General Flight Test Theory Applied To Aircraft Modifications

Acquisition Review Quarterly, Spring, 1999 by Lt Col Lionel D. Afford, Jr., Robert C. Knarr

Any external aircraft modification has potentially far-reaching effects on the capability of the aircraft to succeed or fail in its mission. The authors take a systematic look at the effects that small changes can have upon the whole, with a series of examples that demonstrate why careful review of data or testing is often vital in the assessment of system modifications.

A new design aircraft program always includes an instrumented test to validate the analyses. But a modification program may rely instead on previously collected data for model validation. Such a program must adequately address the effects of the modification on the aircraft and its mission. The user must judge these effects for their desirability--especially when they degrade mission capability. But, to be judged, they must be fully understood. Reviewing historical data or conducting a test are two ways to validate the data by which these effects on aircraft capability are judged.

In this article, we address eight critical test and evaluation considerations for an external aircraft modification. The aircraft design problems covered here represent the fundamental characteristics by which aircraft capability is judged. These design problems, when not properly analyzed and tested (if required), have historically resulted in significant degradation of air worthiness. We define the subject area and explain the importance of each problem by discussing the rationale behind standard design practices and air worthiness and operational considerations for the fleet aircraft. Concrete examples illustrate each case. Although only effects to the C-130 aircraft are discussed in detail, these principles and observations apply to any aircraft.

STRUCTURAL (STRESS AND LOADS ANALYSES)

Rationale. When structural strength proof tests are not performed, it is a standard engineering practice to specify that aircraft modifications be designed for a 25 percent or greater static margin of safety using a factor of safety of 1.5. The modified airframe will then have the strength capability to be released to fly at 100 percent of design capability.

However, if analyses show that an aircraft has a margin of safety between 0 and 25 percent, then the aircraft must be tested with sufficient instrumentation to ensure a positive margin of safety for the ultimate design conditions in order to prevent flight envelope restrictions. Finally, if analyses reveal a negative margin of safety or failure occurs during testing, either the deficient structure must be redesigned or aircraft flight envelope restrictions must be imposed.

Air worthiness and operational considerations. Reduction of the flight envelope means the aircraft must be restricted in airspeed, symmetric or maneuver C-loading, sideslip, or payload to prevent a design load limit (DLL) from being exceeded. Limiting the C-130 flight envelope as a result of any modification will significantly affect the aircraft mission capability. This is to be avoided at all cost.

The ultimate result when an aircraft is not designed to standard engineering practice (or verified by test) is increased likelihood of component or structural failure. An example of this is skin surface antenna mounts that come off in flight due to repeated flights at high airspeeds. Structural modifications that pierce the pressure vessel and are grounded in the load-bearing components of the aircraft are a special threat. This is because those components, when they fail, have a tendency to cause the failure of other load-bearing structures. This domino (a.k.a. zipper) effect can result in the loss of an aircraft. The loss of a modified KG--135 aircraft in the early 1970s was probably attributable to such a failure in a fuselage-mounted radome. Another problem symmetric modification can create is asymmetric loading. As a result of even the most benign maneuvers, the modification may be subject to airloads that cause oscillations in the fuselage. This can result in fatigue failure of structures well forward of the modification. The Beech V-tailed Bonanza is a classic example of this; the shape of the tail caused fishtailing that eventually resulted in fuselage failure.

PRESSURIZATION

Rationale. Pressurization is directly related to the previous discussion. It is in its own category because it is a common and potentially catastrophic failure mode in modifications. Generally, when the aircraft pressure vessel is penetrated, for whatever reason, a full pressure test series (proof and leakage rate) is made on the aircraft. Following a significant modification, a full pressure test must be completed prior to the first flight during which the aircraft will be pressurized. The pressurized portion of the aircraft must be capable of withstanding proof-pressure testing at a level 1.33 times the maximum setting plus tolerance on the safety valve. This test should be performed on each modified aircraft.

Air worthiness and operational considerations. The importance of verifying pressure vessel integrity is evident from the standpoint of the potential consequences of a modification failure which breaches the pressure vessel. Pressure vessel failures have the potential to cause the loss of an aircraft due to an explosive decompression. An example of this is the C-130 flying near Iceland that had a breach near the wing root; it lost most of the top of the center wing and some of the fuselage. This aircraft made it back safely; many crews have not been so lucky. With any depressurization there are additional safety hazards to the crew as well.