Like cruise control, they increase efficiency and safety of flight operationBy Jim Sparks Many new cars sold today come equipped with a cruise control system. This automatic feature allows the driver to set a desired speed and then sit back and enjoy the ride. A cruise control will adjust engine power to correct for conditions such as traveling up hills and minimizes engine changes while in a cruise condition thereby enabling fuel efficiency to be increased, not to mention a reduced workload for the driver. What a concept! But will it work in an aircraft? As a frequent flier who is often bored by the final segment of a flight on the airlines, I often find myself counting the number of audibly recognizable power changes that occur from the time the landing gear is extended to touchdown. On a recent flight I observed 24 rpm changes. By my count this was a new record. This unusual behavior on my part is a result of routinely occupying the jump seat in a Lockheed JetStar that was flown by one of Lockheed's original test pilots. He would pride himself in not having to make throttle adjustments during descent and approach to landing. This true aviator would have his aircraft speed and engines synchronized then deploy secondary flight controls and landing gear when needed, while continuing to fly the approach to landing never touching the throttles.
Throttle systems Like a cruise control in an automobile, thrust management systems (TMS) in aircraft increase the efficiency and safety of flight operations. The TMS is an auto throttle/thrust director system that supplies throttle control. Auto throttle systems have only been around commercial aviation for about 40 years and like most everything else in this industry the evolution of the system has been significant. In some cases the automated adjustment to engine power settings is considered an element of the auto pilot system where other installations are considered a stand-alone device. The significance of this information is that when an automatic throttle system is integrated with the other automatic flight systems it generally means sensors used to measure aircraft movement and response are shared rather than a stand-alone device which would have to depend on separate components for gaining system information. Automatic thrust control systems will need the same information as the pilot to successfully provide adequate thrust for any given aircraft maneuver. This data includes engine mechanical as well as thermal limitations, thrust settings for given temperature and pressure altitude conditions, aircraft situations such as take off, climb, cruise, descent, or landing. There are also safety devices, some automatic and others initiated by the flight crew, that may include a fail safe mode in the event of noncritical malfunctions, an automatic disconnect in the event the system is incapable of performing properly, and a means of manual override in the event the pilot decides to take over control of engine power selection.
Engine power Turbine engine power is determined in several different ways. Turbo fan engines will often use the compensated speed of the fan while other applications will use engine pressure ratio (EPR). This is the differential pressure sensed at the engine inlet compared with the pressure of the exhaust gas. So, when does a pilot need to make power adjustments? This question can take us all the way back to the basic theory of flight. We all know there are four primary forces excluding economics and politics that impact our business: lift, gravity, drag, and thrust. The relationship between these means engine adjustments throughout the flight regime are both frequent and numerous. As an example, when the aircraft levels off and is in a cruise condition, fuel will continuously be consumed resulting in a lighter aircraft. If the automatic flight control system is actively trying to hold altitude it will have to frequently adjust the pitch to keep the now lighter aircraft from gaining altitude. This tendency to lower the nose will then result in an increase in speed or Mach number. If this condition persists unnoticed an aircraft over speed or resulting mach tuck may be encountered. Pilots often develop a feel for the responsiveness of their specific powerplant. They can then use this knowledge to predetermine the amount of throttle change needed to correct for any given condition along with the speed of movement. In many of the engines in use today, thrust increase is more exponential rather than linear with power lever movement. This means a flight crewmember unfamiliar with engine tendencies will definitely impact the comfort of passengers during thrust changes. Understand system capabilities Before attempting to troubleshoot any automatic throttle system it is imperative to have a good understanding of that system's specific capabilities. Some systems have full authority of engine control throughout the flight envelope while other systems have only a certain window of operation during different aspects of flight. For example, in some systems there is a specific maximum amount of rpm change that can be commanded in cruise. If the pilot is not aware of this characteristic, after a sustained high altitude cruise the auto flight system may begin to pitch the aircraft in an attempt to prevent an overspeed and the auto throttle system when it reaches its extreme will automatically disconnect. Then when the pilot restores the aircraft to a normal condition and engages the auto throttle again it works just as it should without a trace of a problem. As with diagnosing many intermittent aircraft problems asking the right questions to the flight crew can often be the key to unlocking what could otherwise be a complicated and expensive problem resolution.
Computers and thrust control Components used in a TMS often include some form of a thrust computer. This may be an internal device within the electronic engine control (EEC) or it may be a stand-alone computer or even a circuit included within the basic auto flight system. Frequently servo actuators are used to manipulate the actual power levers. This means the throttle levers may physically move in response to power changes initiated by the auto throttle system. This feature provides both tactile feedback to what the engines are doing, as well as providing visual reinforcement. Nonmoving throttle levers rely on the engine instruments to provide pilots with information showing what the engines are actually doing. There is also some form of control panel where various modes of auto throttle can be selected and engaged plus a remote auto throttle disconnect switch, giving the flight crew a quick means of eradicating any improper operation and taking immediate command when needed.Performance data Engine performance is another significant factor in the systems used today. EEC can now transmit this data to the thrust management computer via some type of digital data bus. Control laws programmed into the thrust management computer will allow the system to maximize passenger comfort by minimizing throttle operations caused by temporary environmental changes. Environmental changes occur mostly in high-altitude cruise where slow and continuous changes in the throttle operation are applicable to control the aircraft speed. Energy management control laws supply high degrees of performance with a minimum of throttle operation. Measured airspeed coupled with inertial acceleration in different proportions as a function of altitude and the segment of flight, aircraft drag, and the engine thrust data in the autothrottle software let the TMS adjust the position of the throttle levers. These adjustments are necessary to make allowances for aircraft movement and configuration changes. The TMS controls the acceleration and deceleration of the aircraft to smoothly change to a new airspeed selection or new flight aspect. Several modes of disconnecting auto throttles are often used. Should the pilot physically move the throttles, an override is generally perceived by the system, resulting in isolation. There are often auto throttle disconnect switches available to flight crew members. In most cases thrust management systems incorporate a philosophy where any perception of failure to perform in a safe manner an annunciated disconnect will be noticed by the flight crew.
Engine synchronization Engine synchronization systems may operate independently of the auto throttle or be an integral function. When a synchronization mode is engaged, the rpm may be synchronized to the predetermined value established by the system manufacturer during the specific mode of aircraft operation such as climb or cruise. Frequently systems that use a thrust management computer will have the ability to adjust all operating engines enabling the aircraft to maintain a constant thrust while in a sync mode. Other systems will have a master engine and subsequent slaves. Noncomputerized sync systems will often match the slave engine(s) rpm with that of the master engine. In this case thrust may deviate from what is actually needed. When any automatic throttle mode is engaged some form of annunciation should be available to the flight crew. In most cases there are several indications showing mode along with normal and abnormal operations. Checking these indications along with manual overrides and disconnects are often part of the testing required to ensure continued airworthiness for most thrust management systems. Common maintenance problems with auto throttles often involve either improper engine rigging or more frequently high operating forces in the engine control linkage. As many of the electronic systems include a diagnostic mode perceived faults may now be recorded by a computer and then retrieved and analyzed by technicians prior to initiating maintenance procedures. As with most of the sophisticated systems found in the aircraft of today a thorough and sometimes individual debrief of the flight crew may be rewarding especially if the pilots fly more than one type of aircraft utilizing automatic thrust management. Even though the principle is the same the methods may differ. A good friend of mine was recently having a problem with the cruise control in his truck so I offered to help him with it. We found a bad clutch switch. Oh well, so much for comparing cars to aircraft.