Business & GA, Commercial

Safety: Throttles-Only Aircraft Control

By David Evans | November 1, 2004
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The proverbial pucker factor would have been higher had the pilots of the stricken DHL Airbus 300 been aware of the grim findings of a 1997 NASA technical paper: "Manual throttles-only control is exceedingly difficult."

The pilots' airplane had been struck shortly after takeoff Nov. 22, 2003, from Baghdad by a shoulder-fired SA-14 missile. The heat-seeking weapon had missed the left engine but inflicted significant damage to the aft portion of the left wing -- igniting a fire and causing complete loss of all hydraulics. The pilots were left with differential engine thrust to effect a modicum of control over pitch, roll and yaw (add power, nose goes up, etc.).

They were able to complete a straight-in flaps-up approach to Baghdad, veering off the runway, breaking through a fence and coming to a stop on the sandy shoulder. By all accounts, the landing was a superb feat of airmanship, in part because the captain was aware, from his knowledge of a United Airlines DC-10 accident at Sioux City, Iowa, in 1989, that differential engine power could control an airplane with under-wing engines.

The DHL pilots were lucky. The A300 is one of the most responsive transport-category airplanes to changes in engine power alone, and the DHL pilots were hit during daylight and clear weather.

Their feat demonstrates that loss of flight controls need not be a death sentence. 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 throttles-only attempts to control their aircraft, and crashes resulted. While it is difficult for pilots to do so manually, software developed nearly a decade ago by NASA demonstrated that, with some automation assistance, safe landings could be made with engine power alone. The concept of automating throttles-only control (TOC) was dubbed propulsion-controlled aircraft (PCA).

PCA software would have made the DHL pilots' task easier. The software enables controlled 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," says 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, suggests 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 by its advocates 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 backup flight controls."

The cost of missile defenses for commercial airliners was raised in a European Commission (EC) internal report produced earlier this year: "Mandating �1 to 2 million 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 EU report mentions that engine thrust might be employed 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."

As an example, a 10-degree throttle split on the MD-11 produces about 20,000 pounds of differential engine thrust. TOC also can be applied on aircraft with tail-mounted engines, although the effects can be less significant.

In the event of loss of control to the flight surfaces, pilots must cope with two aircraft motions. One is known as phugoid, the longitudinal pitch oscillation of the airplane. This up-and-down motion can occur over a cycle ranging from 30 to 40 seconds to 1 to 2 minutes, depending on the aircraft type, its center of gravity, its trim state and engine thrust setting. Trim state is that condition where an airplane is flying in a balanced condition.

The other problem is known as Dutch roll, in which yaw induces a rolling motion. One experienced captain described the phenomenon as akin to wallowing through the sky in a horizontal figure eight pattern. Yaw dampers connected to the rudder suppress Dutch roll.

Thus, with loss of flight controls, pilots are faced with two problems: restoring lateral/directional control and restoring pitch control. Differential thrust can be used to bring the wings level and control yaw.

Damping the phugoid motion can be tougher. As the nose falls, thrust can be added, or, as the nose rises, thrust can be reduced.

As Lutz says, once pitch and roll are under control, "now you can think."

What sounds straightforward in the abstract can be extremely difficult in an actual emergency. The DHL crew employed manual TOC. However, a January 2004 NASA technical memorandum, based on numerous simulator trials and flights involving different aircraft and pilots, is laced with dark expressions about manual TOC:

"Good phugoid damping is critical for TOC flight."

"After much practice, some safe landings could be made."

"Bounces were typical."

"Of the four TOC approaches, one appeared suitable for a survivable landing."

"The combined TOC task of controlling sink rate, touchdown position, and runway alignment is extremely difficult."

The message from the 2004 NASA report is that manual TOC is always a two-pilot task, and the likelihood of an average crew successfully recovering an otherwise undamaged aircraft to a runway via TOC would be low.

Automated TOC is another matter. The NASA-developed PCA was successfully demonstrated in a series of flight tests using an MD-11 in 1995. The proof-of-concept system consisted of about 300 lines of computer code. In the flight tests, the normal hydraulic flight controls were disconnected and the PCA was switched on to assess its ability to null out Dutch roll and phugoid, and to enable crews to control the airplane to a safe landing. PCA software controlled the flight path to within a few 10ths of a degree and, with a level flight path command, held altitude to within �20 feet.

Fullerton recalls, "The PCA worked better than our dreams."

"It was just like being on autopilot, so long as you did it smoothly" and avoided large changes, he says.

Based on the simulator trials of the PCA, the pilots thought its performance would degrade at higher altitude (the system having been designed for speeds of around 150 to 200 knots at altitudes below 15,000 feet). The system was flight tested under various conditions up to 30,000 feet and performed better than predicted by the computer simulations.

PCA operation coupled to an ILS approach also was tested, both in the simulator and in flight. Landings were at safe sink rates and bank angles, with a significant reduction in pilot workload.

The results with PCA were characterized by phrases virtually the opposite of the descriptions related to manual TOC. For PCA:

"In smooth air, a new pilot could make a successful PCA approach on the first try."

"In smooth air, flight path was held to within less than 0.5 degree of the pilot's command."

"The ILS-coupled approaches were successful in turbulence levels up to at least moderate."

"The PCA system operation is so sufficiently straightforward that extensive training is not required."

The system's use was limited to aircraft with flight control computers (FCCs) and full authority digital electronic controls (FADEC) on the engines--equipment that is on most modern airliners.

There is another important limitation, says Bill Burcham. He was involved in the original PCA software development. Burcham, now retired, says that even with PCA, "you need an airplane that is minimally controllable."

For older airplanes without FCCs and FADEC, Burcham recalls that simplified versions of PCA were developed. One was dubbed "PCA-light" and the other was known as "PCA-ultralight."

In the PCA-light version, the software was coupled to the autothrottles, moving them at a slower rate than in the original PCA version. The PCA-light modification moves the throttles differentially, but not beyond a few degrees.

In the PCA-ultralight version, the software was used for pitch control and the pilot controlled lateral movement of the aircraft by manually jiggling the throttles for asymmetric thrust.

"All versions had major improvements over the pilots grabbing the throttles by hand," Burcham recalls.

Why was the system never developed beyond proof of concept to a deployable system? Burcham says, "By the time we got around to flying the PCA in 1995, it was six years after the United Airlines accident at Sioux City."

Nonetheless, as a result of its investigation into the United crash, the National Transportation Safety Board (NTSB) recommended that backup means of motive power to powered flight controls should be developed for all new widebody airplanes. Lutz says, "PCA provides the same degree of control while remaining completely independent of any hydraulic systems."

"While the NTSB recommended alternate means to power the flight controls, this may be [made] possible by new aircraft designs, such as the Boeing 7E7, but it is unlikely that retrofit would be possible for existing airplanes," he adds.

"Had the NTSB known about PCA at the time of its report on the United crash, the board likely would have seen the great potential for PCA to allow a low-cost control system, independent of existing powered flight controls, and made it a part of its recommendations," Lutz says. He suggests that the federal government could provide funding to advance PCA from the concept stage to the point where it could be made available for retrofit to all airplanes without backup mechanical flight controls.

Burcham observes that the missile strike on the DHL jet has "reignited interest in what could be done in the near term."

David Evans can be reached by e-mail at [email protected].

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