Beginning last January, U.S. cellular telephone companies began deploying C-band 5G networks that operate on frequencies that could cause interference with radar altimeters installed in aircraft.
The FAA is urging aircraft manufacturers to work with avionics suppliers to develop mitigations for the interference such as electronic filters. At least one avionics manufacturer—FreeFlight Systems—is now offering 5G-resistant radar altimeters but it is not a quick process for airlines to replace equipment in large fleets of aircraft. The FAA also issued Special Airworthiness Information Bulletin AIR-21-18R2 in response to this issue.
The FAA is asking pilots and operators to report instances of suspected 5G interference because radar altimeters provide low-level altitude information to many aircraft systems and interference could affect critical equipment such as autothrottles, autopilots, spoilers, anti-lock brakes, head-up displays, and others.
Pilots should file reports on possible 5G interference on the FAA’s “Report a Radio Altimeter Anomaly” website.
While information from these reports isn’t available on the FAA website, AIN has submitted a Freedom of Information Act request to view all the reports submitted through the end of 2022. Meanwhile, one source of information on 5G anomalies is the NASA Aviation Safety Reporting System (ASRS) database.
In a search of the ASRS database for 2022, there were 143 reports containing “5G” in the narrative. A handful were unrelated to radar altimeters. Some were turbulence reports where pilots wrote that they had experienced “5G” bumps. Many others had to do with certain airports being unusable as IFR alternates due to the FAA’s 5G notams warning about the possibility of interference and certain approaches being unavailable. In some cases, pilots and dispatchers found the notams so confusing that they either neglected to file the proper alternate or had to scramble to find a legal alternate after flying a missed approach procedure at the destination. This resulted in some disruption and is one consequence of the C-band 5G cellular network deployment.
Many autoland (CAT II and III) approaches are not available for certain airplanes due to potential 5G issues. One pilot complained in an ASRS report: “Weather at PDX [Portland International Airport in Oregon] was at minimums for ILS CAT I, with autoland not available due to 5G tower location. Fix the 5G issues! This is adding a lot of additional workload and safety hazards to flights that should be easy utilizing a proven safe autoland system.”
In the ASRS 5G results, pilots reported a variety of anomalies that may have been a result of 5G interference. These incidents can’t be considered definitively due to 5G interference, however; many of the reporters cited GPS interference, which could have been caused by intentional jamming tests by the U.S. military.
Yet some reports included worrisome instances of aircraft systems either shutting down or operating in a manner that required pilot intervention. This included an autopilot that nosed the airplane down close to the ground, autothrottle shutoffs, and head-up display (HUD) anomalies.
It should be noted that the majority of these reports are by pilots flying in Part 121 airline operations. For some unknown reason, pilots flying business jets either aren’t experiencing these 5G interference issues or they just aren’t reporting them to the ASRS database.
Although it ought to be helpful for regulators and aircraft and avionics manufacturers to see these reports, much of the useful information about aircraft and avionics types is stripped out, so the reports don’t help identify which products are experiencing the most issues.
'Impossible' HUD
A pilot flying into Sarasota/Bradenton International Airport in Florida in visual meteorological conditions (VMC) had the ILS Runway 14 approach loaded in the avionics to provide a backup, always a good idea to confirm the aircraft is aligned with the correct runway. According to an ASRS report: “At approximately minimums (200 feet agl), HUD indications shimmered, then blinked, and then filled display with all kinds of data/warnings, to include what I believe was the attitude lines of the unusual attitude recovery mode, largely blocking all direct view through the HUD and making it impossible to use and difficult to look through.
“At approximately 150 feet agl (estimated by looking out window), GPWS flare callouts began to count down 50, 40, etc., like aircraft wanted me to flare. I was way too high for that. FO (first officer) pilot monitoring stated: ‘That's not right’ and asked me what was going on. I said I lost HUD and was proceeding visually.
“I was able to ‘fly through the HUD,’ ignoring its information to a normal landing. He said he saw no other indications. I have no idea [of the cause]. HUD was inop from 200 feet down. I talked to another captain, and he told me to treat it like a failure and put in AML [maintenance log], which we did. We both understood that 5G is not supposed to be on yet, so elected to treat as a failure. Maintenance could find no faults in subsequent checks and signed it off. Next leg was no problem. Delayed flight about an hour.
“Maintenance stated that they have gotten several reports on different aircraft this week similar to this but do not know the cause. It is quite possible that 5G testing is going on that we do not know about. Also, quite possible it is just a glitch, another software issue, other interference, or just broken. I suggested to maintenance that they begin tracking these odd events by airport as well as aircraft. Had it been a low-IMC approach, I would probably have had to exercise a missed.”
Autothrottle, Autopilot Issues
After a departure from Oregon's Portland International Airport, a flight crew notified the departure controller that the avionics made a radar altitude callout of 1,000 feet while the aircraft was climbing through 2,300 feet. At 11,600 feet, the callout said “ten” and the autothrottles disconnected. The pilots reported this to the Seattle Center controller, and all would have been fine from this point had the pilots not been distracted by the random callouts.
One of the pilots reported to the ASRS, “I went heads down to report the occurrences to the company via ACARS and failed to recognize we climbed through FL180 and failed to complete the after-takeoff checklist, most importantly failed to reset the altimeters to 29.92. We leveled at FL370 for less than a minute with the altimeters set to the departure airport, and this put us at approximately FL367. We corrected the altimeters and completed level-off at FL370. I got distracted by abnormal radio altitude callouts and autothrottle disconnect due to 5G interference. I was predisposed to think the implementation would be a nonevent and was surprised to experience actual interference events. I then rushed to comply with company reporting requirements, which should have waited until the next phase of flight.”
In VMC during an autoland “confidence check” approach to Runway 25L at Los Angeles International Airport, a pilot reported a problem with the autopilot. “Approach was normal at first, received [autoland] 'Land Green' before 350 feet. On short final, I believe it was below 100 feet agl, near the overrun, the plane started an aggressive nose-over descent. I took control of the aircraft and raised the nose and landed.
“In hindsight with time to process the events, I should have gone around. At the time I was reacting to a sudden and unexpected event and did not have time to think. I did not see an autoland light before the nose over. [This] may be a 5G interference. It was a very alarming pushover by the autopilot. In IMC conditions, it could have caused a crash.”
While nearing Phoenix, a flight crew noticed that the auto brakes disarmed two times after being set, so they finally selected manual braking. After the captain’s radar altimeter flagged red (inoperative), the pilot flying reported, “In the flare I noticed a resistance to pitching up. It almost felt like the autopilot was still engaged. I overcame the resistance to set the landing attitude. Additionally, the [autothrottles] did not go to idle at 27 feet as normal.
“I manually closed the thrust levers prior to touchdown. The spoilers worked normally. As the nosewheel came in contact with the runway, the thrust levers started to move forward. I held them back and disarmed the [autothrottles]. This delayed my engaging the thrust reversers a few seconds. The aircraft was stopped without incident.”
The potential interference from 5G cellphone towers isn’t going away, and these reports suggest that this is a significant safety issue. AIN will follow this up with any additional information on 5G interference and the response from the FAA to the request for reports to the Report a Radio Altimeter Anomaly website.