Expectation bias played a factor in the 2023 ground collision between a Raytheon Hawker 850XP and a Textron Aviation 510 Citation Mustang at William P. Hobby Airport (KHOU) in Houston in 2023.

According to the National Transportation Safety Board (NTSB) final report, on October 24 at 3:20 p.m. CDT in VFR conditions, the Citation had just landed on Runway 13R and was rolling out when its vertical stabilizer was struck by the left wing tip of the Hawker, which was taking off from Runway 22.

No injuries were reported for the two pilots and one passenger aboard the Hawker or to the pilot and three passengers aboard the Citation.

About the Airplanes and Crew

The pilot of the Citation (N510HM) held a private pilot certificate with 1,000 hours total time and 300 hours in the accident airplane.

The Hawker was owned by DuPage Aerospace Corp. The first pilot in the Hawker had 28,000 hours total time and 3,800 hours in the Hawker. The second pilot had an estimated 24,000 hours, of which 300 were in Hawkers.

The Hawker (N269AA) was operating as a Part 135 on-demand passenger flight from KHOU to Waukesha County Airport (KUES) in Wisconsin. The Citation (N510HM) was operating as a Part 91 flight from Fulton County Executive Airport-Charlie Brown Field (KFTY) in Atlanta to Houston.

The Airport

A review of the airport diagram at KHOU shows intersecting runways and areas that the FAA has identified as “hot spots,” which are locations in an airport movement area with a history of potential risk of collision or runway incursion and where heightened attention by pilots is necessary.

These hot spots are marked on airport diagrams with boxes or circles and HS, followed by a number. However, the collision did not take place on a marked hot spot.

Accident Timeline

According to the NTSB, approximately 4.5 minutes prior to the collision, the tower controller cleared the Citation to land. The aircraft was on a 9-mile final. About two minutes later, the Citation reported a 4-mile final.

At 3:21:08 the controller cleared the Citation to land on Runway 13R, and the flight landed uneventfully.

Two minutes prior to the collision, the controller cleared the Hawker to “line up and wait” (LUAW) on Runway 22. The pilot monitoring (PM) correctly read back the clearance with “line up and wait, uh, two two, uh, six nine Alpha Alpha.”

The NTSB report indicated that when the controller issued the LUAW clearance, they did not provide a traffic advisory to either airplane, which is a required procedure. It was suggested that this traffic advisory “would have provided more context and awareness for both crews about the location and activity of the other airplane.”.

However, as the PM acknowledged the LUAW clearance, the report indicated that it “should be sufficiently clear that a delay was required before takeoff could commence, regardless of the reason.”

However, the crew of the Hawker taxied onto the runway and began the takeoff without a clearance from the control tower.

View of both airplanes’ flight tracks with the area the ground collision occurred highlighted in yellow. The blue line represents the N510HM flight track, and the red line represents the N269AA flight track. [Credit: NTSB]View of both airplanes’ flight tracks with the area the ground collision occurred
highlighted in yellow. The blue line represents the N510HM flight track, and the red line
represents the N269AA flight track. [Credit: NTSB]

The airport is equipped with Airport Surface Detection Equipment-Model X (ASDE-X), and at 3:19:49 the automated airport surface detection equipment sounded an alarm warning of a perceived collision. The controller twice instructed the Hawker to stop and hold position, but the crew did not respond.

According to the pilots aboard the Hawker, as they began the takeoff roll, they noticed that the rudder bias system had activated, which they resolved by adjusting the thrust such that both engines were set to similar power settings. The pilot flying (PF) did this, and the rudder bias system deactivated. Second, the elevator trim warning system activated, and the PM then adjusted the pitch trim (by rolling it nose down about one-sixteenth inch), which extinguished the warning.

The NTSB report suggests that these two events likely distracted the pilots and prevented them from recognizing the instructions from the controller to stop.

During the postaccident interviews, the pilots aboard the Hawker reported having an issue with the flight management system (FMS) and feeling rushed to reprogram it just as ATC issued the LUAW clearance. During the postevent interview the PM aboard the Hawker recalled hearing the LUAW clearance and a subsequent clearance for takeoff. The pilot flying recalled hearing only a clearance for takeoff.

However, a review of the certified air traffic control (ATC) voice recordings revealed that there was no takeoff clearance issued to the Hawker or other airplanes at that time. The NTSB noted that “this discrepancy was likely due to the pilots’ expectation bias, a cognitive phenomenon where individuals perceive what they expect to hear or see and act accordingly.”

The Hawker was equipped with a cockpit voice recorder (CVR), which, according to company officials, was supposed to be tested for operation prior to every flight. After the accident, the CVR was found to be inoperative due to activation of the impact or G switch, which interrupts its electrical power and its control unit in the cockpit. 

This can occur for several reasons, including hard landings or during maintenance operations. Review of the recording revealed audio consistent with maintenance activities.

NTSB investigators indicated the CVR “likely became inoperative at some time prior to this crew’s pairing, which began two flights prior to the accident flight.” 

Postaccident testing of the CVR and the impact switch revealed they operated as designed. The NTSB said the flight crew should have been aware of the CVR’s nonoperational status during the before start checklist prior to the accident and the two previous flights, had they pressed the CVR test button and noticed that none of the indicator lights on its control unit had illuminated, because the control unit and CVR were not powered.

The only methods for the flight crew to determine if the unit is functioning are to use the self-test function or by monitoring the audio through the headset jack on the control panel. Since the CVR was inoperative, the relevant crew conversations that would have provided additional insight to the investigation were not captured.

The NTSB noted “this demonstrates the importance of properly testing the CVR before each flight.”

In the postaccident interview, the Citation pilot said that he did not see N269AA, but during the landing roll he heard a “sound similar to a truck tire blowing out on a highway.” The airplane did not yaw or have controllability issues rolling down the runway or taxiing to the ramp, and it wasn’t until the pilot exited the airplane that he saw the damage to the tail.

The postcollision inspection revealed the rudder sector mount was torn from the structure at the fastener locations, the rudder torque tube was separated, and the upper and lower left and lower right rudder control cables were broken in a manner consistent with tensile overload. The left strake was impact fractured from the tail cone stinger and was recovered from the runway along with various sheet metal fragments from the stinger.

The pilots of the Hawker said they did not see the Citation until one second before impact and described the event as a “thud.” The pilots continued the takeoff and requested to return for landing. The crew noticed damage to the wingtip after landing.

The NTSB cited the probable cause of the accident as the takeoff by the flight crew of Hawker N269AA without a takeoff clearance, which resulted in a collision with Citation N510HM that was landing on an intersecting runway. Contributing to the accident was the Hawker crew’s expectation bias and distraction.

The full NTSB final report can be found below: