Photo: Air Midwest Flight 5481 crash site from CLT terminal, from "Lookout2" (released to public domain)

Eddie Sez:

Anyone involved in aircraft maintenance is subject to many of the same pressures felt by pilots, but in some areas the pressures are worse. Both professionals are subject to becoming complacent and the temptation to take short cuts. Both can be adversely impacted by the effects of fatigue. While both receive initial training and some level of oversight, mechanics do not always receive ongoing (recurrent) training and regular practical examinations (check rides). With both populations, as experience and reputation grow, the temptations associated with complacency grow too. This is one of those cases.

A quality assurance inspector was wearing two hats the day before this mishap, he was instructing a mechanic new to the airplane's rigging procedure and inspecting that same mechanics work. The only functional check of the work would occur the next day with the airplane filled with passengers. The inspector assured the mechanic in training several steps of the process could be skipped and the elevator was left with inadequate nose down pitch authority.

The report doesn't go into why the nose down authority was compromised other than to compare the end state of two turnbuckles, devices used to adjust the tension of the elevator cables. The mechanic apparently adjusted the nose up turnbuckle much tighter than the nose down turnbuckle. Since the cables wrap around pulleys and are connected indirectly, adjusting one turnbuckle impacts the tension on both. Several of the steps they skipped could have impacted the apparent tension, could. But two of the skipped tests prevented them from detecting their error. They could have determined the precise amount of available elevator travel had they measured flight data recorder pitch data and had they checked actual elevator travel using a "travel board" from its level. But they didn't. The quality assurance instructor didn't think any of that was necessary.

The airplane was also loaded with a CG that was too far aft.

Either one of these problems might have been recoverable, but combined they resulted in killing everyone on board.

I call these mishaps cases of maintenance malpractice because of the short cuts taken, the lack of adequate training and oversight, and the possible impact of fatigue. Whatever the cause, the aircraft was rendered unflyable because of it.

What follows comes from the references shown below and my comments shown in blue.


Accident Report


Narrative

The Flight

[NTSB AAR-04/01, ¶1.1

Maintenance Events Prior to the Flight

[NTSB AAR-04/01, ¶1.1


Analysis

Personnel

[NTSB AAR-04/01, ¶1.5

The Pitch Control System

Figure: Beech 1900D pitch control system, from NTSB AAR-04/01, figure 4.

[NTSB AAR-04/01, ¶1.6

  • The Beech 1900D airplane is equipped with a mechanically operated pitch control system. The three primary elements of the pitch control system are the elevators, the control column, and the connecting rods and cables. The elevators (left and right) are attached to the trailing edge of the horizontal stabilizer, which is mounted on top of the vertical stabilizer in a T-tail configuration. A pilot pushes forward on the control column to move the elevator trailing edges down (resulting in the airplane pitching AND) and pulls back on the control column to move the elevator trailing edges up (resulting in the airplane pitching ANU).

  • The inboard end of each elevator has a control horn that is connected to the elevator surface by a shaft. Four primary stop bolts (left upper, left lower, right upper, and right lower) are mounted on airplane structure. The limit of travel for each of the elevator control horns is contact with an up stop bolt or a down stop bolt. The Beech 1900D AMM, section 27-30-02, indicates that the elevator primary stop deflection settings are 20° +1j/-0° up from the neutral position and 14° +1°/-0° down from the neutral position. The elevator's neutral position is the point at which the position of the trailing edge of the elevator is aligned with the chord plane of the horizontal stabilizer.

  • Control cable assemblies (one ANU and one AND) connect the aft bellcrank to the forward bellcrank. The control cable assemblies have two cable sections joined by turnbuckle assemblies located in the base of the vertical stabilizer. Each control cable assembly comprises seven spirally wound strands and has one long and one short component. The turnbuckle assemblies establish the correct tension in the cables. Each turnbuckle assembly consists of a barrel and two threaded cable terminals.

  • The accident airplane was equipped with a mechanically operated pitch trim control system. The pitch trim control system includes a single movable trim tab for each elevator and a cable-driven jackscrew actuator for each tab. The trim tabs, which are located on the inboard trailing edge of each elevator, relieve the force a pilot must hold on the control wheel to provide longitudinal control (for example, angle of attack or pitch) of the airplane. According to the Beech 1900D AMM, the trim tabs move from 5.75° ±0.25° up to 17° ±0.5° down. Movement of the trim tabs in the downward direction creates an upward aerodynamic moment on the elevator and results in an ANU pitching moment.

