What you need to know about 5G interfering with aircraft

You may have seen some recent news stories about 5G wireless technology affecting aircraft, and how some flight operations might become difficult during periods of time when the weather is less than ideal.

So what does 5G have to do with aircraft?

First, it's important to talk about altimeters — the instruments that pilots use to determine an aircraft's altitude or height. The airliners familiar to TPG readers have two types of altimeters on board: a barometric altimeter and a radio altimeter. Pilots use the barometric altimeter almost exclusively, and it's used to determine altitude above mean sea level.

The other type of altimeter is called the radio (or radar) altimeter. This altimeter determines an aircraft's precise height by bouncing radio waves off the terrain below. It only works from the ground up to a few thousand feet.

Pilots use radio altimeters when conducting approaches to airports in low visibility, like when airports such as Houston (IAH) or Seattle (SEA) get socked in by fog. These approaches are predicated on a minimum height at which pilots must see at least the approach lighting system — it's called the decision height. If they don't see the lights or runway, they must execute a missed approach and wait until the weather improves, and if it doesn't improve, must divert to an alternate airport.

For these types of low-visibility approaches, pilots use the radio altimeter to provide precise guidance on exactly how high they are, since they often won't be able to see anything until seconds before touchdown. Aircraft conducting automatic landings (exactly what it sounds like) in low visibility also rely on this data. Finally, radio altimeters power aural digital height callouts ("50, 40, 30, 20, 10") on aircraft, increasing a pilot's situational awareness.

On Jan. 5, Verizon and AT&T will switch on a new spectrum of 5G wireless communications — the C-Band spectrum. The problem is that a study conducted last year shows that 5G transmissions in this spectrum interfere with radio altimeters on aircraft, saying there is a "major risk" that these systems "will cause harmful interference to radar altimeters on all civil aircraft." These radio altimeters operate in an adjacent spectrum to 5G C-Band and are susceptible to interference.

More: What happens when you don’t put your phone on airplane mode?

The Federal Aviation Administration views this as a major safety issue, and on Thursday, formally published what's known as an airworthiness directive, or AD — a formal order requiring operators to take action for safety reasons. This AD says that the FAA will publish Notices to Air Missions, or NOTAMs — important, time- and safety-critical information that's related to operations at a specific airport — when it's determined that 5G C-Band towers will be in the vicinity of an airport. Those NOTAMs will prohibit certain operations requiring radio altimeter data at those airports.

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To be clear, the FAA is taking issue with 5G C-Band towers, and not passenger cell phones. While your personal phones should always be in airplane mode, and the captain can order all devices turned off if any sort of interference is detected, the FAA views towers as the threat, as they can't easily be turned off like a personal device.

Imagine this scenario: You're on an Alaska Airlines flight into SEA, and as is common, fog is rolling in. Normally, your pilots would be able to conduct one of these low-visibility approaches. But there's a 5G C-Band tower near SEA, and the FAA has published a NOTAM prohibiting this type of approach because the tower interferes with your aircraft's radio altimeter. Your aircraft is placed in a holding pattern for a bit, but unfortunately, the weather does not improve, and you must divert to your flight's alternate airport in Portland, Oregon (PDX).

It's this scenario that could play out in the coming weeks and months, and a major pilot union is sounding the alarm.

"There are ways that 5G can be deployed while maintaining our high level of aviation safety, and time is running out for the wireless industry and the broader aviation community to work together on implementing mitigations that will ensure that every passenger and cargo flight arrives safely without severe disruptions to aviation operations," said Capt. Joe DePete, president of the Air Line Pilots Association, which represents pilots at Delta, United and smaller carriers.

As ALPA and other industry stakeholders keep negotiating over 5G C-Band, it seems highly unlikely that the FAA's AD will be the final word on it.