Figure: USAir 5050 N416US, from NTSB/AAR-90/03, Figure 5.

Eddie Sez:

Reading the NTSB probable cause we are told the captain failed to "exercise command authority in a timely manner to reject the takeoff or take sufficient control to continue the takeoff, which was initiated by a mistrimmed rudder." Let's translate that into plain pilot-speak. The pilot got ample clues that something was wrong before V1 and should have aborted the takeoff. The need to abort was caused by his inadequate rudder inputs, placing more strain on the nosewheel tires than they could handle, causing one of those tires to separate from the rim. But the airplane was perfectly flyable even with the failed tire and mistrimmed rudder. He was a former Air Force pilot where the mantra beaten into our heads was "first, fly the airplane." Had this pilot done that, the aircraft would not have been destroyed and the two fatalities would have made it to their destination alive.

Sure, it is easy enough to say "fly the airplane," and that can become a bit trite. We pilots tend to tune these things out. But let's try saying it this way: "Don't become a passenger in the pilot's seat." The first officer, as the pilot flying during the first part of the takeoff, did not apply sufficient rudder pressure to overcome the wind and rudder trim. The captain, when aborting the takeoff, failed to apply much rudder at all to correct to centerline, or enough brake pressure to stop the airplane in the confines of the runway.

What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.


Accident Report


Narrative

[NTSB/AAR-90/03, ¶1.1]

[NTSB/AAR-90/03, ¶1.7.1] The Weather Service Contract Meteorological Observatory at LaGuardia issued the following observation: Time--2334; type--local ; ceiling--estimated 500 feet overcast; visibity--5 miles; weather--light rain and fog; temperature--73°F.; dewpoint--73°F.; wind--210 degrees at 4 knots; altimeter--30.20 inches; remarks--aircraft mishap.


Analysis

Figure: USAir 5050 Rejected Takeoff Braking Levels, from NTSB/AAR-90/03, figure 2.

[NTSB/AAR-90/03, ¶1.10] Based on runway surface characteristics, crown, and texture depth values, the drainage analysis indicates that runway 31 had excellent water drainage capability (excluding the painted nongrooved blast pad [overrun] area) particularly for the rainfall rate occurring at the time of the accident (.008in./hr.). With the transverse grooving, and the good tread condition of the main gear tires, hydroplanning was not a significant factor. Except for approximately 1,200 feet near runway 4/22 intersection, the accident aircraft RTO track was established by the white tire erasure marks found on both the asphalt and concrete deck surfaces of the runway. In terms of tire friction performance, the Saab friction tester results indicate that the lateral displacement of the aircraft left of centerline actually provided better friction performance compared to that measured closer to runway centerline.

It appears the captain did not use all off the braking capability he had available.

Figure: USAir 5050 Cockpit aisle stand, from NTSB/AAR-90/03, figure 6.

[NTSB/AAR-90/03, ¶1.16.2] On-scene examination of the rudder trim system showed that the rudder trim control knob rotated freely in both directions with no evidence of binding or sticking. This control is located on the aft end of the center pedestal. The trim indicator, which is also on the center pedestal, showed an "off" flag, and there was no evidence of sticking. Extension of the rudder trim actuator rod was 10.5 inches, corresponding to a trim position of 16° left deflection of the rudder.

[NTSB/AAR-90/03, ¶1.17.1.2] Regardless of the trim setting the pilots' force on the rudder pedals can still move the rudder to its full deflection of 26° either side of center.

The misset trim should not have been an issue. The pilot's job is to position the aircraft where it is supposed to be.

[NTSB/AAR-90/03, ¶1.17.5] The Safety Board collected about 90 reports of rudder trim anomalies for the Boeing 737-300/400 series airplane. [ . . . ] Many reports described the inadvertent setting of rudder trim by the foot of a jumpseat occupant behind the captain's seat. The reports imply that casual visitors to the cockpit did not strap in, sat sideways and used the end of the center pedestal as a footrest for their right foot. This allowed their shoe sole to push the trim knob counterclockwise and set left rudder trim. The Pan American captain who had visited the cockpit before departure of flight 5050 said that he did not rest his foot on the center pedestal at any time. [ . . . ] Other reports show that placing objects on the center pedestal can inadvertently turn the rudder trim knob. While the Pan American captain said that he believed the first officer placed his chart holder on the pedestal during prestart activities, the first officer did not recall having done so.

