A King Air 90 collided with the Capitan Mountains during a moonless night, killing all four on board. The final accident report is yet to be published but the information available shows that GPS jamming from the nearby White Sands missile range could be a contributing factor.
The crew reported losing GPS and requested radar vectors to Sierra Blanca Regional Airport. Albuquerque Center began vectoring the aircraft while a supervisor contacted the military to stop the jamming. Later, 31 nautical miles from the airport, the crew reported “visual on Ruidoso” adding “we can go visual”. ATC cleared the visual approach, transferring responsibility for terrain separation to the crew. A few minutes later, the aircraft collided with terrain. More details are available from the NTSB preliminary report.
Unclear capabilities, unclear intentions
Radio transmissions and phraseology are important here. The crew first reported losing GPS and requested vectors, then reported visual contact with the city and added “we can go visual”. This does not imply that their GPS was usable or that they expected to have it available for the visual approach. Stopping the jamming does not mean GPS functionality is immediately restored: receivers may need time to reacquire or require crew action or a reset. The preliminary report contains no transmission in which the crew reported that GPS capability had been restored.
Both the ILS and GPS approaches require a local altimeter setting, but the airport’s automated weather station was NOTAMed out of service. The crew may have expected that the arrival would need to be completed visually, although the preliminary report does not establish their original plan.
About two minutes after the crew reported they could go visual, the supervisor informed the military that jamming could resume. ADS-B reporting degraded again shortly afterwards. Yet the preliminary report contains no transmission from the crew confirming that their GPS capability had been restored.
Workload, timing and positioning
After being assigned a heading of 350°, the aircraft crossed the ILS final approach course and continued north for about 20 nautical miles. The controller advised the crew to expect right turns to bring them back towards the approach. While on this heading, the crew reported “visual on Ruidoso” and were cleared for a visual approach before the controller started the right turns towards the ILS. They then initiated a left turn which brought the aircraft towards the Capitan Mountains.
The preliminary report does not mention whether the controller detected the proximity to terrain or whether a ground-based safety net generated an alert. In any case, the responsibility for navigation and terrain avoidance was with the crew. What the report does mention is that the controller had to deal with three other aircraft with lost GPS, and at least one had issues locating a ground-based navigation aid and required assistance, creating additional workload.
Going visual and removing safety layers
Was the crew fully aware of their position when they reported “visual”? Was the ILS receiver set, showing their position relative to the approach? Flight recorders are not normally required for this type of operation, but investigators may recover information from avionics and other recorded data.
By accepting the visual approach, the crew gave up an important safety layer: ATC was no longer responsible for providing terrain clearance through radar vectoring. This also gave them the freedom to make a more direct left turn.
Deciding to go for a visual approach is a perfectly sound decision as long as navigation can be ensured visually. After being radar-vectored north of the final approach course, and possibly still without reliable GPS, the crew’s precise awareness of their position relative to the terrain could become an important question for investigators.
GNSS interference beyond conflict zones
Whatever role the final report attributes to GPS jamming, this accident shows that the issue is not restricted to conflict zones. This was an announced case of jamming and the crew detected and reported it.
The dependency of aviation on GPS is significant and while the military need for jamming is understandable, the additional workload it creates should not be underestimated. The preliminary report does not mention that the controller was overloaded but operations were not running under normal conditions.
