annual recurrent training at FlightSafety International in Wichita this spring, my sim instructor, King Rhiley III, gave me an interesting problem with no absolutely correct answer. I was flying the CitationJet on an RNAV approach into Runway 9 at Memphis. The ILS to the runway was out of service, so the RNAV was the only approach available to the runway. The controller's radar had also failed so I was flying a "manual" approach with procedure turn required.

A clogged fuel filter had killed the left engine. Fuel filters clog a lot in simulators, but almost never in real flight, but it could happen. There is another simulator phenomenon that involves generators. When one engine quits, the generator on the operating engine suddenly has a mean time between failure of about three minutes. It could happen. And, of course, it did happen to me. The generator on the good engine failed as I was nearing the final approach fix outbound, leaving no operating generator.

In the CJ the standard procedure if both generators fail is to switch to the emergency bus that powers only the "essential" items for a demonstrated 30 minutes on battery power only. The essential items are the things you absolutely must have to continue to fly in night IFR conditions, but don't include such nice things as the primary EFIS displays or the flight management system (FMS). All you have for navigation is the number one VOR/ILS receiver. To stay right side up you need to fly by the two-inch "peanut" attitude gyro that is powered by its own dedicated battery. I had flown an ILS using only the emergency bus equipment earlier, and it's a lot of work, but is doable. So, here's the problem King gave me. If I followed the standard procedure and moved the battery switch to emergency bus, what would be the fallout? The FMS is not on the emergency bus so I would have to abandon the RNAV approach I was already on. If I followed standard procedure, the question was could I fly on my own navigation an ILS or VOR approach to another runway, or to another airport, in less than 30 minutes, when power to the emergency bus could expire? The other option was to keep power to the main bus and continue the GPS approach I was already established on.

The book says that the aircraft battery will power the equipment and systems used for normal IFR flight for approximately 10 minutes. I estimated that I could complete the approach, including a procedure turn, in less than that, so I decided to continue the approach on main battery bus. And to extend the battery as far as possible, my copilot and I started turning nonessential equipment off. It was daylight, so we killed all lights. It was warm, so pitot heat went off. So did unused avionics.

I started my watch just after the second generator had failed and I made the decision to continue the GPS approach. It took seven minutes and 40 seconds to get to the runway, and the battery held out. Could I have flown another approach to Memphis without radar vectors in 30 minutes on the emergency bus? Probably, but it takes longer than you think. If I had exhausted the aircraft battery, I would still have had the emergency peanut gyro, plus airspeed and altitude, but no approach guidance or communications. But I made the choice to get on the runway as quickly as possible, and that required continued use of the FMS, which eliminated using the emergency bus. An interesting choice, and that's what good training does because it makes you think. A double generator failure is extremely rare in real jet flying, but it could happen.

I also got to appreciate the usefulness and reality of simulator training on my flight back home to Westchester County Airport near New York City. A tight low-pressure system had cranked up just off the East Coast and the wind was ripping from the east, causing lots of turbulence and a classic wind shear situation. It was a Saturday evening and traffic was light, but the tower controllers relayed wind shear reports from airplanes that had landed a half hour or more before. A King Air 200 was on the ILS to Runway 16 ahead of me and reported a loss of 20 knots of airspeed and moderate turbulence about 100 feet above the runway. The frequency was quiet, so the King Air crew had time to warn me that if I thought it was bumpy now-and I sure did-wait until "you get to the bottom of this thing."

The classic indication of wind shear, and the profile I had just flown the day before in the simulator, is a rapidly increasing headwind that will certainly be followed by an equally rapid decrease, or even change in direction to a tailwind. The increasing headwind causes you to gain airspeed on the approach and to climb above the glideslope. If you see that in a jet, you should begin an immediate go-around. In a piston twin like my Baron, a go-around isn't usually necessary because it lacks the mass and drag of a jet, so it can accelerate quicker when the wind shear hits, but you sure need plenty of extra airspeed to fly through.

At about 500 feet above the ground, without any power change, the airspeed in my Baron increased from 120 to 150 knots, and I soared three dots above the glideslope. By this point the runway was in sight, so I didn't reduce power to try to regain the glideslope centerline. I expected the airspeed to go away, and as the King Air crew warned, it did just before reaching the runway threshold. I was glad to have all those extra knots, because the sinking spell was dramatic. I have flown through wind shear at Westchester many times, but this was the most classic case I had seen and could have come right out of a Level C or D simulator. We're supposed to train the way we fly, and fly the way we train, but I'd be happy to keep that kind of weather locked up in the simulator.