Between June 27, 1958 and January 14, 1999, there had been 74 crashes of KC-135 aircraft and the few related models used by the U.S. Air Force. That's nearly two a year. Several of those could be attributed to the airplane's negative lateral stability and absence of a yaw damper. KC-135A crews had to be experts on the subject of dutch roll.
Figure: KC-135R 63-8877 Dutch Roll, from Accident Investigation Report, figure 6.
- Date: 3 May 2013
- Type: Boeing KC-135R Stratotanker
- Operator: United States Air Force - USAF
- Registration: 63-8877
- Fatalities: 3 of 3 crew
- Aircraft Fate: Destroyed
- Phase: En route
- Airports: (Departure) Bishkek-Manas International Airport (FRU/UCFM), Kyrgyzstan; (Destination) Bishkek-Manas International Airport (FRU/UCFM), Kyrgyzstan
A US Air Force Boeing KC-135R Stratotanker aircraft left Bishkek-Manas Airport at 14:30 LT for a combat aerial refueling mission. Upon takeoff, a flight control system malfunction, the board found, generated directional instability, causing the aircraft's nose to slowly drift from side-to-side or "rudder-hunt." This condition, not fully diagnosed by the crew, progressed into a more dangerous oscillatory instability known as a "Dutch roll." The board identified that a poor layout of key information in the inflight manual and insufficient crew training contributed to the mishap by detracting from the crew's ability to act on critical information during their troubleshooting to turn off either of two cockpit switches which may have eliminated the malfunction.
Having not recognized the Dutch roll condition, the crew initiated a left turn to remain on-course along the planned route of flight and used a small amount of left rudder to coordinate the turn. The use of rudder, while in a Dutch roll, increased the aircraft's oscillatory instability. The ensuing large side-to-side movements of the aircraft varied the crew member's foot pressure on the rudder pedal which caused inadvertent fluctuations in rudder position. These fluctuating rudder movements, coupled with slight right rudder use while rolling out of the turn, compounded the Dutch roll severity and produced extreme airframe stress that caused the KC-135's tail section to separate from the aircraft. The subsequent, uncontrollable descent resulted in an in-flight explosion.
Figure: Dutch Roll Illustration, from Accident Investigation Report, figure 7.
[Accident Investigation Report, page 49]
- The cause of the mishap was the MA's [Mishap Aircraft] tail section separating due to structural overstress as a result of the MC's [Mishap Crew] failure to turn off either the SYD [Series Yaw Damper] or the rudder power and oscillating dutch roll-induced forces translating through the MP's [Mishap Pilot] feet as the MP used rudder during the unrecognized dutch roll condition.
- The MC's failure to turn off either the SYD or the rudder power was contrary to the Inflight Manual procedure for correcting rudder hunting. Per the Inflight Manual, the procedure is to sequentially disengage the autopilot, EFAS, SYD, and the powered rudder until the oscillations stop. Flight data indicates the SYD was engaged the entire flight. The MC deviated from the procedure by leaving the SYD engaged.
- The MP's use of rudder during a dutch roll was contrary to the Inflight Manual that prohibits rudder use in this manner. Rudder movements overstressed the tail section and initiated the MA's breakup sequence that caused the crash. The rudder inputs of the MP were causal in the failure of the tail section.
- The MP and MCP [Mishap Copilot] diagnosed a directional control problem as well as a problem with the SYD. The Lateral and Directional Control Difficulty Due To SYD Malfunction and Rudder Hunting sections in the Inflight Manual describe appropriate crew actions in response to these conditions. The Inflight Manual contains warnings that state:
- The MA was in dutch roll, but still in a flyable condition, when the MP took control of the MA. In the last minute of the mishap flight, the MP applied and varied the left rudder pressure multiple times, then reversed pressure to the right pedal. These rudder pedal movements, confirmed by the FDR data analysis, reveal the growing sideslip oscillations of the MA represent the application and multiple variations of left rudder pressure, as well as an abrupt right rudder reversal in the final 10 seconds leading to a progressively increasing sideslip. Due to the timing of the rudder inputs, each peak rudder input occurs in the same direction as the sideslip. Instead of decreasing the sideslip, the MP's rudder pedal movements compounded it. The final rudder input of 11 degrees coincides with the structural failure of the tail section and end the of the FDR / CVR data, about one second before the MA pitches over.
"Manual damping of dutch roll is to be accomplished with lateral (aileron) control. Do not attempt to damp dutch roll manually with the rudder...the sudden reversal of rudder direction at high rudder deflections, due to improper rudder application or abrupt release, can result in overstressing the vertical fin. This condition could be brought about during recovery attempts from a flight condition induced by a lateral control malfunction."