Flight Test Programs for Aircraft

The first test flights were seat of the pants affairs performed by the best and bravest pilots. Now, many millions of dollars are at stake, possibly even the company’s survival. Every detail of the program is well conceived and carried out. But the basic goals of a test flight program remain the same.

The flight test process begins long before the momentous first flight. By the time the airplane actually flies it will have been subjected to a bewildering number of tests already.

Test pilots with superior flying skills are selected. They must be ready for life threatening surprises and be able to lead the way for future pilots. They will work closely with the engineers from the outset to better predict the likely handling and performance characteristics. They are quite knowledgeable about the aircraft before it is even built.

New aircraft are usually designed with specific customers in mind, whether they are commercial or military. Each will have performance and capability requirements that must be met. Proving and quantifying these requirements are an integral part of any test flight process. Payload, take off and landing distances, speed and fuel burn rates are some of the important test areas.

The prototype is a beehive of activity as it is assembled. As each system becomes functional it goes through static testing. The structural components are often tested to destruction in hydraulic test rigs. Any structural problems in the airframe, such as cracking, must be discovered and redesigned during static testing. Landing gear is tested in test rigs then performs function tests on the aircraft. Control surfaces and other moving parts also go through similar processes. Realistic passenger evacuation trials are carried out. Once the avionics and electronics are fully tested and debugged the aircraft is deemed to be airworthy and released for taxi trials.

Taxi trials let the test pilots get a “feel” for the new aircraft. Some of the parameters future pilots will need to know are determined, such as minimum takeoff speed. Operation of the engines, nose wheel steering, braking and control surfaces are tested at increasing speeds without risking the aircraft. Inevitable deficiencies are corrected as they are discovered. Usually more than one aircraft is used to facilitate this process. The stage is set for the first flight once the pilots and engineers are satisfied.

The first flight marks the beginning of a lengthy and sometimes dangerous flight test program.

Understandably, the first flight is usually quite conservative. Each flight will continue to prove the planes performance and safety at increasing speeds and G’s. The development of the aircraft’s training and flight manuals continues through the entire program.

Today’s test flights are much safer with advanced flight simulators in common use. While they are very realistic, the test flight of the real aircraft will help to calibrate them for even better accuracy. Like other areas of the program, the test pilots lead and prepare the way for all future ones.

Two of the test items are riskier than the others: stall characteristics and Velocity Never Exceed (VNE). To illustrate the elevated risk, the first Airbus A380 was fitted with an escape chute in the cockpit. The crew wore parachutes and was prepared for an “in-flight exit” in the event of structural failure or unrecoverable stall. It was a small consolation knowing the unlikelihood of escaping safely.

The aircraft designers will have a good idea of what the maximum safe speed will be. Regardless of careful design and planning, aircraft are sometimes lost during the VNE and stall tests. Unexpected aerodynamic loads can cause varying degrees of structural failure. “Flutter” is a most unpredictable and insidious type of malfunction. If a control surface begins to flutter it can increase to a predictable outcome – the component will wrench itself free of the aircraft.

Every aircraft will stop flying, or “stall”, at some combination of airspeed and angle of attack. The flight test program must define what the safe limits are to avoid a stall. The stall characteristics are explored from initial symptoms to a deep and very dangerous stall. Equally important, procedures to get out of a stall must be developed and documented in the flight manuals.

A successful flight test program will have confirmed the structural and operational safety of the new aircraft. Deficiencies will have been discovered, remedied, and retested. Safe limits of operation will be established, as well as procedures to recover the aircraft in the event of a problem. Finally, a complete set of flight manuals will be prepared for pilot training and operation of the aircraft. Thanks to the highly skilled pilots and flight test team, the new aircraft will be one big step closer to its Certificate of Airworthiness. When it’s our turn to fly in the plane, we can rest assured that it will be a pleasantly uneventful and safe flight.