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The Tri-City Airport has standard instrument landing system equipment. Both the localizer and the glide slope are equipped with dual transmitters with a provision for automatic switchover in the event of a malfunction of any kind in the main equipment. They are also provided with standby power units in the event of commercial power failure. Finally, both the localizer and glide path are self-monitored and any malfunction in the equipment will actuate a red light and sound a buzzer in the tower.
In addition, the ILS installation includes an outer marker and a middle marker beacon. These VHF marker beacons are located on the ground in line with the localizer path at specific distances from the end of the runway to enable the pilot to orient himself in relation to the distance to the runway threshold. As an additional assistance to the pilot there is a low frequency compass locator at each marker beacon location which is normally utilized by means of the ADF. The compass locators for the middle and outer markers transmit nondirectional signals on frequencies of 201 kcs. and 239 kcs., respectively.
The marker beacons and the compass locators are single-transmitter installations and do not have standby power provided. However, like the localizer and glide path they are automatically monitored and any malfunction will actuate equipment failure alarms in the tower. The components of the ILS and the compass locators were functioning properly on the night of the accident. No such equipment failure alarm occurred in the tower. In addition, another Southeast flight executed an ILS approach subsequent to Flight 308 and reported the system normal. A flight check by the Federal Aviation Agency was made of all the Tri-City radio facilities the following day and all were operating normally.
At the Tri-City Airport the middle marker and outer marker were located on the localiser path 5 and 3.7 nautical miles, respectively, east of the approach end of runway 27. Eight and six-tenths nautical miles farther east on the localizer path (12.3 nautical miles east of the runway threshold) there is another low frequency nondirectional "H" facility known as the Emmett beacon. Two additional radio aids are a low frequency radio range and a VOR (VHF omni directional range).
There are two specified altitudes at which procedure turns are authorized for an ILS approach to Tri-City. Originally the minimum altitude at which the turn could be made was 5,500 feet within 10 miles east of Emmett "H." Later an alternative procedure turn was authorized at an altitude of 3,000 feet within 5 miles east of the outer marker. The 5-mile restriction is necessary because beyond this area the terrain rises rapidly to elevations well above 3,000 feet.
According to a company witness, the Southeast training program covered the Tri-City ILS procedures. He said that the procedure taught is to cross Gray intersection, proceed to the middle marker, and thence to the outer marker. All Southeast pilots have been instructed that the flying time from Gray to the outer marker is approximately three minutes. They are also instructed that the procedure turn must be started 30 seconds after passing the outer marker to ensure that it is completed within the 5-mile radius of the LOM (outer marker and compass locator). Shortly after the accident Southeast Airlines sent the following message to all of its pilots: "Effective immediately positive identification of Gray intersection will be made utilizing the facilities that comprise the intersection. Namely, the 275° radial of TRI-VOR and Southwest course of TRI-LFR (Tri-City Low Frequency Range). Descent below 5,000 prohibited until positively identified."
