What does low visibility mean?
What does low visibility mean?
Alternatively, what happened in SFO this morning? NOTAMs were still effective and altimeters were not approved.
No boilerplate needed about explainers for this one, though we still want to welcome new readers—we’re grateful you’re here!
Instead, we’ll use this space to solicit suggestions: with the holiday travel season having wrapped up, we’re on the lookout for future travel occasions we can cover. Please drop any suggestions in the comments (we’re quite happy to write about more localized events).
Even more so than holiday cancellations, the impacts of 5G deployment on aviation is a well-covered topic. We could link to a different article from each preceding word and still have a stack leftover. The vast majority—if not all—of these articles reference “low visibility,” but don’t go on to define what that means. We don’t fault them for this: they’re charged with appealing to a readership in the millions, in some cases, and to explain it would require more than a sentence or two. Our burgeoning subscriber base numbers in the… well, not millions. More germanely, it’s a technical and/or curious readership—we’ll take advantage of this and steer into some deeper waters. While it will take us a few paragraphs to get there, we think we’ll demonstrate that it’s an important definition.
Instrument operation categories
In our airport capacity explainer, we’d distinguished between visual and instrument conditions:
If visibility is less than 3 miles and/or ceilings are less than 1,000', then aircraft can no longer maintain visual separation from other aircraft.
You’d be forgiven for thinking low visibility means any non-visual conditions—we’d initially made the same mistake, ourselves. To glean the applicability to 5G, we need to head to the FAA’s NOTAM search site. NOTAMs, or Notice to Air Missions1, communicate the abnormal status of procedures or facilities to national airspace system users (i.e. operators). On January 13, the FAA blanketed airports with NOTAMs pertaining to 5G, which would become effective at midnight ET on January 19 (or Jan 19 05:00 GMT) to coincide with the deployment of 5G. Most airports would receive two NOTAMs: one that affects the airport (or aerodrome) and a second that concerns the approach procedures required during instrument conditions (i.e. instrument landing system, or ILS).
Let’s consider the procedural NOTAM first. It’s important to note the parenthetical in lines 2, 3 and 4—these procedures are rendered unavailable for only certain categories (or CAT) of instrument approaches, categories II and III. Disregarding special authorization2 (SA), there’s three categories of instrument landings. CAT I, which remains available, is at the higher end of instrument conditions in terms of visibility; CAT III is at the low end.
But we’ve landed back at our titular question! When it comes to determining CAT, a unique measure of visibility is used: runway visual range, or RVR. RVR reflects the distance a pilot should be able to see down a runway and is determined by an automated system of sensors3. At or above RVR 1,800 feet, operations are considered CAT I. CAT II and III operations are required when RVR’s are less than 1,800 feet. Among other prerequisites, aircraft must be equipped with an autoland system, which automates the landing while pilots monitor, or augmented imaging (i.e. EFVS4) to complete CAT II and III approaches.
Both autoland and EFVS rely on the radio altimeter, which has garnered so much attention owing to the first class of NOTAMs we mentioned, the one that more broadly applies to the airport. These aerodrome NOTAMs prohibit the use of autoland and EFVS because 5G interference betrays the altimeters reliability.
How often would this be a problem?
So we’ve answered what’s meant by low visibility. Naturally, we’d want to know how frequently airports are afflicted by these conditions. Historical data around the more commonly used measures of ceiling and prevailing visibility (measured in miles) is readily available, though decidedly less so for RVR. For that we had to turn to METARs, the raw surface observations taken at least hourly at airports. We scraped RVRs from over a quarter million METARs to estimate the incidence of CAT II/III conditions at the FAA’s core 30 airports. The punchline? Probably not as often as you would think—about 0.24% of hours on average, or just less than 21 hours per year.
Though SFO made headlines5 this morning, Seattle (SEA) finds itself atop our list, with 100 hours spent in CAT II/III during 2019. Oppositely, there were three airports that did not record one hour with RVR’s below 1,800 feet; we’re a bit suspicious of IAH, especially, in this respect and future efforts could expand the sampling window6, perhaps to match NOAA’s 15- or 30-year climate normals.
What happened in SFO this morning?
You may have noted that the NOTAMs excepted aircraft that were using an alternative means of compliance (or AMOC—aviation loves our acronyms). On January 16, the FAA approved the radio altimeter models installed on, most notably, the Boeing 737 and Airbus 320 family, essentially clearing these aircraft types to operate in the presence of 5G. Early Wednesday, the FAA approved another three models of radio altimeters, though regional fleets still lacked an AMOC when operations spooled up this morning.
While weather across the system was generally benign on Wednesday, the first day of deployment, today would prove less cooperative. SFO bucked 2019 averages and would report RVR’s of less than 1,300' feet from 5:30 a.m. to 11 a.m. PT. United Airlines requested a company only ground stop at 7:54 a.m. PT—holding their SFO-bound planes on the ground at their origin station—and the FAA would extend it to all carriers at 8:33 a.m. But the damage was done: more than 20 flights operated by regional aircraft7 were scheduled to depart for SFO before 07:30 a.m. PT. Unable to accept the RVR upon reaching the Bay Area, these airborne regional flights entered holding stacks. Occasionally, a regional flight would test their luck, hoping that RVR’s would briefly fluctuate above 1,800, only to execute a go-around during the last couple hundred feet of the approach. In many cases, with hold fuel exhausted, the only remaining option was to divert. The first regional flight would not land at SFO today until 10:30 a.m. PT. Diversion recovery was expected to last through 4:30 p.m. PT, per an FAA Air Traffic Control System Command Center advisory.
It should be noted that the initial forecast called for conditions to improve around 8:30 a.m. PT. That said, part of us—the cynical, Machiavellian part—suspects the industry launched flights this morning perhaps not hoping for disruption, but believing that it’d be useful to prove a [painful] point. We also think the FAA was slated to approve radio altimeters equipped on “some Embraer 170 and 190 regional jets” this afternoon independent of any disruption in SFO this morning, but it makes for some coincidental timing. The latest approvals clear 74.4% of capacity inbound to US airports; we’ve added a column to the table above that reflects the percentage of seats8 inbound to that airport operated by an aircraft equipped with a not-yet-approved altimeter.
Formerly called Notice to Airmen.
A little too technical even for our publication. SA CAT I and II differ from their parent category by allowing lower minima. These operations may require additional aircraft equipment and aircrew qualifications, and may allow the use of runways with less than the normal lighting facilities required for their approach category.
CAT is also a product of decision altitude (DA), however it’s judged by pilots, not regularly reported and not readily tracked.
RVR is measured at the touchdown point, midpoint of the runway and rollout (or end).
Sorry about the footnote-heavy paragraph. Enhanced flight vision system may rely on forward-looking infrared, millimeter wave radiometry, millimeter wave radar, low-light level image intensification or other real-time imaging technologies.
A couple other technical notes. RVR is often reported as varying between a high and low value, in which case we took the low value. RVR also fluctuates within an hour and we also took the minimum from each hour. As a result, not all 60 minutes of hours we considered CAT II/III were necessarily spent below RVR 1,800’.
Embraer E175 and Bombardier CRJ-200/700.
For the period 1/19/22 to 3/31/22.