Unbalanced in Charlotte

Air Safety Week, March 8, 2004

Judgement has been rendered. The Air Midwest twin-turboprop that crashed Jan. 8, 2003, in Charlotte, N.C., was overweight - and out of balance, to boot. The tailheavy situation meant that mis-rigged elevator control cables left the pilots with insufficient pitch control authority, so the accident has been deemed another example of maintenance-related mayhem (see ASW, March 1). The National Transportation Safety Board's (NTSB) findings in the case mark the denouement of an apprehension expressed in this publication five years ago about the growing potential for a crash stemming from maintenance error.

The crash of the Air Midwest Beech 1900D has wider implications, as evidenced by post-accident corrective actions. Consider three issues:

Design:

The Beech 1900D design culminates the process of "growing" the original Beech 200 planform (see ASW, June 9, 2003). As the variants grew in succession, control correctives were added, setting something of a record. The 1900D design, featuring a number of additions to the tail structure, was described by one industry observer as "playing Mr. Potato Head," a reference to a child's game in which plastic pieces are stuck into a potato to create humorous faces.

The design features a fixed horizontal stabilizer, as opposed to a trimmable stabilizer controlled by a jackscrew. Hence, all the pitch control had to be invested in the elevator control and the trim and servo tabs. The fences hanging down from the horizontal stabilizer, called "tailets," are meant to inhibit outward airflow toward the tips and make the elevator more effective.

The anhedral strakes beneath each side of the empennage are intended to provide more stabilizing aft keel surface area (for directional stability), as well as for increasing the horizontal stabilizer wetted area. The same applies to the mini-stabilizers, called "stabilons," apparently for airflow streamlining at high angle of attack and horizontal stabilizer augmentation. It seems obvious that this aircraft's pitch stability was a concern - hence all the awkwardlooking adornments.

In the evolution from the original B200 to the 1900C and then to the 1900D, the basic design changed substantially. Our illustrative and tabular discussion of the "stretch" to the 1900D from the original Beech 200 design in the June 9, 2003, issue of Air Safety Week speaks volumes. The two airplanes don't look anything alike and certainly don't behave aerodynamically the same way. Yet both have the same type certification number. At what point is the design different enough to warrant going back to square one for certification purposes?

The Federal Aviation Administration (FAA) produced an answer after the accident at Charlotte. On April 28, 2003, the agency issued advisory circular number 21.101-1, "Establishing the Certification Basis of Changed Aeronautical Products" (see ASW, June 9, 2003). This document shows that there is a point at which derivative designs have exceeded the original parameters for certification and the process must begin anew.

Payload weights:

Investigators found that average passenger and checked bag weights used to calculate takeoff weight and center of gravity greatly underestimated actual weights. Was the FAA unaware of the widespread obesity problem in the Western World, and its obvious impact upon assumed average weights for takeoff calculations? This rhetorical question aside, it is a known fact that the FAA issued a notice (No. 8400.40) on Jan. 27, 2003, calling upon operators to revalidate their weight and balance programs (see ASW, Feb. 3, 2003). That notice, issued 19 days after the accident, culminated in a final notice (No. 8300.112), issued Dec. 12, 2003, increasing average passenger, carry-on and checked bag weights substantially.

Yet even the upwardly revised weights may not be sufficient, as the NTSB pointed out that the Beech 1900D could still be loaded outside of its aft center of gravity limit using the new average weights. The picture that emerges is one where the loading of passenger aircraft remains under-regulated, unlike military aircraft where loadmasters meticulously weigh everything going aboard.

Maintenance:

In military terms, any general has to depend on mid-level commanders to do the right thing. It is evident that second- and third-hand maintenance can set a death trap if everybody up the chain of command wrongly assumes that the troops and sergeants on the line know what they are doing.

Hence, at the coalface of the operation, sledgehammer-wielding tyros who haven't done the job before are trying to follow a poorly written, much amended and badly illustrated manual, while not really understanding the nuances and pitfalls of the cable-rigging task at hand. The harried supervisors give the job a quick look-over and sign off on it.

Again, history repeats itself. Following the Jan. 31, 2000, fatal crash of Alaska Airlines [ALK] Flight 261, the carrier was almost grounded for subsequently revealed shortcomings in its Continuing Analysis and Surveillance System (CASS) program (see ASW, July 3, 2000). The program did not catch the improperly maintained elevator jackscrew that led to the crash of the MD-83.