When discussing personal minimums with pilots, the aspects we discuss the most are ceiling above the runway, cross wind / gust factor limitations and sometimes we discuss currency / proficiency minimums. The aspect that is often overlooked is visibility minimums.
Let me ask you to answer honestly: What is the most limiting factor in your assessment of your own flying abilities as it relates to personal minimums? My bet is that most of you will say cross winds and low ceilings. Right? It is a leading question with this topic heading, but I think visibility may be overlooked as a very, if not the most, limiting issue. And I think this is true for IFR/IMC conditions, but even for VFR pilots. Limited visibility can throw mean curveballs even when the airport shows ‘green’ on ForeFlight.
Taxiing out to the runway this weekend, in the rain and still on the ramp, I received a call from ground ‘Caution, visibility rapidly decreasing’. This was already entirely true for me, as I could hardly see a thing ahead of me with the rain hitting the wind screen the moment I pushed the throttles forward a bit to get going on my taxi journey to the runway. This situation gave me a few seconds pause and I tried to internalize the ‘caution’ I just received.
From my position I could see the tower still without trouble. Runway length of 7700, estimating to be about half way, this was about 3000 ft to the tower, plus perhaps another 2000-3000 ft to the tree line behind the tower. So, about at least a mile visibility was my estimation. ATIS was alerting me to ceilings of 2500 bkn, 3900 bkn, 5500 ovc, and Foreflight showed ‘Blue’. It was rapidly changing though, according to ATC.
ICAO defines visibility as the greatest of:
- The distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background.
- The distance at which lights in the vicinity of 1000 candelas can be seen and identified against an unlit background.
That clears that up. Practically, it is very difficult to estimate visibility. Visibility is usually measured using known landmarks, exactly like I did above. Or from the vantage point of the cockpit in the air, using visible landmarks in the landscape or towards runways. A VFR 3sm visibility, on the ATIS last year at INT for several weeks due to fires and smoke particles, is remarkably tight. Considering that you would not see the runway from the final approach fix on the ILS into runway 33, you would not see the airport while coming from your usual Mount Airy practice run until you are parallel to the city. And remember that 3 sm is 0.86 x 3 nm =2.58 Nm.
To make things even more complex, seeing the ground below does not mean that you can see forward when lower to the ground. The density of the particles forward viewing is difficult to estimate, and the distance to the ground may give a false sense of security. I found this to be a good write up about these issues.
So, with the plane now moving (and without a fan on the nose on the Baron blowing the rain off the windshield) and without window wipers, my visibility of the yellow lines on the apron disappeared almost entirely. Using the airport diagram on the avionics and looking forward and alternating out of the side window, I found my bearing to taxiway C, and turned onto Alpha to the runway. Sloooooow taxi.
Taxiing low visibility operations are very tricky. I found this write up on boldmethod.com to be great. Perhaps click on this link for some further education on this topic. ICAO put out this awesome slide show:
During the run up phase and prior to calling ‘Ready’ I triple checked the avionics and autopilot settings: Heading, altitude, transponder set. I had my HAT on. I anticipated turning on the autopilot once I safely got up, ceiling 2500, but likely immediately in IMC. Autopilot set for 2000 with flight director in GoAround mode. Large Jet parks behind me, and surely I got a right turn out amendment upon calling tower to give way to the American Airlines big boy behind me. ‘After departure turn right heading 190, cleared for takeoff runway 17. Perfect Swiss cheese holes lining up for spatial disorientation because of loss of visibility with this kind of turn.
After take off the avionics thankfully did their job. I hand flew to about 1000 ft, in immediate IMC / rain upon rotation and looking up towards gray clouds, and entered into a turn to the right while in this climb. ATC transfer to Approach, with an immediate reminder by ATC to turn to 190, to get the 200 impatient passengers behind me on their way to wherever. A quick verification of the flight director and the autopilot came on. This was the start of an amazingly uneventful flight back home.
Had something happened to my PFD and had I have to switch to my standby little G5, with minimal forward visibility this would have been a all hands on deck, super high octane departure with a very difficult return to the airport. Perhaps the best option would have been to then fly straight out, call an emergency, and climb to 8000 ft without turns. That’s what I had briefed myself I would do during my departure brief. No head turns, no level offs, and no returns to the airport. Verify climb and direction of flight as the emergency aircraft by asking ATC, and once above the cloud deck (which I knew to be at 7000) I would fly west to VFR conditions.
So, boring story about a non-event, brought on by an ATC ‘caution’. Regular fare for airline pilots flying Cat III approaches with autoland. At the same time, I felt that the pitfalls of low visibility -even with plenty of a ceiling- would be something to contemplate when we discuss personal minimums next!
Cheers, and happy, safe flying!
David
REENO Aviation Flying Club 
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