777-200LR Flight Test Journal: Archives

02 August 2005

Aero

Nick Ferrera

Aero Performance Engineer and 777-200LR team leader

Jim Blohowiak

Airplane Performance Engineer

'Aero performance' is a big area of flight testing for the 777-200LR. When we fly on the test airplanes, we think of it as measuring the airplane as an integrated whole, not as individual parts or systems.

Performance can be broken up into two broad categories: high speed and low speed. High-speed performance data is not used for airplane certification, but is critical for the cruise and mission performance guarantees we give our customers. It's almost exclusively limited to fuel mileage, or how much fuel they need to get to their destination. That's because it is what the airlines pay attention to and what they need to know for operations and mission planning.

Calculating fuel mileage means determining many things very precisely. We inspect the airplane for contour to make sure it's rigged and trimmed properly so it's not flying in some sort of asymmetric configuration. We have test samples of fuel to measure fuel properties and energy capacity. And the airplane is weighed very carefully. Then we fly the airplane in very calm air to capture data on fuel flow, our exact speed and altitude. By accounting for all these things, we try to remove variability from the fuel mileage calculations and come up with the best determination of airplane cruise performance.

The category of low speed testing is primarily for airplane certification. Among other things, it includes takeoff testing, where we simulate an engine failure and record the time and distance it takes for the airplane to rotate, lift off and fly away. We develop a speed schedule by doing a series of tests at different weights and power settings to determine the speeds we should use for takeoff. Along with verifying or adjusting speeds, we also look at what we call flare times. This refers to the time it takes the airplane to fly from liftoff to 35 feet. The speeds and distances are very precisely computed. Its part of the takeoff field-length calculation that's required for every takeoff the airplane will do in service.

Most of the data we gather during flight testing ends up in the flight manual we give to the airline customers. It's like the owner's manual for the airplane. We boil our test data down into parameters such as takeoff speed schedules, landing approach speeds, and airplane drag. These parameters get reassembled in a large database that links them with a thrust model from our propulsion group. With computational tools, we can then calculate the airplane's performance at any weight, temperature, altitude and thrust setting.

Much of our performance flight testing has to be done at remote desert airfields with long runways and little else to get in the way. As a group, we've been supporting testing for years on all the Boeing commercial products.

We now look forward to the nasty taverns and breakfast joints in the middle of the desert when we support flight testing. Its part of our group's culture on these remote tests to go through the rituals of bad food, early wake-up calls and learning too much about our co-workers. It's those experiences that we look back on fondly as a unique part of our jobs.