[NTSB/AAR-90/03, ¶2.2] Data from the DFDR showed that rudder trim on N416US was neutral after arrival at the USAir gate at LGA. The DFDR subsequently shut down. When repowered after engine start for flight 5050, the DFDR showed that the rudder had moved to the full left trim position. Electrical and hydraulic power from the Auxiliary Power Unit (APU) were available during the intervening period to change the rudder trim position, if commanded. The time to run trim from neutral to full left is about 30 seconds, so momentary knob rotation would not have produced full left trim.

[NTSB/AAR-90/03, ¶2.3]

  • The captain could have noticed the misset rudder trim almost immediately after engine start, even before he began taxiing away from the gate, because the rudder pedals were offset from each other by 4 1/4 inches. He did not mention the offset to investigators 2 days after the accident. However, after the DFDR evidence indicated the trim anomaly, he said that he had noticed the offset pedals, adding that the offset did not bother him because he was used to taxiing with offset pedals in the C-130.

  • Although the captain had more than 2,600 flight hours in the B-737, he had only about 140 hours in the left seat. Since taxiing is performed only from the left seat, his taxi experience was somewhat limited. However, the full rudder trim would also have turned the nose wheel about 4° left.

  • According to Boeing's Chief B-737 Test Pilot, the nosewheel steering is effective only until the rudder gains aerodynamic authority. Afterward, the nosewheel tires cannot produce sufficient cornering force to redirect the airplane in opposition to the aerodynamic force resulting from rudder deflection. Using Flight 5050's gross weight, an engineering simulator showed that, at more than 81 knots on a dry runway and 64 knots on a wet runway, nosewheel steering alone could not turn the airplane in opposition to 16° of rudder deflection.
  • We often think about VMCG (Minimum control speed - ground) as the speed where we no longer need nosewheel steering to keep the airplane on the runway following an engine failure. Another way to look at it is the speed where the rudder will do the job for us. Something we don't often think about is at what point is the nosewheel steering no longer effective on its own. But underlying it all, is the fact we should be using aerodynamic controls as soon as they are effective.

  • The captain testified that he tried to halt the leftward track of the airplane by using both rudder pedal and the nosewheel steering tiller prior to rejecting the takeoff. But the DFDR data refutes such an occurrence because it indicates a maximum rudder deflection of only 1 degree right during the 4 l/2 seconds from "got the steering" to the captain's signaling his rejection of the takeoff by saying "let's take it back."

  • Although 1° nose-right rudder required about 58 lbs on the right rudder pedal, neither pilot applied the 71 lbs. of force needed for full right rudder.

The captain was applying significant pressure on the nosewheel to turn the airplane right just as airplane's large vertical fin was pushing it in the other direction. The rudder would have been a much more effective tool.

[NTSB/AAR-90/03, ¶2.6]

[NTSB/AAR-90/03, ¶2.7]

[NTSB/AAR-90/03, ¶2.9]

[NTSB/AAR-90/03, ¶2.10] The Safety Board views the absence of a comprehensive departure briefing, the absence of airspeed callouts, the failure of the first officer to clearly communicate his directional control problem, and the non-assertive manner in which the captain communicated his intent to reject the takeoff as indications of poor cockpit coordination. That the pilots of flight 5050 were ineffective as a team is probably the result, in part, of their lack of any formal training on cockpit resource management (CRM). Both of the flight crewmembers were hired and trained by Piedmont Airlines before the merger with USAir had been completed. Piedmont Airlines did not provide formal CRM management training to either pilot involved in this accident.


Probable Cause

[NTSB/AAR-90/03, ¶3.2] The National Transportation Safety Board determines that the probable cause of this accident was the captain's failure to exercise his command authority in a timely manner to reject the takeoff or take sufficient control to continue the takeoff, which was initiated with a mistrimmed rudder. Also causal was the captain's failure to detect the mistrimmed rudder before the takeoff was attempted.


See Also

Normal Procedures & Techniques / CRM

Technical / V-1

Technical / V-MCG


References

NTSB/AAR-90/03 Aircraft Accident Report, USAir, Inc., Boeing 737-400, La Guardia Airport, Flushing, New York, September 20, 1989