Only a few years ago, a fully integrated automatic flight control system (AFCS) with an autothrottle was the sole domain of the air transport aircraft and heavy iron business jets. However, today’s AFCS with autothrottle (AT) are becoming common on single engine turboprops. Are you ready?
The trend toward more automation will continue to migrate downward into piston aircraft. Soon, all pilots will have the opportunity to fly fully integrated systems with autohrottles. Since autohrottles are new to many pilots, this article will attempt to remove some of the mystery.
Cockpit automation has radically reduced the physical workload for pilots and consequently improved the safety of flight. Today’s pilots operating a fully integrated automatic flight control system with autothrottles are truly system managers. There is no other piece of avionics that helps provide a relaxed flight like a fully integrated AFCS. The relaxation comes from having an extra set of hands that do what you command. Many pilots have thought that the autothrottle was a gimmick until they have had a flight with the additional automation. An autothrottle truly completes the picture.
Why is it such a hit? Primarily because it is simple and straightforward to operate, even more so than the flight director/autopilot (FD/AP). Because the autothrottle integrates seamlessly with the flight director, there is no contention on what system is controlling airspeed. It’s automatic without having another controller to manage. The only control to contend with is simply engage/disengage and selection of the desired airspeed. Everything else in the AT operation is determined from the FD/AP selection plus the engines and configuration of the aircraft. However, to get the most out of the automation, you need to know its limitations and quirks.
There are a lot of modes for a FD/AP, but there are only two modes for an autothrottle. The first is speed (SPD) and the second is power (PWR). Having said that, there are some ATs which have a hold mode (HLD) which is basically disengaged. The AT is also less complex because all the mode determination is already done in the FD/AP. The AT is the slave to the FD. The AT monitors the pitch mode of the FD/AP to determine if it should be in SPD or PWR. Either the FD/AP or the AT can control airspeed, but not both simultaneously. The AT has logic that knows if the AP is controlling airspeed then the AT will control the power. If the FD/AP is not controlling airspeed, then the AT will control to the selected airspeed. The side bar below shows all the AFCS vertical modes and what is being controlled by the FD and what is controlled by the AT.
The operation of the autothrottle is straightforward and intuitive, but there are a few areas to be aware of. Most authrottles have a takeoff mode. The majority of these require the pilot to arm the system for takeoff and then push the levers forward, where the AT takes over and completes the forward movement to takeoff power. If another FD/AP mode is not selected within the time limit for takeoff power, the AT will retard the levers to climb power.
Normally the pilot will select VS or FLC for the initial climb. As can be seen from looking at the sidebar, selecting FLC will transition the AP to flying the selected airspeed and the AT will transition to climb power. If you have 250 knots selected, the autopilot will pitch the aircraft to capture and maintain 250 and the autothrottle will reduce to climb power. However, this is an area that can catch a new autothrottle manager by surprise. The combination of 250 knots and climb power in many business jets will result in a very high rate of climb. If you have been cleared to a low intermediate altitude, pitch and power changes will happen very quickly! While I don’t like the use of VS mode in a climb, for the initial climb, VS provides a more comfortable ride.
In the cruise segment it is very straightforward. The AP will be in altitude hold and the AT in SPD. For the descent, there are several options for the pilot. Consider a FLC (airspeed) descent. Different manufactures handle it differently. One brings the power back an appropriate amount but allows the pilot to adjust the thrust levers to modify the descent rate. Others bring it back to idle and still others reduce the power to the minimum to permit adequate bleed air for de-icing. A VNAV descent is another option. Here the autopilot is controlling the pitch to stay on the vertical profile and the autothrottle is adjusting the power to maintain the selected airspeed.
Another area to be alert is on the approach and landing. The autothrottle can be a huge asset in establishing a stabilized approach. Many autothrottles also have an engine retard function during the landing flare. Be sure to check the conditions for the flare retard are satisfied and the retard function is annunciated as being armed. Otherwise you could be expecting a retard that never happens.
For an ILS or VNAV approach, the autopilot controls the pitch to maintain the glideslope (glidepath) and the autothrottle adjusts power to maintain the selected airspeed. If the FD is knocked off-line on the approach, most autothrottles are designed to continue to fly the selected airspeed, but it will not arm for the landing retard.
A way to ruin a good stabilized approach is to inadvertently hit the go-around switch during a coupled approach. The AP will pitch up and most autothrottles will advance the throttles/thrust levers to go-around power. An inadvertent activation of go-around is evidently what started the tragic set of circumstances in the Amazon-Atlas Air Boeing 767 accident in Houston.
As opposed to the autopilot, most air transport autothrottles have clutches that are easy to over-ride without disengaging. Additionally, most of these autothrottle systems do not disengage when over-ridden. That can lead to a situation where the pilot over-rode the system, but as soon as he or she releases the thrust levers, the AT will revert to its previous state. NOTE: “Most” is not “ALL.” You need to check the operation of your specific autothrottle to see if it disengages when over-ridden. Several of the latest autothrottle designs have eliminated independent clutches and instead electronically control the servomotor, simulating a clutch. Many of these will disengage when over-ridden. Consult your specific Flight Manual Supplement.
Compared to other avionics, the AT requires the least amount of study to understand its operation, however it is essential to know the mode transitions and to continuously monitor the operational status. Knowing the system will prevent any “why is it doing that?” questions and assure a smooth, relaxed flight.