Page:CAB Accident Report, Slick Airways Flight 25C.pdf/7

-7- and landed on runway 1R. The next aircraft, a Boeing jet, abandoned an approach to runway 28R at 1159 due to fog and eventually landed at another airport. The third aircraft, a Convair 340, which immediately preceded Slick 40Z, landed on runway 28R at 1202. Each of the crew members of these flights agrees that the approach light system (ALS) was operating, but none could recall seeing the sequence flashing lights (SFL), nor could they state that they were operating. The local controller in the San Francisco Tower stated that the runway lights and the ALS light switches were on position No. 5 full up and the SFL switch was on.

To guard against an undetected malfunction of the ALS/SFL system, the control panel in the tower cas has an alarm feature to indicate a failure of the system. This alarm contains a buzzer which sounds when there is an interruption in power or failure of a given number of lights. The volum of this buzzer is controlled by a rheostat which can be controlled from the tower cab. The buzzer can be turned down to a point where no sound can be heard. None of the tower controllers recall hearing the buzzer immediately prior to, during, or subsequent to the time Slick 40Z was making its approach. The lights were found to be inoperative 3 hours and 20 minutes after the accident occurred.

A flight check of the ILS at the San Francisco International Airport was made by the FAA on February 3, 1963, as a result of the accident. The facility performance was satisfactory.

All the weather observing equipment at San Francisco International Airport was in operational order at the time of the accident. There were two methods of obtaining the official visibility at San Francisco International Airport for determining landing minimums of an aircraft operating under Part 42 of the Civil Air Regulations: Prevailing visibility and RVR.

Prevailing visibility is the greatest visibility which is attained or surpassed throughout half of the horizon circle, not necessarily continous. To determine prevailing visibility under non-uniform visibility conditions, the horizon circle is divided into several sectors of equal size in each of which the visibility is substantially uniform. The prevailing visibility is then the highest value that is equal to or less than the visibility of sectors that cover at least one-half of the horizon circle. The prevailing visibility at 1155 and 1208 was reported as 3/4 mile over half of the horizon circle and more specifically to the east or toward the threshold of runway 28R.

Runway 28R at San Francisco International Airport is equipped with RVR. The equipment used in this system includes a transmissometer, a digital display, and a recorder which charts the transmissivity of the atmosphere. The transmissometer of the San Francisco RVR system is located parallel to runway 28R; the projector is 1,500 feet from the threshold, and the receiver is 500 feet further west. The initial impact point was 2,570 feet east of the projector.

The applicable minimums for a straight-in approach to runway 28R at San Francisco International Airport are 200 feet ceiling and 1/2 mile visibility provided all components of the ILS installation and related airborne equipment are operating satisfactorily. The Operations Specifications for Slick Airways specify that a landing may be made at an airport when the local visibility is reduced to not less than 1/2 mile by purely surface weather conditions such as smoke, hazs, cust, ground fog, blowing snow or sand, provided the ceiling is not less than 1,000 feet, the aircraft is aligned with the runway of intended landing is plainly visible allowing the pilot to have adequate visual reference to the line of forward motion at all times during final approach and landing.