What caused the Angels Flight Accident?
Shortly after noon on February 1, 2001, an accident occurred during the
operation of the Angels Flight funicular railway when a passenger car (named
Sinai) broke loose, rolled down the inclined tracks and impacted the other car
(named Olivet) that had been brought to a halt by the inclined railway’s braking
system, Figures 1 through 3. Unfortunately, as a result of this accident, one
passenger was killed and seven others suffered injuries.
Angels Flight is a historic funicular railway in downtown Los Angeles
that was originally built in 1901 to move passengers between the commercial
district at the bottom of Bunker Hill and the residential area then at the top
of the hill. It was dismantled in 1969, but the original cars, named Olivet and
Sinai, and other equipment were placed in storage. The funicular was rebuilt
close to its original site and was re-opened in 1996 using the original two cars
still with wire-rope haul cable, the same track layout, but a different haul
Disassembly of Sinai’s cable drum from the gear box showed that its female spline which transmitted the rotational motion from the gear box to the cable drum had suffered a massive failure of its splines, Figures 6 through 9. The loss of these splines explained why the motion of Sinai was not controlled by the gear drive system, but did not explain why the spline failure had occurred or why the emergency brake system failed to activate and prevent Sinai from accelerating down the incline into Olivet.
There were two independent braking systems used on Angels Flight: a) the service brake, mounted next to the electric motor, used to stop and hold the cars in the stations at the end of each trip, and b) the emergency brakes, mounted on each car’s cable drum. In the days immediately following the accident, it was found that the service brake system worked as designed, but that the emergency brake system did not operate at all and, based on the oil film on the emergency brake rotors, had not operated for some time. Follow-up investigation found that the solenoid valve used on the emergency brake system had a burnt-out valve activation solenoid coil mounted on a normally-closed valve, rather than the required normally-open valve (see the brake system schematic, Figure 10, and photos of the system equipment, Figures 11 through 13). This resulted in the valve staying in the closed position, unable to relieve the hydraulic pressure in the emergency brake system, causing the brake calipers to remain open.
In addition, our examination showed that the solenoid and valve did not fit properly and had actually been forced together. We contacted the manufacturer regarding this observation and learned that in 1997 the solenoid valves had been redesigned such that the component parts produced after the redesign were not interchangeable with those produced prior to the design change. Our research of the identification numbers on the solenoid coil and on the valve showed that the solenoid coil had been manufactured prior to the design change in 1997, whereas the valve had been made after the design change. Because of the dimensional changes involved in the redesign, the new valve could only have been assembled into the old solenoid by forcing it in, and the nut used in the redesign to hold the new valve in the solenoid can was barely larger than the hole in the old solenoid, which had been designed for the larger threaded shaft of the old design. Wrench marks were found on the exterior of the solenoid, indicating that the valve body had been forced into the solenoid coil, although with properly matched parts, the solenoid was supposed to be assembled to the valve without the use of any force, Figures 14 and 15.
Examination of Angels Flight maintenance records showed
that the emergency brake solenoid valve had been worked on or changed in
December, 1998, and again in September, 1999. The solenoid valve assembly in
that system on the day of the accident was found to be the forced assembly of a
burned-out (electrically open) solenoid coil to a normally closed valve instead
of to a normally open valve. With these inoperative and incorrect components,
neither Sinai nor Olivet was protected by the emergency brake system on the day
of the accident. Based on our analysis of these records, the emergency brake
system on Angels Flight had been non-functional from either December 1998 or
September 1999 (the maintenance records lacking sufficient detail) until the
date of the accident on February 1, 2001. The incompetent maintenance of Angels
Flight had resulted in its operating for 17 to 26 months without the emergency
brake protection that it was designed to have, and in this period the
maintenance personnel never noticed that the emergency brake system was
inoperative. Had the maintenance personnel ever checked the brake pads visually
or verified the hydraulic pressure in the two systems, by looking at the gauges
shown in Figures 11 through 13, when they conducted the daily brake test, they
would have seen that the emergency brake system was not operating.
A careful study of the maintenance evidence, including records
and depositions, did reveal one difference in the utilization of the two cars,
and that occurred every morning when the entire braking system for Angels Flight
was tested operationally by the maintenance personnel. To perform this daily
test, the maintenance person would position Sinai at the bottom of the
incline and, with Olivet at the top, start the cars in motion and then hit the
emergency button, which cuts off electrical power from the entire system,
stopping the drive motor and activating both the emergency and service brakes.
Our analysis of the motion of the two cars during this test showed that Sinai’s
momentum continues to move it uphill after the power is cut, allowing its cable
to slacken until it comes to a stop. But with the cable slack and the emergency
brake disabled by the faulty maintenance, there is nothing that prevents Sinai
from sliding back down the hill until the cable snaps taut, causing a sudden
impact force to be applied to the entire Sinai drive system. This would not
occur on the downhill-bound Olivet system because its cable is never allowed to
go slack during this procedure and its momentum is retarded by a slight stretch
of its always taut control cable.
As a final part of our investigation, we wanted to determine
whether the female spline would have lasted longer than the subject failed
spline if it had been fabricated using the surface-hardened AISI 8822 alloy
steel, which most metallurgists would have preferred to use instead of the soft,
un-heat-treated AISI 1018 steel. Interestingly, metallurgical testing of
surface hardened AISI 8822 alloy steel coupons showed that, if this material
had been used for the female spline, the thin, hardened case would have been
less resistant to the impact loads resulting from the improper daily “brake
tests” and would have resulted in the cracking and failure of the female spline
sooner than it did with the subject, softer (but tougher), AISI 1018 steel
There was one other maintenance procedure mandated by the Operations & Maintenance Manual for the Angels Flight system that had not been adhered to. This maintenance procedure required that oil samples be withdrawn at regular intervals (about every six months) and sent to a laboratory for the analysis of wear particulate matter. This type of analysis is a normal procedure for large equipment that may suffer wear problems. By regularly conducting this type of analysis, the maintenance personnel would have been able to get non-invasive advanced notice if some portion of the system began to wear. This was done during the first years of Angels Flight operation, but for unknown reasons was discontinued long before the subject accident. The last oil analysis report was performed in May 1998 and, because the amount of particulate material was found to be increasing compared with prior analyses, the contract laboratory recommended that the time intervals between analyses be reduced. Instead, the oil analyses were discontinued. Had these analyses been continued and had the maintenance personnel realized the import of these analyses, a complete inspection of the Angels Flight system could have been undertaken to determine the cause of the particulate debris and could have corrected the problem, thus completely averting this tragic accident.