HISTORY OF FLIGHTOn May 11, 2022, about 1419 central daylight time, SkyWest flight 5069, a Mitsubishi CL-600-2B19, N965SW, sustained minor damage to the right-wing flap skin and fairing when it struck an airport sign during a runway excursion after landing on runway 8R at George Bush Intercontinental Airport (IAH), Houston, Texas. The 16 passengers and crew aboard the airplane were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 121 scheduled domestic passenger flight from Victoria Regional Airport (VCT), Victoria, Texas, to IAH. The CVR recorded the sound of touchdown at 1418:59. The captain, who was the pilot flying, reported that, after a normal touchdown, the initial ground roll and deceleration were uneventful as the airplane tracked the runway centerline. The captain recalled that shortly after the first officer’s 90-knot callout (which occurred at 1419:09), he stowed the reversers. The tower controller informed the flight crew of landing traffic 2 miles in trail and requested thatflight 5069 turn right toward high-speed taxiway NK. The captain complied with this instruction, but the airplane continued to turn right with the magnetic heading continually increasing. At 1419:14, the CVR recorded the captain commenting about the lack of nosewheel steering.
The captain attempted to stop the turn using the rudder pedals and the nosewheel steering tiller but was unsuccessful. Shortly thereafter, the captain noticed a “STEERING INOP” caution message on the engine indicating and crew altering system (EICAS) display. The CVR captured an EICAS message chime and the DFDR recorded a steering fault after the airplane departed the runway. At 1419:18, the CVR recorded sounds that were consistent with the airplane departing a paved surface. About 4 seconds later the FDR recorded a Nose Wheel Steer Status of Failed/Not Armed, and an EICAS chime was recorded by the CVR 1 to 2 seconds later. The captain continued to apply braking until the airplane came to a full stop in the grass alongside the right side of the taxiway, which occurred about 3 seconds after the EICAS chime.
A photograph provided by the Federal Aviation Administration showed tire marks on the runway that veered to the right toward the airplane’s resting location in the grass. The marks were consistent with the left and right main landing gear tires.
Figure 1. Runway Tire Marks. (Courtesy FAA) AIRCRAFT INFORMATIONAccording to the Mitsubishi Heavy Industries Regional Jet – (MHIRJ, the type certificate holder) Flight Crew Operating Manual, the nosewheel can be steered up to 70° left or right via the tiller handwheel and up to 7° left or right via the rudder pedals. Further, if a failure is detected by the nosewheel steering (NWS) electronic control unit (ECU), which is also known as the analog/digital steering controller, or if hydraulic system No. 3 fails, the NWS would revert to a shimmy-damping mode to allow free-castering of the nosewheel. The pilot could then maintain directional control on the ground using the rudder, differential braking and differential thrust. The ECU captures detected fault codes in nonvolatile memory (NVM). The codes are stored in reverse chronological order without dates or timestamps.
According to the MHIRJ Fault Isolation Manual, NWS is deactivated when the weight on wheels (WOW) signal indicates “air.” When the airplane is airborne, the steering command signals are replaced with a “straight ahead” reference. Also, a mechanical cam mechanism inside the nose landing gear (NLG) strut ensures that the NLG is straight when the strut is fully extended. FDR data indicated that the WOW switches and hydraulic system No. 3 pressure were operating normally. If the tiller handwheel is not centered during touchdown – and the nosewheel steering angle is less than 7°, the ECU would not detect a fault. If this situation were to occur, when the nosewheel WOW switch changes to ground, the ECU would combine the tiller commanded position with the rudder pedal position, to compute a steering command for the nosewheel.
The NWS Status FDR parameter changes to FAIL/Not Armed if the ECU detects a fault with one or more NWS system components or the NWS is not armed. This condition is annunciated on the EICAS display in the cockpit as a caution (amber) “STEERING INOP” message with an accompanying aural chime. Faults include system component failures; improper voltages; degraded performance; or unexpected operating conditions such as valve positions inconsistent with commanded position, handwheel commanded steering angle (greater that 7°) during touchdown, or excessive rates of operation, that is, moving the tiller handwheel or rudder pedals too quickly (sensors rotated more than 500°per second).
