Losing situational awareness on any flight is a bad thing, but it’s particularly bad in IMC.
When you can’t see the horizon because of clouds or darkness, it can become a potentially deadly situation. This is why instrument flight candidates are taught to fly by “partial panel,” which means the instructor “fails” an instrument, often the attitude indicator, by covering it up.
The learners expect this during training. But something they aren’t trained for, if anecdotal evidence is correct, is a loss of Remote Autonomous Integrity Monitoring (RAIM), which is the function that tells us the GPS information is reliable. If you lose RAIM during an approach, it is much like having a call drop when using a cellphone—when you realize the call has been dropped, you may not know where in the conversation you were because you kept going, or the other person kept going.
Either way, you are missing information.
RAIM is one of the functions pilots are supposed to check before flight to make sure it will be there when you need it. For example, if you determine it will take 23 minutes to get to the destination where you plan to execute a GPS approach, put the estimated arrival time into the GPS and check to see if RAIM is available. Even if it will be (in theory), it’s still a good idea to have a backup approach in mind—like the VOR or ILS/LOC.
Losing RAIM means you are no longer guaranteed GPS accuracy. You could literally be “flying blind,” losing situational awareness. If this happens en route, the FAA recommends and instructors teach that you immediately switch to an alternate approved form of navigation, such as asking for radar vectors if in radar contact or use a VOR if able. If you haven’t begun the approach yet, notify ATC as soon as possible about the loss of RAIM and be prepared to proceed to the missed approach point and perform the published missed.
Losing RAIM and therefore GPS signal reliability has been discussed at the airport of late because the National Transportation Safety Board (NTSB) has released the preliminary report on the crash of an air ambulance in New Mexico in the predawn hours of May 14 that killed both pilots and two flight nurses.
According to the NTSB report, the crew of the King Air 90 repeatedly lost GPS services while en route to Sierra Blanca Regional Airport (KSRR ) in Ruidoso, New Mexico, due to the military jamming the GPS. The route of flight took the aircraft over mountainous terrain and a Military Operations Area (MOA).
At the time of the flight a NOTAM had been issued advising pilots that the automated weather observation system (AWOS) at KSRR was out of service and there would be GPS interruptions caused by the military jamming the signal. This information was allegedly provided to the crew through the ForeFlight briefing and NOTAM information.
Details of the Flight
The aircraft was based at Roswell Air Center (KROW) in New Mexico. The airport sits at an elevation of 3,671 feet msl. The aircraft was under the operational control of Generations Jets, which employed the two pilots. The flight nurses were employed by Trans Aero MedEvac.
The aircraft was dispatched to KSRR, some 51 nm away, to pick up a patient for transport to Albuquerque. The field elevation of KSRR is 6,813 feet.
The NTSB report indicates visual meteorological conditions (VMC) prevailed with no moon, so “dark night conditions” prevailed.
There are two instrument approaches at KSRR—an ILC or LOC for Runway 24 and an RNAV (GPS) for 24. The notes section on the instrument approach procedures contain the following statement: “When local altimeter setting is not received, procedure NA (Not Authorized).”
The NTSB report noted the King Air was equipped with Spidertracks flight tracking equipment, which reported GPS-generated altitude, heading, airspeed, and position for the duration of the flight.
The NTSB used this information and ABS-B data to create a narrative. It was noted that the recorded Spidertracks GPS altitude was generally about 600 feet higher than the recorded ADS-B altitudes. Also, there were large gaps in the recorded ADS-B data.
On May 13 at 11:52 p.m. MDT the flight took off from KROW. The pilot had filed an instrument flight plan and contacted Albuquerque Air Route Traffic Control Center (ARTCC) and requested an instrument clearance. The aircraft was cleared “as filed” to KSRR and assigned an altitude of 12,000 feet msl.
Shortly after midnight, ATC advised the flight crew that they were 1,000 feet above their assigned altitude. The crew replied they were correcting and stated they had lost GPS capacity. The pilot asked for heading and was subsequently instructed to fly at 275 degrees toward KSRR. ATC asked what approach they would like at the airport, and the pilot requested the RNAV approach to Runway 24.
At 12:01 a.m. the ATC controller telephoned their operations supervisor and requested the military to stop jamming the GPS, then subsequently cleared the flight to REYOK, an initial approach fix approximately 21.1 nm from the airport. The pilot read back the clearance, then turned to the north. A few minutes later, the pilot requested the ILS approach into KSRR because of the loss of GPS navigation capability.
The NTSB report stated the controller had other traffic that night, including three additional aircraft reporting a loss of GPS. One of these aircraft required extra assistance from ATC as the pilot was having difficulty identifying a ground-based navigation aid.
At 12:04 a.m. ATC advised the accident flight crew they would be getting radar vectors to align the aircraft for the straight-in approach over REYOK “in a couple of minutes.”
At 12:05 a.m. the ATC operations supervisor contacted the military to stop jamming. At 12:07:34 a.m. ADS-B data recording resumed.
At 12:08:06 a.m., while flying a northwest heading and about 31 miles northeast of KSRR, the flight crew reported they had the airport in sight but their transmission was stepped on by other radio traffic. The crew repeated the transmission and indicated they would “go visual.”
ATC subsequently cleared the flight to KSRR and for a visual approach, and informed them they could cancel IFR in the air above 9,000 feet msl or they could cancel their clearance via the flight service station after landing. The flight crew acknowledged the clearance and responded that they would cancel IFR “in just a couple of minutes.” There were no more transmissions from the crew.
The accident aircraft turned toward KSRR. Between the airplane and the airport were the Capitan Mountains, extending to an elevation of 10,201 feet msl.
At 12:10 a.m. the ATC operations supervisor informed the military that the airplane was on a visual approach and that they could resume jamming.
At approximately 12:10:27 a.m. ADS-B again began collecting data as the airplane began a descent toward the airport followed by a slight right turn. The Spidertracks GPS altitude indicated 9,820 feet msl, then a descent to 9,400 feet, then a climb.
The last Spidertracks readout captured at 12:15:26 a.m. indicated the aircraft at 9,823 feet GPS altitude on a heading of 250 degrees with a ground speed of 150 knots. The aircraft collided with terrain at an elevation of approximately 9,950 feet, and about 730 feet east and 230 feet below the Capitan Mountains Summit Radio Facility, which is located at an elevation of about 10,180 feet. A postimpact blaze occurred that initiated a forest fire.
Throughout the report the NTSB cautions that this information is preliminary and subject to change. The final report is likely 18-24 months away.
That being said, this is one of those accidents that savvy instructors will be following closely and likely using for discussion in instrument ground schools. The only positive thing that comes out of events like this is that it gives other pilots a chance to learn from the mistakes and misfortune of others—and make it home safely.
The information provided in this NTSB report inspires discussion for aeronautical decision making given a challenging situation. What would you do if you were the pilot and lost GPS during the approach?
As a seasoned instructor used to tell me, “GPS works wonderfully as a means of navigation until it doesn’t.” He was both strict and thorough and made sure I knew how to use all the navigation tools in the cockpit and could fly all the published approaches, which at the time were ILS/LOC, VOR, NDB, SDF, LDA, and RNAV with both full and partial panel.
I can still hear his voice telling me to climb if I lost RAIM during an approach, because a loss of situation awareness in IMC or at night can lead to a collision with “cumulo granite”—that is running into a mountain, or closer to the airport you might encounter an antenna or even a building if you continue the descent. If you lose GPS during the approach, don’t hesitate— climb.
As he noted, statistically speaking, there is usually less of a chance of running into something in the air than close to the ground.
