National Transportation Safety Board
Aviation Accident Final Report

 

 

Location:

Salt Lake City, UT

Accident Number:

WPR11LA058

Date & Time:

11/23/2010, 2220 MST

Registration:

N614SK

Aircraft:

BOMBARDIER INC CL-600-2C10

Aircraft Damage:

Substantial

Defining Event:

Ground handling event

Injuries:

69 None

Flight Conducted Under:

Part 121: Air Carrier - Scheduled

Analysis

The airplane was operating 2 hours behind schedule and had been dispatched with its auxiliary power unit (APU) inoperative, a condition that was permissible based on the approved Minimum Equipment List. The ramp area was covered in snow and ice, and it was snowing during aircraft loading and the subsequent pushback attempt. The captain started both engines while the airplane was at the gate, utilizing an external air-start cart in lieu of the APU. The captain stated that he started both engines due to a concern that, by starting one engine only, he would encounter control problems taxiing in the slippery conditions. He recognized that he could also have attempted to start the second engine in the alleyway after pushback using a cross-bleed engine start, but was concerned that attempting such a start would require increased engine thrust on the operating engine and could be disruptive to ground personnel and other traffic within the alleyway.

During pushback, the tug was unable to gain enough traction to move the airplane and was subsequently replaced with a larger tug. The larger tug was able to push back the airplane; however, a short time later the thrust from the airplane's engines, which were running at idle, exceeded the traction available to the tug. The captain, seated in the left seat, experienced a sensation of unusual movement as the airplane began to overpower the tug, and he asked the tug driver if the driver still had control of the airplane. The tug driver confirmed that he had control; however, the airplane subsequently moved forward while still attached to the tug, which rotated to the right striking the airplane's fuselage.

According to ground personnel and flight crew guidance, if the ramp surface conditions were such that adequate traction was not available, engine start should have been delayed until the pushback was complete. However, an engine start was required prior to pushback due to the inoperative APU. No guidance existed for either the flight or ground crew regarding pushback procedures in low traction ramp conditions with an inoperative APU.

Probable Cause and Findings

The National Transportation Safety Board determines the probable cause(s) of this accident to be:
The tug driver’s loss of airplane control during pushback due to a loss of traction in slippery conditions on the ramp. Contributing to the accident was the ground crews’ and airline operators’ inadequate guidance with regard to pushback procedures during low ground traction conditions with an inoperative airplane auxiliary power unit.

Findings

Aircraft

 

Personnel issues

Task performance - Ground crew (Cause)

Environmental issues

Snow/slush/ice covered surface - Effect on equipment

Organizational issues

Adequacy of policy/proc - Operator (Factor)

Adequacy of policy/proc - Not specified (Factor)

Factual Information

HISTORY OF FLIGHT

On November 23, 2010, about 2220 mountain standard time, a Bombardier CL-600-2C10 (CRJ), N614SK, operated by SkyWest Airlines as Delta Connection flight 4543, collided with a tow tractor during pushback from gate B4 at Salt Lake City International Airport (SLC), Salt Lake City, Utah. The 2 flight deck crewmembers, 2 flight attendants, 65 passengers, and ground handling crew were not injured. The airplane sustained substantial damage to the lower fuselage skin and multiple belly stringers. The scheduled passenger flight was operating on an instrument flight plan and was destined for Will Rogers World Airport (OKC), Oklahoma City, Oklahoma, under the provisions of 14 Code of Federal Regulations Part 121. Night instrument meteorological conditions prevailed. The flight was delayed prior to the accident, with a scheduled departure time of 2000.

SkyWest Airlines is a regional airline headquartered in St. George, Utah. The airline serves as a feeder airline and operates under contract with various major carriers. The accident flight was performed under a code sharing agreement with Delta Air Lines, Inc. Ground handling duties at the time of the accident were performed by agents from Delta Air Lines, Inc. The airplane was dispatched with the auxiliary power unit (APU) inoperative, a condition which is permissible based on the airplane's Minimum Equipment List (MEL), and which requires that one or both engines be started utilizing an external air-start cart or a precharged air bottle.

The airplane load agent, who was responsible for driving the tow tractor, stated that snow was falling during the loading and pushback sequence. He reported that during loading he observed a member of the flight crew performing an external preflight inspection.

Once the airplane had been loaded, he began to prepare for pushback. He was seated in the tow tractor, which was configured as a pushback tug and connected to the airplane's nose wheel utilizing a tow bar. He was in communication with the flight crew via the interphone, and stated that the crew could see him from their position on the flight deck. The captain requested an, "air-start" at the gate, and reported that he was going to start both engines. The engines were then started and the captain reported that the brakes were released and they were clear to commence the pushback. The driver then began the process of pushing the airplane back, but the wheels of the tug slipped and the airplane would not move. He subsequently informed the flight crew, and he decided to use a larger tug. With the larger tug attached, he began the pushback. The airplane subsequently started moving, and they began to back out of the gate area towards the alleyway. He was about to begin the turn into the alleyway, when the airplane started to skid to the left in the direction of the turn. He reported that the airplane then began to move forward, towards the tug, pushing it to the right side of the airplane. The driver stated that about this time, the captain asked if the airplane was pushing the tug, to which he responded yes. The airplane continued to move forward, subsequently overtaking the tug, which remained connected to the airplane's nose wheel. He further reported that the tug struck the belly of the airplane about the same time as tow bar severed at the nose wheel connection.