According to the SkyWest Dispatch Procedures Manual, the airplane’s crosswind component limitation was 27 knots for a dry runway.
Post-incident Examinations
After the event, SkyWest maintenance downloaded the NVM from the ECU, which contained the NWS fault codes shown in table 1.Because no dates or times were associated with these codes, the investigation could not directly determine which (if any) of the codes were related to the event. However, the codes are stored in the sequence in which they occurred in (reverse) chronological order. The table below was provided by MHIRJ, the potential cause(s) of the codes are referenced from the MHIRJ Fault Isolation Manual STEERING INOP Message section. The most recent code is listed first.
Code Description Potential cause 033 PILOT_HANDWHEEL_RATE_FAULT Detection of handwheel position greater than 8° from center during a 12.5-second system test that occurs immediately after nosewheel extension. Movement of the handwheel at a rate greater than 500° per second 191 WARM_BOOT_OCCURRED ECU 115 AIR_TO_GROUND_MODE_CMD_FAULT Handwheel- commanded steering angle (upon air-to-ground transition) exceeds 7° of NWS angle 109 SSV_STUCK_CLOSED_PS1 SSV (solenoid selector valve), or PS1 (pressure switch) or NLG (nose landing gear) priority valve or NLG selector valve or NWS system check valve blockage or wiring 170 PILOT_HANDWHEEL_RVDT_FAULT Pilot’s handwheel control unit or wiring 036 FEEDBACK_TRACKING_FAULT R feedback RVDT (rotary variable differential transformer) or L feedback RVDT or grease contamination or steering collar wear
105 SSV_COIL_BIT_FAULT SSV or ECU or wiring or NWS system check valve blockage Table 1: Nose wheel steering electronic control unit fault codes. (Source: MHIRJ)
SkyWest maintenance also performed the instructions in the MHIRJ Fault Isolation Manual for the STEERING INOP caution message but was unable to determine why the message occurred. A timing check was performed to determine whether the NWS handwheel would return to center from the full-left and full-right positions within the 2- to 4-second range, and the check showed that recentering was within the specified range.
SkyWest maintenance then replaced the NWS handwheel control unit (tiller) and ECU, checked the voltages for the left and right actuator feedback rotary variable differential transducers (RVDT), and the rudder pedal RVDT; all were within limits, and the NWS system operated normally (RVDTs are angular position sensors).
The ECU, as well as the handwheel control unit, electrohydraulic servo valve, steering selector valve, rudder RVDT, a wiring harness, hydraulic manifold, and compensator were shipped to their respective manufacturers/suppliers for examination and acceptance test procedure (ATP) testing.
The ECU, rudder RVDT, and compensator passed all ATP test elements. The handwheel control unit was tested at its supplier’s (Safran Landing Systems) facility before the start of the NTSB’s investigation, and no test report was generated. Safran reported that the unit failed the return-to-center element of the ATP. The unit exceeded the allowable 4 seconds to return to center after it was fully displaced in both directions (left and right). The unit was cleaned, and then it passed the ATP test.
The electrohydraulic servo valve operated normally and met ATP specifications except for null bias. The null bias shift was small (measured at 0.21 mA, maximum allowable was 0.16 mA). The steering selector valve passed all functional elements of the ATP but failed one nonfunctional element (the insulation resistance test at 500 VDC). The resistance criteria was 500 megohms or greater; the tested value was 7 megohms. The NLG wire harness passed all ATP elements except for the dielectric strength test at 550 VDC. Disassembly of the harness found two broken shield wires, which are used for a health monitoring function and do not affect steering. The hydraulic manifold passed all ATP elements except for the “Flow1” test, which required the hydraulic fluid flow to be between 0.95 and 1.40 gallons per minute. The flow was measured as 1.41 gallons per minute.
