System Enables Safe Landings With Total Failure of Normal Flight Controls

Air Safety Week, Sept 13, 2004

Loss of flight controls need not be a death sentence, as engine thrust can be used as a fallback means of bringing a stricken airplane to a reasonably safe landing.

More than 1,000 aircrew and passengers have been killed over the years when primary flight control systems failed, pilots were forced into throttleonly attempts to control their aircraft, and crashes resulted. However, through careful manipulation of engine thrust, sufficient control over the airplane can be maintained to complete an emergency descent to landing. While it is difficult for pilots to do so manually, software developed nearly a decade ago by the National Aeronautics and Space Administration (NASA) demonstrated that some automation assistance safe landings could be made with engine power alone. The concept of automating throttles-only control (TOC) was dubbed propulsioncontrolled aircraft (PCA).

The Nov. 22, 2003, surface-to-air missile attack on a DHL A300 cargo jet has heightened interest in PCA as an alternate fallback flight-control system. The airplane was struck by an SA-14 shoulder launched missile on climb out of Baghdad. The heat-seeking missile missed the left engine but struck the wing, causing a complete loss of hydraulic power. In what is regarded as a phenomenal feat of airmanship, the two pilots resorted to throttles-only control of their aircraft to make turns, descend and land at Baghdad.

PCA software would have made their task easier. As one advocate of the concept said, PCA can make every pilot faced with complete loss of flight controls "a hero."

PCA is not an automatic landing system. Rather, using its automation allows the pilot to more precisely control aircraft heading and rate of descent. As such, it enables controlled safe runway landings, as opposed to a controllable landing, not necessarily on a runway.

PCA supporters said the history of system failures, plus the added threat of ground-to-air attacks on airliners evidenced by the DHL event, add impetus for incorporating such an emergency flight control system into transport category aircraft.

"With just a few hundred lines of computer code, the system could be put in the airplane for less than the price of fueling the airplane," said Capt. David Hayes, Director of Certification Programs for the Air Line Pilots Association (ALPA).

Gordon Fullerton, the senior NASA test pilot involved in the original PCA flight tests, suggested that developing a PCA system for a specific aircraft model "might cost a couple tanks, but it doesn't add a single ounce to the aircraft weight."

With 1,098 lives lost in cases of flight control failure in which engine throttles were or could have been used, PCA is seen as a valuable emergency backup. In a recent briefing, Hayes and Capt. Terry Lutz, ALPA Director of Aircraft Development and Evaluation Programs, noted that of 3,447 airplanes among U.S. carriers, 1,607 - or about 47 percent - "have no mechanical back-up flight controls."

In August 25 testimony to the House Aviation Subcommittee, Capt. Duane Woerth, ALPA president, said PCA should be considered as a lower cost option to the installation of missile defense systems on airliners:

"The response to this [missile] threat must be very carefully considered, because airborne countermeasures are tremendously expensive to purchase, maintain, and operate ... In addition to any other measures eventually taken, we believe that the government should look seriously now at developing, certifying and funding deployment of NASA's propulsion controlled aircraft on airliners. This system, which would cost far less than an electronic [missile] countermeasures system, would allow flight crews to fly an aircraft to a safe landing in the event of hydraulic failure or damage to flight control components."

The cost of missile defenses for commercial airliners was raised in a European Commission (EC) internal report produced earlier this year: "Mandating a C1-2 million (Euro) equipment per aircraft, only to find out three years later that new missiles have been developed for which the installed equipment is not capable to defend against, is not the suitable way forward."

The EC report suggested that fuel tank protection and additional fire prevention "associated with the wing-engine assembly" should be considered "as viable research candidates able to save an aircraft in a post impact phase."

The EC report also mentioned engine thrust as a backup in the event of damaged or lost flight control systems: "The vertical or horizontal movements can be substituted by either engine thrust or control of other available flight surfaces."

Most airliners today feature engines slung under the wings. The placement means that adding thrust can raise the nose, while retarding power can lower the nose. Applying more thrust to one side can also change the direction of flight. For example, a 10[degrees] throttle split on the MD-11 produces about 20,000 pounds of differential engine thrust. TOC can also be applied on aircraft with tail mounted engines, although the effects can be less significant.