The tug driver reported that two wing-walkers were present at the time of the accident, and that neither tug was equipped with snow chains or traction devices.

The captain recounted similar observations regarding the initial pushback attempt. He became concerned that disconnecting the tug with both engines running could cause the airplane to move forward in the slippery conditions. He considered shutting down one or both of the engines; however, when he attempted to communicate his concerns with the driver, he had already departed to get the larger tug. By this time, the air-start equipment had also been removed from the gate area. The airplane appeared to remain firmly in position at the gate, and as such, he decided to keep both engines running.

The driver returned with a larger tow tug, and the airplane was successfully pushed back from the gate. Shortly thereafter, the captain experienced a turning and slipping sensation. He asked the tug driver if he still had control of the airplane, to which he replied in the affirmative. He then observed the tug rotate to the right and felt a collision. He reported that although he did not believe the airplane was moving forward, the events transpired so quickly that it was possible he may have missed any forward motion. As such, he could not definitively confirm if the tug struck the airplane, or if the airplane struck the tug.

With regard to starting both engines instead of one in lieu of a functioning APU, the captain stated that he made a judgment call based on a concern that if he started one engine only, he would encounter control problems taxing in the slippery conditions. He reported that he could also have attempted to start the second engine in the alleyway after pushback using a cross-bleed start. However, he was concerned that attempting such a start would require increasing the power to the running engine and could be disruptive to ground personnel and other traffic within the alleyway.

With regard to weather, the captain stated that although snow was falling at the time, the ramp area at the gate was plowed and mostly clear, and that the alleyway was contaminated with snow and ice. His view was not obscured by precipitation during the maneuver, and he could see the tug.

Examination of a photograph taken following the accident revealed that the airplane's nose gear had become canted to the right over-steer limits, with the tow bar bracket remaining partially attached to the left side of the wheel by the axle engagement pins. The bracket separated from the tow bar at its shank, which remained within the tow bar collar. The tow bar remained attached to the tug, which had come to rest impinged against the right forward fuselage, and opposite the direction of travel. Tire-shaped snow tracks were observed in the snow behind the nose wheel, the tracks continued in a sweeping arc aft towards the left side of the airplane. No snow disruption was present forward of the nose wheel.

METEOROLOGICAL INFORMATION

A special automated surface weather observation for SLC was issued at 2222. It indicated winds from 020 degrees at 6 knots; 2 miles visibility, with light snow and mist, a broken cloud ceiling at 1,600 feet, scattered clouds at 700 feet and overcast at 6,000 feet; temperature minus 7 degrees C; dew point minus 9 degrees C; and an altimeter setting of 29.81 inches of mercury.

FLIGHT RECORDERS

The airplane was equipped with both a Flight Data Recorder (FDR), and a Cockpit Voice Recorder (CVR). Both units were sent to the NTSB Office of Research and Engineering for data extraction. Examination of the CVR revealed that the unit was not de-energized subsequent to the collision, and as such, the accident sequence had become overwritten.

The FDR recorded about 77 hours of data, including the accident sequence. The data revealed that both engines were started about 2158 and powered to an N1 (Fan rpm) speed of 25% rpm while the airplane remained on a heading of 142 degrees magnetic. At 2220, the heading decreased until it reached 57 degrees, 45 seconds later, with a coincident jump in lateral and longitudinal acceleration of 0.07 and -0.08 g, respectively. The brake pedals were not applied until about 2 seconds after the acceleration event.

ADDITIONAL INFORMATION

SkyWest CRJ Standard Operating Procedures (SOP), in part, required the following prior to commencing the pushback:

"When ramp surface conditions would prevent adequate traction for ground equipment, engine start will be delayed until pushback is complete, and the pushback crew is clear of the aircraft. The pushback operator is responsible to notify the captain accordingly."

The corresponding SkyWest "Below-Wing Stations Operating Manual" made the following reference with regard to pushback/dispatch procedures with the tow bar and tow unit:

"WARNING - WHEN RAMP SURFACE CONDITIONS PREVENT ADEQUATE PUSHBACK UNIT TRACTION, ENGINE START MUST BE DELAYED UNTIL PUSHBACK IS COMPLETE. THE PUSHBACK OPERATOR IS RESPONSIBLE TO NOTIFY THE PIC ACCORDINGLY."

Neither the SOP nor the Below-Wing Operations Manual provided guidance for pushback procedures with an inoperative APU during low traction ramp conditions.

Subsequent to this accident, SkyWest issued a Ground Operations Bulletin, and Delta issued an Immediate Action Bulletin requiring that when ramp surface conditions prevent adequate pushback unit traction, only one engine may be started prior to pushing back a regional jet aircraft.