None of the minor discrepancies would have prevented normal operation of the NWS system.
Maintenance Records
A review of the airplane’s maintenance records over the 12 months preceding the incident related to the NWS system revealed the following pilot-reported discrepancies:
One report of the tiller not being centered and the airplane pulling “excessively” to the right during taxi, along with the STEERING INOP EICAS message. The discrepancy was cleared after the handwheel control unit was inspected, and the nose wheel steering health check was performed. Neither of those tasks found any anomalies. Six additional reports of the STEERING INOP EICAS message were noted. Three were cleared after performing a reset of the NWS system, two were cleared after NWS system operational checks were performed, all with no anomalies found. One was cleared after the right feedback actuator RVDT was found out of rig. A review of the maintenance records related to the NWS over the 6 months following the incident revealed a pilot-reported discrepancy on December 15, 2022, which stated, “excessive pull to the left and over correction with both tiller and rudder.” After the report, the left feedback actuator RVDT voltage was found to be out of tolerance. The discrepancy was cleared after replacement of the RVDT.
A second pilot-discrepancy occurred on December 19, 2022. The discrepancy stated, “on taxi in, during a left turn, received N/W STEERING INOP caution msg. Turned off and on the steering switch and the msg cleared, however, for the rest of the taxi in the airplane pulled hard to the left.” Afterward, the right feedback actuator RVDT voltage was found to be within tolerance, even though the mechanic noted that it needed rigging according to the airplane maintenance manual. This discrepancy was cleared after replacing the right feedback RVDT. METEOROLOGICAL INFORMATIONA review of the wind data recorded by the automated surface observing system at IAH revealed that, between 1419 and 1420, the 2-minute average wind speed and direction varied between 12 and 14 knots from 125° to 133°, and the 5-second gust speed and direction varied between 16 and 20 knots from 134° to 137°. The runway 8L magnetic heading was 087°. The right crosswind component for the 2-minute (steady) and 5-second (gust) average wind ranged from 7 to 10 knots and between 12 and 15 knots, respectively. The most recent meteorological aerodrome report, at 1353, reported that the wind from 120° at 9 knots. Visual meteorological conditions prevailed at the time of the event, and the runway was dry. AIRPORT INFORMATIONAccording to the Mitsubishi Heavy Industries Regional Jet – (MHIRJ, the type certificate holder) Flight Crew Operating Manual, the nosewheel can be steered up to 70° left or right via the tiller handwheel and up to 7° left or right via the rudder pedals. Further, if a failure is detected by the nosewheel steering (NWS) electronic control unit (ECU), which is also known as the analog/digital steering controller, or if hydraulic system No. 3 fails, the NWS would revert to a shimmy-damping mode to allow free-castering of the nosewheel. The pilot could then maintain directional control on the ground using the rudder, differential braking and differential thrust. The ECU captures detected fault codes in nonvolatile memory (NVM). The codes are stored in reverse chronological order without dates or timestamps.
According to the MHIRJ Fault Isolation Manual, NWS is deactivated when the weight on wheels (WOW) signal indicates “air.” When the airplane is airborne, the steering command signals are replaced with a “straight ahead” reference. Also, a mechanical cam mechanism inside the nose landing gear (NLG) strut ensures that the NLG is straight when the strut is fully extended. FDR data indicated that the WOW switches and hydraulic system No. 3 pressure were operating normally. If the tiller handwheel is not centered during touchdown – and the nosewheel steering angle is less than 7°, the ECU would not detect a fault. If this situation were to occur, when the nosewheel WOW switch changes to ground, the ECU would combine the tiller commanded position with the rudder pedal position, to compute a steering command for the nosewheel.
