Executive Summary
On Saturday, February 15, 2014, about 11:30 a.m. local time,
a 2004 Kia Sorento sport utility vehicle (SUV), occupied by a 37-year-old
driver and four passengers, was traveling westbound on US Route 90 (US-90) near
Centerville, Louisiana.
The SUV was traveling in the right lane at a
witness-estimated speed of 70 mph (roadway speed limit) when the driver lost control
of the vehicle due to a tread separation and rapid air loss in the left rear
tire.
The SUV veered into the left westbound lane, rotating in a
counterclockwise direction, and departed the roadway into a 64-foot-wide
depressed earthen median.
It continued through the median, now rotating in a clockwise
direction; entered the eastbound traffic lanes of US-90; and collided with a
2005 IC Bus (66-passenger school bus), which was traveling in the right lane.
The left front of the SUV contacted the right front of the
school bus, resulting in the rapid clockwise rotation of the SUV and secondary
impacts along the right side of the school bus. The driver of the SUV and three
rear passengers were ejected. The fourth passenger remained in the vehicle.
Kia Left Rear Tire and Wheel
The left rear tire tread separated from the tire carcass
prior to the collision sequence.5 The left rear rim and tire carcass were still
attached to the Kia postcollision. The Louisiana State Police later removed
these components from the vehicle and transferred them to the NTSB materials
laboratory. NTSB investigators recovered the separated tread from the scene for
further examination.
The subject tire was a Michelin Cross Terrain manufactured
in late October 2003.6 The tire tread was composed of five ribs with zigzag
grooves between each rib. Tread depth measurements were taken in all four
grooves, every 4 inches along what would have been the circumference of the
tire (figure 4).
The average tread depths for grooves 1 through 4 were 4/32,
2/32, 2/32, and 3/32 inch, respectively.7 The tire had minimum tread depth
values of zero at some locations (bald spots), where wear had exceeded the
depth of the grooves.8
The state of Louisiana requires vehicle inspections every 2
years. The inspection requires that tires have at least 2/32 inch of tread
measured in any two adjacent tread grooves and that there are no knots, bulges,
sidewall cracks, or tread exposure. According to the Louisiana Office of Motor
Vehicles, the Kia passed inspection on April 11, 2013.
The tread separated from the tire carcass near a bald spot
located toward the inboard edge of the tire. The separation started as a
delamination between the inner and outer steel belts on the inboard side of the
tire. It evolved into a thumbnail-shaped pocket that extended along the
shoulder of the tire.
The difference in wear between the delaminated region and
the remainder of the tread indicates that the pocket was likely present for
several hundreds to thousands of miles. Additional delamination formed along
the outboard shoulder and progressed toward the inboard shoulder. The tread
separated from the tire when the delaminations merged. The tire carcass
subsequently split in the radial direction (from the outboard shoulder to the
inboard shoulder), and the tire lost pressure (figure 5).
Three foreign objects had penetrated the tread and the outer
steel ply. Figure 6 shows one object found approximately 46 inches clockwise
from the tip of the leading edge flap.9 The steel wires in the outer ply were
deformed where the objects had penetrated the tread.
Other physical damage to the subject tire included U-shaped
compression set marks on the outboard sidewall and cracking along the sidewall,
buttress, and shoulder area (figure 7, left).10 The wheel weight clip, used to
balance the tire, left multiple impression marks near the bead on the inboard
side of the tire (figure 7, right), indicating that the tire had likely been
operated in an underinflated condition at some point during its lifetime. No
precollision damage was found on any of the rims.
Driver Information
Kia Driver
At the time of the crash, the driver of the Kia was
traveling from her home in Patterson, Louisiana, to the “Krewe of Head Start
Recruitment Parade” in Franklin. She held a class E driver’s license.15 According
to relatives, her vision, hearing, and general health were good. NTSB
investigators examined cell phone provider records and determined that she was
not using her cell phone at the time of the crash. Postcrash toxicological
testing did not indicate the presence of alcohol or drugs.16
According to the surviving passenger in the Kia, after the
tire failure, the vehicle drifted to the left and the driver responded by
braking and steering. According to a truck driver traveling behind the SUV,
when the tire “popped,” the vehicle moved to the right, then overcorrected to
the left, and crossed the median.
Field tests have found that drag forces are generated when a
rear tire experiences tread separation. These forces pull the vehicle toward
the side of the affected tire.17 Noise and vibration were present in each field
test before the vehicle began pulling to the side of the affected tire,
providing an auditory cue to the driver. In most cases, experienced test
drivers were able to keep the vehicle in the travel path with small steering
corrections.18
In other cases, the drag forces were significant and resulted in
large vehicle deviations.19 Other tests have reported that a higher vehicle
speed and a longer duration of the tread separation are associated with greater
lateral deviation from the travel path.20 Tests have also found that drag
forces ended once the tread fully separated from the tirewhereas, for a
partial tread separation, drag forces continued until the vehicle came to rest.
If the affected tire lost air, drag forces also continued until the vehicle
came to rest.21
Although an experienced test driver in a simulated
environment who is aware of an impending failure may be able to control a
vehicle following a tread separation, it can be difficult for a typical driver
to have the same control in some real-world traffic environments.