 

History of Flight

Prior to flight

Sys/Comp malf/fail (non-power)

Pushback/towing

Ground handling event (Defining event)

 

 

Pilot Information

Certificate:

Airline Transport; Commercial

Age:

48, Male

Airplane Rating(s):

Multi-engine Land; Single-engine Land

Seat Occupied:

Left

Other Aircraft Rating(s):

None

Restraint Used:

Seatbelt, Shoulder harness

Instrument Rating(s):

Airplane

Second Pilot Present:

Yes

Instructor Rating(s):

Airplane Multi-engine; Airplane Single-engine

Toxicology Performed:

No

Medical Certification:

Class 1 With Waivers/Limitations

Last FAA Medical Exam:

07/19/2010

Occupational Pilot:

Yes

Last Flight Review or Equivalent:

07/25/2010

Flight Time:

10066 hours (Total, all aircraft), 6846 hours (Total, this make and model), 5135 hours (Pilot In Command, all aircraft)

 

 

Co-Pilot Information

Certificate:

Commercial

Age:

31, Male

Airplane Rating(s):

Multi-engine Land; Single-engine Land

Seat Occupied:

Right

Other Aircraft Rating(s):

None

Restraint Used:

Seatbelt, Shoulder harness

Instrument Rating(s):

Airplane

Second Pilot Present:

Yes

Instructor Rating(s):

Airplane Single-engine

Toxicology Performed:

No

Medical Certification:

Class 1 Without Waivers/Limitations

Last FAA Medical Exam:

03/26/2010

Occupational Pilot:

Yes

Last Flight Review or Equivalent:

01/07/2009

Flight Time:

3317 hours (Total, all aircraft), 2135 hours (Total, this make and model)

 

 

Aircraft and Owner/Operator Information

Aircraft Make:

BOMBARDIER INC

Registration:

N614SK

Model/Series:

CL-600-2C10

Aircraft Category:

Airplane

Year of Manufacture:

 

Amateur Built:

No

Airworthiness Certificate:

Transport

Serial Number:

10051

Landing Gear Type:

Retractable - Tricycle

Seats:

69

Date/Type of Last Inspection:

11/22/2010, Continuous Airworthiness

Certified Max Gross Wt.:

75000 lbs

Time Since Last Inspection:

 

Engines:

2 Turbo Fan

Airframe Total Time:

20574 Hours as of last inspection

Engine Manufacturer:

General Electric

ELT:

C126 installed, not activated

Engine Model/Series:

CF34-8C5B1

Registered Owner:

ATLANTIC SOUTHEAST AIRLINES INC

Rated Power:

9140 lbs

Operator:

SKYWEST AIRLINES INC

Operating Certificate(s) Held:

Flag carrier (121)

Operator Does Business As:

Delta Connection

Operator Designator Code:

SWIA

 

Meteorological Information and Flight Plan

Conditions at Accident Site:

Instrument Conditions

Condition of Light:

Night

Observation Facility, Elevation:

SLC, 4227 ft msl

Distance from Accident Site:

0 Nautical Miles

Observation Time:

2222 MST

Direction from Accident Site:

 

Lowest Cloud Condition:

Scattered / 700 ft agl

Visibility

2 Miles

Lowest Ceiling:

Broken / 1600 ft agl

Visibility (RVR):

 

Wind Speed/Gusts:

5 knots /

Turbulence Type Forecast/Actual:

/

Wind Direction:

10°

Turbulence Severity Forecast/Actual:

/

Altimeter Setting:

29.81 inches Hg

Temperature/Dew Point:

-7°C / -9°C

Precipitation and Obscuration:

Light - Snow; Mist

Departure Point:

Salt Lake City, UT (SLC)

Type of Flight Plan Filed:

IFR

Destination:

Oklahoma City, OK (OKC)

Type of Clearance:

None

Departure Time:

2150 MST

Type of Airspace:

 

 

Airport Information

Airport:

Salt Lake City (SLC)

Runway Surface Type:

 

Airport Elevation:

4227 ft

Runway Surface Condition:

 

Runway Used:

N/A

IFR Approach:

None

Runway Length/Width:

 

VFR Approach/Landing:

None

 

Wreckage and Impact Information

Crew Injuries:

69 None

Aircraft Damage:

Substantial

Passenger Injuries:

N/A

Aircraft Fire:

None

Ground Injuries:

N/A

Aircraft Explosion:

None

Total Injuries:

69 None

Latitude, Longitude:

40.788333, -111.977778 (est)

 


 


Administrative Information

Investigator In Charge (IIC):

Eliott Simpson

Report Date:

03/08/2012

Additional Participating Persons:

Skeeter Gehring; Federal Aviation Administration FSDO; Salt Lake City, UT

Michael Eisenstat; SkyWest Airlines; St. George, UT

Shannon M Masters; Delta Airlines; Atlanta, GA

Publish Date:

03/08/2012

Investigation Docket:

http://dms.ntsb.gov/pubdms/search/dockList.cfm?mKey=77874