The NWS Status FDR parameter changes to FAIL/Not Armed if the ECU detects a fault with one or more NWS system components or the NWS is not armed. This condition is annunciated on the EICAS display in the cockpit as a caution (amber) “STEERING INOP” message with an accompanying aural chime. Faults include system component failures; improper voltages; degraded performance; or unexpected operating conditions such as valve positions inconsistent with commanded position, handwheel commanded steering angle (greater that 7°) during touchdown, or excessive rates of operation, that is, moving the tiller handwheel or rudder pedals too quickly (sensors rotated more than 500°per second).
According to the SkyWest Dispatch Procedures Manual, the airplane’s crosswind component limitation was 27 knots for a dry runway.
Post-incident Examinations
After the event, SkyWest maintenance downloaded the NVM from the ECU, which contained the NWS fault codes shown in table 1.Because no dates or times were associated with these codes, the investigation could not directly determine which (if any) of the codes were related to the event. However, the codes are stored in the sequence in which they occurred in (reverse) chronological order. The table below was provided by MHIRJ, the potential cause(s) of the codes are referenced from the MHIRJ Fault Isolation Manual STEERING INOP Message section. The most recent code is listed first.
Code Description Potential cause 033 PILOT_HANDWHEEL_RATE_FAULT Detection of handwheel position greater than 8° from center during a 12.5-second system test that occurs immediately after nosewheel extension. Movement of the handwheel at a rate greater than 500° per second 191 WARM_BOOT_OCCURRED ECU 115 AIR_TO_GROUND_MODE_CMD_FAULT Handwheel- commanded steering angle (upon air-to-ground transition) exceeds 7° of NWS angle 109 SSV_STUCK_CLOSED_PS1 SSV (solenoid selector valve), or PS1 (pressure switch) or NLG (nose landing gear) priority valve or NLG selector valve or NWS system check valve blockage or wiring 170 PILOT_HANDWHEEL_RVDT_FAULT Pilot’s handwheel control unit or wiring 036 FEEDBACK_TRACKING_FAULT R feedback RVDT (rotary variable differential transformer) or L feedback RVDT or grease contamination or steering collar wear
105 SSV_COIL_BIT_FAULT SSV or ECU or wiring or NWS system check valve blockage Table 1: Nose wheel steering electronic control unit fault codes. (Source: MHIRJ)
SkyWest maintenance also performed the instructions in the MHIRJ Fault Isolation Manual for the STEERING INOP caution message but was unable to determine why the message occurred. A timing check was performed to determine whether the NWS handwheel would return to center from the full-left and full-right positions within the 2- to 4-second range, and the check showed that recentering was within the specified range.
SkyWest maintenance then replaced the NWS handwheel control unit (tiller) and ECU, checked the voltages for the left and right actuator feedback rotary variable differential transducers (RVDT), and the rudder pedal RVDT; all were within limits, and the NWS system operated normally (RVDTs are angular position sensors).
The ECU, as well as the handwheel control unit, electrohydraulic servo valve, steering selector valve, rudder RVDT, a wiring harness, hydraulic manifold, and compensator were shipped to their respective manufacturers/suppliers for examination and acceptance test procedure (ATP) testing.
The ECU, rudder RVDT, and compensator passed all ATP test elements. The handwheel control unit was tested at its supplier’s (Safran Landing Systems) facility before the start of the NTSB’s investigation, and no test report was generated. Safran reported that the unit failed the return-to-center element of the ATP. The unit exceeded the allowable 4 seconds to return to center after it was fully displaced in both directions (left and right). The unit was cleaned, and then it passed the ATP test.
The electrohydraulic servo valve operated normally and met ATP specifications except for null bias. The null bias shift was small (measured at 0.21 mA, maximum allowable was 0.16 mA). The steering selector valve passed all functional elements of the ATP but failed one nonfunctional element (the insulation resistance test at 500 VDC). The resistance criteria was 500 megohms or greater; the tested value was 7 megohms. The NLG wire harness passed all ATP elements except for the dielectric strength test at 550 VDC. Disassembly of the harness found two broken shield wires, which are used for a health monitoring function and do not affect steering. The hydraulic manifold passed all ATP elements except for the “Flow1” test, which required the hydraulic fluid flow to be between 0.95 and 1.40 gallons per minute. The flow was measured as 1.41 gallons per minute.
