Many maneuvers and procedures of modern, multi-engine aircraft are controlled by on-board automatic control systems. An automatic throttle (A/T) system is an example of such a system. An A/T system receives various engine and avionic signals and controls the amount of thrust produced by the engines. More specifically, in response to its input signals, an A/T system produces a thrust control signal for each engine. The thrust control signals control the position of the throttle for each engine so that each engine produces the thrust called for by a suitable source, such as an autopilot, for example.
Some maneuvers, such as an aircraft landing maneuver, require equal thrust from the engines mounted on opposing sides of the aircraft. Unequal, i.e., asymmetrical, thrust levels may cause undesirable lateral excursions of the aircraft during landing. During automatic landing maneuvers, the A/T system rather than the pilot issues thrust commands to the engines designed to cause the engines to produce equal thrusts.
During automatic landing maneuvers when the A/T system is engaged, a failure in the A/T system will, most likely, cause the engines to produce unequal thrusts. More specifically, a failure in a hardware component in the A/T system or a problem with the software used to control the A/T system may cause the A/T system to produce a throttle control signal that rapidly drives one of the throttles fully on or fully off. This throttle action produces a corresponding engine thrust that is substantially different from the thrust level of the outer engine(s), resulting in an excessive lateral excursion by the aircraft.
One technique used in the prior art to detect unequal thrust levels during automatic landing maneuvers monitors the position of the throttles. This prior art techniques interprets a difference in throttle positions as a difference in thrust levels since throttle position is related to the thrust produced by the corresponding engine. In other words, a difference, commonly called a split, in throttle position is interpreted by a throttle split monitor as unequal thrust levels between the related engines. Typically, such a technique allows for a maximum split between throttles to accommodate for system and component variations. The maximum split is usually a fixed and predetermined value. Accordingly, variations in thrust levels are permitted as long as the actual throttle split is less than the predetermining maximum throttle split. Once the actual throttle split exceeds this predetermined throttle split, the throttle split monitor trips and disengages the A/T system.
While the just described prior art technique works well with aircraft having identical engines, it does not work well with aircraft having intermixed, i.e., dissimilar, engines. As is well known in the aircraft industry, dissimilar engines most likely have different operating characteristics. Because of the different operating characteristics, the throttles associated with the dissimilar engines will have different positions for a given thrust level. Accordingly, when an aircraft having intermixed engines is performing an automatic landing maneuver, which, as noted above, requires that the thrust produced by the engines be equal, there will be a "normal" throttle split between the engine throttles. Unfortunately, this "normal" throttle split may exceed the maximum throttle split provided for by prior art throttle split monitors and, thus, cause an A/T system to be inadvertently disengaged. Once the A/T system is disengaged, the automatic landing maneuver must be aborted and the aircraft landed manually by the pilot. Even though pilots are trained to land aircraft, such a change in a landing maneuver is highly undesirable, especially when an aircraft is in its final approach, and very near to the ground.
Another prior art technique for detecting unequal thrusts during a landing maneuver involves directly comparing the thrust levels of the different engines. In this technique, when unequal thrust levels are detected, the A/T system applies appropriate thrust control signals to the throttle servos, resulting in throttle settings being changed to balance out engine thrust. Unfortunately, there is an inherent lag between throttle change and engine response thereto. The lag in engine response may cause a monitor to trip, resulting in the A/T system's being disengaged. In an attempt to prevent the monitor from inadvertently disengaging the A/T system, this type of prior art technique incorporates a delay that allows the engine time to respond to the throttle motion. Unfortunately, this delay may result in excessive lateral excursions of the aircraft before a failure in throttle motion is detected. Thus, this technique is undesirable regardless of whether the aircraft engines are similar or dissimilar.
As will be readily appreciated from the foregoing discussion, there has developed the need for a monitor that detects unequal thrust levels on multi-engine aircraft having intermixed engines. Such a monitor should discriminate between normal throttle splits that are the result of the different operating characteristics of the dissimilar engines and excessive throttle splits that cause the engines to produce unequal thrusts, such as might be caused by a failure in an aircraft's automatic throttle control system. The present invention is a throttle split monitor designed to achieve these results.