[NTSB AAR-04/01, ¶1.6.3.1] ELEVATOR CONTROL SYSTEM RIGGING

The procedure that follows includes several steps that are not applicable and some that were intentionally skipped.
Green on black items are not applicable to this airplane.
Black on amber items were applicable but intentionally skipped.

  1. Disconnect the autopilot servo cables.

  2. Locate and remove all access panels from the vertical and horizontal stabilizers to gain access to the aft elevator bellcrank and the elevator cables.

  3. Locate and remove the flight compartment seats, carpet, and floorboards to gain access to the forward elevator bellcrank.
  4. It was possible to view a required rig pin without removing the first officer's seat but it was easier with the seat removed.

  5. Locate and remove the passenger seats, carpet, and floorboards on the right side of the passenger compartment to gain access to the elevator cable turnbuckles.

  6. Install an elevator travel board on each elevator at station 50.00.
  7. A travel board is a calibrated template that attaches to the trailing edge of the horizontal stabilizer that allows a person positioned at eye level to see exactly how far up or down the elevator has moved. The text of the report's factual information does not note this step was skipped but the analysis and findings section imply that it was. It makes sense that it was, otherwise the mechanics would have detected the resulting limited elevator travel.

  8. Adjust the center-to-center length of the push-pull tube assembly between the control column and the forward elevator bellcrank to a dimension of 15.12 ± 0.06 inch.

  9. Adjust the surface stop bolts on the elevator control horn support for up-travel of 20° + 1° - 0 and down-travel of 14° + 1° - 0°.

  10. Verify the bob weight stop bolt clearance is 0.5 ± 0.06 inch. Adjust if necessary.

  11. Adjust the forward bellcrank stops for 0.37 ± 0.06 inch clearance from the stop bolts.

  12. Verify the forward bellcrank stop bolts make contact before the bob weight stop bolts make contact with the weight.

  13. Install a rig pin in the aft elevator bellcrank.
  14. NOTE: Verify threads are visible through the inspection holes at the end of the pushrods after adjustments are made.

  15. Adjust the pushrods between the aft elevator bellcrank and the elevator to position the elevator at neutral (0° deflection).

  16. Remove the rig pin from the aft elevator bellcrank.

  17. Remove the safety clips from the turnbuckles and release cable tension.

  18. Move the control yoke to install the rig pin in the forward elevator bellcrank.

  19. Tighten the elevator-up cable until the elevator rises to neutral (0° on the travel board).

  20. Tighten the elevator-down cable until the average tension of the up- and down-cables is 66 ± 8 pounds (the sum of up-cable and down-cable tensions, divided by two). Refer to Figure 203.

  21. Continue to balance the adjustment of the two cables until the average tension is 66 ± 8 pounds while maintaining 0° deflection of the elevator.

  22. Perform the CONTROL COLUMN SUPPORT ROLLER INSPECTION procedure.

  23. Install safety clips on the turnbuckles.

  24. On aircraft equipped with the F1000 Flight Data Recorder, calibrate the Pitch Position Potentiometer. Perform the FLIGHT DATA RECORDER (FDR)̃ PITCH ADJUSTMENT procedure.
  25. The mechanic thought this unnecessary and the quality assurance inspector thought the airplane didn't have an FDR.

  26. Remove the travel boards from the horizontal stabilizers.
  27. See step e.

  28. Connect the autopilot servo cables to the elevator primary control cables.

  29. Install the seats, carpet and floorboards.

  30. Replace all access panels.

Step a, to disconnect the autopilot servo cables, and step w, to connect the autopilot servo cables to the elevator primary control cables, were not applicable to the accident airplane because it did not have an autopilot. Step d, to locate and remove the passenger seats, carpet, and floorboards on the right side of the passenger compartment to gain access to the turnbuckles, was not applicable because the elevator cable turnbuckles for the Beech 1900D model are not located under the passenger compartment floorboards.

[NTSB AAR-04/01, ¶1.6.3.2]

Photo: Turnbuckles as found in the wreckage, from NTSB AAR-04/01, figure 6.

[NTSB AAR-04/01, ¶1.12.4]

[NTSB AAR-04/01, ¶2.3]

[NTSB AAR-04/01, ¶2.4]

Findings

[NTSB AAR-04/01, ¶3.1]


Probable Cause

[NTSB AAR-04/01, ¶3.1]


See Also:

Abnormal Procedures / Maintenance Malpractice

Pilot Psychology / Complacency

Technical / Fault Tolerance


References

NTSB Aircraft Accident Report, AAR-04/01, Loss of Pitch Control During Takeoff, Air Midwest Flight 5481, Raytheon (Beechcraft) 1900D, N233YV, Charlotte, North Carolina, January 8, 2003