None of the minor discrepancies would have prevented normal operation of the NWS system.
Maintenance Records
A review of the airplane’s maintenance records over the 12 months preceding the incident related to the NWS system revealed the following pilot-reported discrepancies:
One report of the tiller not being centered and the airplane pulling “excessively” to the right during taxi, along with the STEERING INOP EICAS message. The discrepancy was cleared after the handwheel control unit was inspected, and the nose wheel steering health check was performed. Neither of those tasks found any anomalies. Six additional reports of the STEERING INOP EICAS message were noted. Three were cleared after performing a reset of the NWS system, two were cleared after NWS system operational checks were performed, all with no anomalies found. One was cleared after the right feedback actuator RVDT was found out of rig. A review of the maintenance records related to the NWS over the 6 months following the incident revealed a pilot-reported discrepancy on December 15, 2022, which stated, “excessive pull to the left and over correction with both tiller and rudder.” After the report, the left feedback actuator RVDT voltage was found to be out of tolerance. The discrepancy was cleared after replacement of the RVDT.
A second pilot-discrepancy occurred on December 19, 2022. The discrepancy stated, “on taxi in, during a left turn, received N/W STEERING INOP caution msg. Turned off and on the steering switch and the msg cleared, however, for the rest of the taxi in the airplane pulled hard to the left.” Afterward, the right feedback actuator RVDT voltage was found to be within tolerance, even though the mechanic noted that it needed rigging according to the airplane maintenance manual. This discrepancy was cleared after replacing the right feedback RVDT. DAMAGE TO AIRCRAFTThe airplane sustained minor damage to the right wing trailing edge flap skin and fairing. FLIGHT RECORDERSFDR data revealed that, at 1418:59, the airplane touched down at an airspeed of 133 knots and a magnetic heading of 92° with both engines near idle at 30% N1 (engine fan speed). About 4 seconds later, the thrust reversers were deployed, and both engines thrust settings increased from near idle to about 50% N1 with some asymmetry (the right engine thrust was higher). The largest asymmetry occurred as reverse thrust was increased from idle; the right engine speed (N1) was about 12% higher than the left engine. Although the captain recalled (in a post-incident statement) stowing and redeploying the reversers, FDR data indicated that the reversers remained deployed with the thrust settings consistent (near 50% N1 for the right engine and 46% N1 for the left engine) for the remainder of the rollout.
As the reverse thrust reached 50% N1, the captain began to apply the brakes. About 3 seconds later, while at an airspeed of about 71 knots, the airplane began a right turn. At the start of the turn, the left wheel brakes reached a (hydraulic application) pressure of about 750 psi, and the right brakes reached a pressure of about 500 psi (which provided a left-turning tendency). During the braking, two successive left rudder inputs occurred during a period of about 3 seconds; the first input was about 20° followed by an input of 15°. The brakes were released about 1 second after the 15° left rudder input, as the airplane decelerated through 58 knots. After the brakes were released, full left rudder (25°) was applied and held continuously. The rate of turn reduced slightly starting at the time of the full left rudder application. About 4 seconds after the full rudder input began the CVR captured a rumbling sound and increase in ambient noise, consistent with the airplane departing the runway into the grass, which occurred at a speed of 50 knots. About 4 to 5 seconds later, the Nose Wheel Steer Status parameter changed from “OK” to “Failed/Not Armed.” and an EICAS chime was heard on the CVR one to two seconds later.
The FDR did not record the nosewheel steering angle or tiller handwheel position. The CVR recording duration was 124 minutes, the audio was of good to excellent quality, and it captured the entire flight. The recording began at 13:06:07 as the aircraft was being boarded and preflighted at the Victoria Regional Airport (VCT), in Victoria, Texas. It ended at 15:10:08, about 51 minutes after the airplane came to a stop.