In June 2019, a King Air 350 crashed on takeoff at Addison (Texas) Airport, killing 10 people. At the time, the NTSB preliminary report focused on a loss of power on the left engine and a loss of directional control.
Now, nearly two years later, the NTSB final report issued on May 18 points to a lack of pilot discipline and manual handling skills and the potential that a single aircraft component can contribute to a crash. The latter, a simple power lever friction lock, according to the NTSB and King Air experts, might have contributed to this and many past inadvertent thrust reductions on this popular turboprop.
In its final report, the NTSB determined the probable cause to be “the pilot’s failure to maintain airplane control following a reduction of thrust in the left engine during takeoff. The reason for the reduction in thrust could not be determined. Contributing to the accident was the pilot’s failure to conduct the airplane manufacturer’s emergency procedure following the loss of power in one engine and to follow the manufacturer’s checklist during all phases of operation.”
From the report, let us dissect the NTSB probable cause statement to provide a little more clarity on this event. Since the aircraft did not have a flight data recorder, investigators had to rely on sound spectrum analysis from the cockpit voice recorder (CVR), video images from external sources, and physical evidence such as “witness marks” on the left engine and propeller to determine that there was a large reduction in thrust.
It was determined through performance calculations that it was “likely” the pilot initially applied left rudder in response to the reduction of thrust on the left engine; this action ultimately caused the aircraft to roll inverted and crash. Had the pilot applied the correct rudder—the right side—the report concluded that it would have been possible to maintain directional and lateral control of the aircraft.
Post-accident functional evaluations of the engine and propeller found no condition that would have prevented normal operation. Both engines were operating at the time of impact. In the absence of engine malfunction, the investigation considered that the reduction in the left engine power was caused by another means such as “an uncommanded throttle movement due to insufficient friction setting of the airplane’s power lever friction lock.”
Heavy fire and impact damage precluded the determination of throttle lever position or friction setting on the accident aircraft, thus the reason for the thrust reduction was not conclusive.
The last element of the probable cause statement is the most troubling and easiest to determine based on the CVR recordings and interviews: the accident pilot simply did not use checklist. The investigation concluded that “given the lack of callouts for checklist on the CVR and the pilot’s consistent reported history of not using checklist, it is possible that he did not check or adjust the setting of the power lever friction locks before the accident flight, which led to the uncommanded movement of the throttle.”
Likewise, following the reduction of power during takeoff, “the pilots did not discuss any emergency procedures.”
According to the report, several pilots who knew the accident pilot and had flown with him were interviewed during the investigation. One pilot reported that the accident pilot “was not strong on the use of checklists.” Another said, “He just jumped in and went.” Several others mentioned “he was bad at using checklist” and would not accomplish tasks such as weight and balance or V-speed calculations.
Now, let us have a deeper discussion on King Air power lever friction locks and a phenomenon known as “power lever migration.” In the NTSB docket related to this accident, there is a great article by Tom Clements titled “Power Loss versus Engine Failure” (reprinted from King Air Magazine).
Clements is a pilot, instructor, and expert with more than 15,000 hours of King Air flight time and surmises that “for every true engine failure [in a King Air]…I believe that there have been at least 10 times as many power rollbacks—maybe even 100 times as many. Of these rollbacks, a sizeable number have been due to mechanical failures beyond the pilot’s control.”
But, he said, compared to those mechanical reasons, there is one cause of power rollbacks more than any other that is completely within the pilot’s ability to control: power lever migration.
Clements describes power lever migration as “the tendency for the power lever to spring back toward idle caused by a spring on its connection to the beta cam box.” The power lever friction control exerts enough resistance to the spring force to prevent the power lever from moving towards idle. Thus, Clements has stressed the importance of this control in numerous articles, forums, and in his book, “The King Air Book.”
The NTSB final report also included additional information on friction lock checklist procedures. According to FlightSafety Textron Aviation Training, “which emphasizes the risk of an unintended power lever migration and the potential loss of control if the friction lock setting is adjusted incorrectly, also provides manufacturer checklist procedures.” Engine control friction locks are an item on both the “Before Engine Start” and “Before Takeoff” (runup) checklists.
Outside checklist use and proper adjustment of the friction lock controls, there are other techniques to prevent inadvertent power lever migration. One technique, in any aircraft, is to “guard” the throttle (or thrust levers) during critical phases of flight—takeoff, initial climb, and approach and landing. If the pilot flying (PF) isn’t guarding the throttle, the pilot monitoring should back up the PF.
Checklist usage is a foundation to both good airmanship and safety. As demonstrated in this accident, the lack of checklist discipline can have a catastrophic outcome. In addition, revisiting the basics of aircraft handling with an inoperative engine, such as “dead foot, dead engine,” is paramount to handling these scenarios.
Pilot, safety expert, consultant, and aviation journalist Stuart “Kipp” Lau writes about flight safety and airmanship for AIN. He can be reached at firstname.lastname@example.org.