777-200LR Flight Test Journal: Archives

21 October 2005

Protecting your tail

Steve Louthain, lead engineer and equipment manager for the Primary Flight Computer

For the Flight Controls teams, FAA certification testing is a long process that began last winter in the earliest stages of the 777-200LR flight test program.

New software is being developed and tested for the Primary Flight Computer (PFC). For the 777-200LR program, the Primary Flight Computer group's focus is to test new software in the primary flight control computer. In addition, we collect data during flight test which cannot be accurately predicted in the simulator in order to modify and tune the functions and control laws.

Changes to the airplane itself are primarily what drive the development of new software. We had relatively few significant changes for the 777-200LR PFC relative to its predecessors. For the 777-300ER we introduced several new features to the Flight Controls system, and new hardware was introduced to bring the hardware design up to the state-of-the-art. For the -200LR we are activating those features and customizing them for the new airframe.

Tail strike protection is a function designed to reduce the probability of the tail of the airplane striking the runway on takeoff and landing. The tail strike protection system was first introduced on the 777-300ER and has now been incorporated on the 777-200LR.

The challenge in the tail strike protection application is the calculation of the height of the tail above the runway during takeoff and landing. This calculation is fairly complex because there is no sensor on a production airplane that measures the height of the tail. Tail strike protection provides a command to the elevator in order to prevent a tail strike only if a tail strike is imminent. If the airplane is in danger of a tail strike then the proper amount of elevator command must be applied, and it must be transparent to the flight crew. Therefore the calculation of the tail height must be very accurate and timing is important.

In flight test, there are specific takeoff and landing conditions that evaluate the PFC's calculation of the tail height. The flight test airplanes have a laser sensor installed at the tail of the airplane that measure the actual height of the tail above the runway to within a couple of inches. We analyze and compare the calculation of tail height versus the actual measurement. This allows us to optimize the PFC's calculation with a software update during the flight test program. We then perform additional tests to confirm the optimized calculation is performing well.

On a fly-by-wire airplane like the 777, the flight controls function interfaces with multiple airplane systems. We also work closely with the Aerodynamics and Dynamic Loads groups. Many of our system requirements are driven by those groups. This makes for a pretty dynamic work environment.

One of the primary flight controls functions driven by requirements from the Aerodynamics/Stability and Control group is Landing Attitude Modification (LAM). The PFC drives multiple wing surfaces to modify the pitch attitude of the airplane on high speed approaches. The LAM design went through several iterations during flight test, and was one of our biggest challenges for the program. Changes and problems with the LAM design were the biggest source of long days and late nights.

One of the other big challenges during the program was troubleshooting problems squawked on the airplane and troubleshooting problems identified while lab testing software updates. The pressure was on to find solutions quickly. We analyzed flight test data, analyzed requirements and software implementation, and ran lab tests in our standalone lab and Flight Controls Integration Bench.

The -200LR flight test program has been a great and exciting experience. One of the most rewarding aspects of the program has been working with the talented engineering and flight test team.