All four crew members were killed as were 35 residents of a settlement located beyond the end of runway 26 after the Turkish-registered aircraft overshot the runway in low-visibility conditions and attempted a go-around at too low an altitude. Another 37 residents were injured.
Russia’s Interstate Aviation Committee (IAC), which is investigating the crash along with the airline, Turkish investigators, the US National Transportation Safety Board (NTSB), Boeing and others, made six safety recommendations as part of the preliminary report, published March 24.
Included are calls for pilots to pay closer attention to charted details for instrument landing system (ILS) approaches and for controllers, when equipped, to monitor and inform pilots to “significant altitude deviations” from the charted approach paths. The report does not offer a probable cause or contributing factors for the crash.
Information from the flight data recorder (FDR) showed that Flight TK6491, inbound to Manas from Hong Kong for fuel and a crew change before departing for Istanbul, was at too high an altitude for its autoflight system to capture the glideslope (vertical reference signal) from the ILS.
The normal intercept occurs at a level altitude of 3,400 ft. above sea level at approximately 4 nm from the runway end, beyond which the glideslope signal provides for a 3-deg. descent angle to the touchdown point. Flight 6491, however, was approximately 600 ft. too high and descending at that point. The aircraft continued its descent to 3,400 ft., an altitude it reached when approximately 2.5 nm from the runway. The glideslope display in the cockpit was in the “full down position,” indicating the aircraft was too high, according to the IAC.
The pilots continued in level flight at 3,400 ft. until capturing a 9-degree angle “false glideslope” at approximately 1.1 nm. from the runway end. The guidance system transitioned then captured the false glideslope and began a steep descent. The aircraft during the approach had passed two ground-based position markers—the outer marker and the middle marker—that are keyed to critical altitudes on the approach chart. Despite receiving those indications on the cockpit displays, which would have made clear the aircraft was significantly higher than allowed at both locations, the pilots did not abort the approach.
False glideslopes are a known shortcoming of the ILS, caused by “sidelobes” of the main signal along the ideal 3-deg. slope. Sidelobes will typically have a slope of 6 deg. and 9 deg. To avoid capturing a false glideslope, pilots are taught to intercept the 3 deg. glideslope at the proper entry altitude and position. For Runway 26 at Bishkek, the intercept occurs at 3,400 ft. altitude and approximately 4 nm. from the runway.
The IAC is recommending that pilots “pay attention” to approach chart information, including the monitoring of distance and altitude at reference points along the path to the airport.
Two other recommendations involve the “inertial mode” operation of Boeing autopilots, a feature on virtually all Boeing aircraft to protect against a loss of external glideslope or localizer (lateral position) ground station signals. Using the inertial mode, the autopilot will continue an approach at a 3 deg. glideslope angle using internal guidance until a valid glideslope signal is regained or until the crew shuts off the autopilot.
At Bishkek, the IAC said an “FMA Fault 2” event that occurred 15 sec. after capturing the false glideslope sent the autopilot into inertial mode in addition to notifying the pilots with textual and aural cautions. Fault 2 indicated that the aircraft autopilot can no longer track the glideslope; however the autopilot remained engaged, as designed. Several other alerts occurred in the cockpit, including an Enhanced Ground Proximity Warning System Mode 5 alert indicating excessive deviation from the glideslope.
With autopilot engaged and tracking an internally computed 3-deg. glideslope, the descending 747-400 crossed the departure end of Runway 26 at 110 ft. above the ground. Seconds later, at the decision height of 99 ft., the first officer called “minimums” and reported no visual contact with the ground. The aircraft continued to descend to 58 ft. before the crew initiated a go-around. The descent stopped 3.5 sec later, but too late—the aircraft struck upsloping terrain and obstacles at 165 kt. airspeed.
According to the IAC, the housing was located approximately 3,200 ft. from the runway end. IAC recommended that airport administrations analyze the “acceptability of construction in the immediate vicinity of airdromes.”
Regarding the Boeing inertial mode autopilot feature, the IAC recommended that airlines provide theoretical and practical training covering the “awareness, procedures and aspects of flight operations” when the autopilot switches to inertial mode during a glideslope descent.
The IAC also wants the FAA and Boeing to consider changing the autopilot logic to prevent the system from following the inertial mode guidance “in cases when the approach path does not allow landing in the appropriate area” on the runway.
(John Croft - ATWOnline News / Aviation Week)