A rear tread
separation at high speed reduces the lateral forces generated by the rear
tires, which can result in excessive rotation and lateral motion for a given
steer input. These changes in vehicle response and maneuverability
characteristics can make it difficult for a driver to safely steer following a rear
tread separation. In addition, drag forces could lead a driver to respond in a
manner that further destabilizes the vehicle, resulting in a loss of control.22
The Louisiana Office of Motor Vehicles provides guidelines
for new and current class D and E drivers on how to respond to a tire blowout,
including staying calm, firmly holding the steering wheel and maintaining the
vehicle in the same lane (noting that there may be a strong pull from the right
or the left), slowing the vehicle, gently applying the brakes, and pulling off
the road to a safe area.
Probable Cause
The National Transportation Safety Board determines that the
probable cause of the Centerville, Louisiana, crash was the Kia SUV driver’s
loss of control due to the tread separation and rapid air loss of the left rear
tire, which altered vehicle handling characteristics.
Contributing to the crash was the deteriorated condition of
the tire due to inadequate maintenance.
Footnotes/References
5 The portion of the tire designed to contact the road
surface (tire tread) separated from the rubber-bonded cord structure of the
tire (tire carcass).
6 Michelin Cross Terrain P245/70R16 106H; tire
identification number: DOT 0CBU JDWX 4403.
7 The original tread depth for the subject tire, according
to manufacturer literature, was 11/32 inch.
8 The minimum tread depth values measured for the left
front, right front, and right rear tires were 11/32, 11/32, and 4/32 inch,
respectively.
9 The leading edge flap is the side of the tread that forms
a triangular wedge of tread pointing toward the direction of rotation, as shown
in figure 4.
10 Cracking can be associated with overdeflection, foreign
materials, ozone deterioration, loss of rubber elasticity over time, and
temperature.
11 At the time of inspection, the driver’s seat belt was
buckled and the seat belt webbing showed cloth color transfer, scuffing, and
cupping. The front seat passenger seat belt showed heat abrasion to the
webbing.
12 If seatback failure occurs, use of the restraint system
may not prevent the occupant from being ejected from the vehicle. See TES
Limited. 1989. Accidents Involving Seatback Failures, report no. C1322/2,
prepared for Transport Canada. Bedford, UK.
13 Louisiana Rev Stat §32.295.1 (www.legis.la.gov).
14 Louisiana Rev Stat §32.295 (www.legis.la.gov).
15 A class E license is a personal vehicle license and
permits the operation of any single motor vehicle under 10,000 pounds gross
vehicle weight rating.
16 Blood was tested for ethanol, acetone, isopropanol, and
methanol. Blood was screened for amphetamines, antidepressants, barbiturates,
benzodiazepines, cannabinoids, cocaine/metabolites, lidocaine, methadone,
nonopiate narcotic analgesic, opiates, phencyclidine, phenothiazines,
propoxyphene, acetaminophen, salicylates, oxycodone, fentanyl, and oxymorphone.
17 C. P. Dickerson, M. W. Arndt, and S. M. Arndt. 1999.
“Vehicle Handling With Tire Tread Separation,” paper no. 1999-01-0120. SAE
World Congress and Exhibition. Warrendale, Pennsylvania: SAE International.
18 R. F. Fay, R. D. Robinette, J. Smith, T. Flood, and G.
Bolden. 1999. “Drag and Steering Effects From Tire Tread Belt Separation and
Loss,” paper no. 1999-01-0447. SAE World Congress and Exhibition. Warrendale,
Pennsylvania: SAE International.
19 D. F. Tandy, K. T. Tandy, N. J. Durisek, K. J. Granat, R.
J. Pascarella, L. Carr, and R. Liebbe. 2007. “An Analysis of Yaw Inducing Drag
Forces Imparted During Tire Tread Belt Detachments,” paper no. 2007-01-0836. SAE
World Congress and Exhibition. Warrendale, Pennsylvania: SAE International.
20 S. M. Arndt and M. W. Arndt. 2001. “The Influence of a
Rear Tire Tread Separation on a Vehicle’s Stability and Control,” paper no.
258. Proceedings, 17th International Technical Conference on the Enhanced
Safety of Vehicles (ESV), Amsterdam, The Netherlands. Washington, DC: NHTSA.
21 G. Beauchamp, D. Koch, and D. E. Thornton. 2013. “A
Comparison of 25 High Speed Tire Disablements Involving Full and Partial Tread
Separations,” paper no. 2013-01-0776. SAE World Congress and Exhibition.
Warrendale, Pennsylvania: SAE International.
22 Loss of control can occur for a variety of reasons. For
the purpose of this report, “loss of control” refers to a change in vehicle
response characteristics beyond which a driver can adapt and steer along the
intended path.
23 See web01.dps.louisiana.gov/DPSForms.nsf, accessed June
12, 2015.
24 A class B license is a commercial driver’s license that
permits the operation of heavy straight vehicles. A straight vehicle is a
vehicle that does not bend or have a moveable joint in its frame between the
driver seat and the cargo or passenger compartment. The driver had the school
bus and passenger endorsements.