Friday, November 27, 2009

Low IFR Departures

Wednesday morning I devoted substantial attention to a strategy for avoiding icing issues during my flight to the DC area and planning to arrive at a time when I'd have good chance of completing the approach at KVKX.

I confess I didn't pay a lot of attention to the weather there in Bridgeport. When I looked out of my window I saw pretty good visibility and an overcast that had been running about 900 to 1,100 feet MSL for hours. So I was a little surprised that it was actively drizzling when I headed over to the airport and the ceiling and visibility had certainly deteriorated.

By the time I had pre-flighted N631S, picked up my IFR clearance and started the engine, this was the current METAR:
KBDR 251835Z 08004KT 1 1/2SM BR OVC003 09/09 A3000 RMK AO2 P0001

A mile and a half visibility and a 300 foot overcast qualifies as Low IFR in anyone's book. My late instructor, Bob Parks, had gotten me to practice "zero/zero" takeoffs "under the hood". But he also told me, "Don't take off from an airport that you can't get back into."

The wind was favoring Runway 6, which is the ILS runway at KBDR. I looked at the approach plate for the ILS RWY 6. It specified a Decision Altitude of 307 feet and minimum flight visibility of 1 mile. I decided to accept the 7 foot difference between the approach DA and the reported ceiling. Off I went, feeling that I could get back in using the ILS if something surprising happened.

Edit, 11/28: A couple of friends have pointed out that since the DA on the plate is in feet MSL and the ceiling in the METAR is in feet AGL, the values in this case were exactly the same in terms of feet above the 7' TDZE (Touchdown Zone Elevation). It's not a big factor at a near-sea-level airport like KBDR, but with a more substantial field elevation the difference would be more important (and presumably, more obvious).

I was promptly into the clouds. After the New York Approach controller got me radar identified she cleared me to 5,000 feet. I broke out of the lowest layer at about 3,500 feet and was into the clear between layers.

The rest of the flight worked out as planned. On this day, no need to return to the airport emerged. If one had (like, maybe, if my vacuum pump failure had picked 25 November instead of 12 October) I was pretty comfortable that I could fly the ILS to minimums and get back on terra firma. Of course, that comfort derived in part from the fact that I know the airport well and have probably done that approach 20 times.

Looking back I ask myself:

  • Would I have been comfortable departing from an unfamiliar airport?
  • How about if the ceiling was at 200 instead of 300?
  • What if I was in an unfamiliar airplane (like a rental)?
  • Suppose N631S had just come out of significant maintenance?
  • What if it had been a couple of months since I'd flown an approach in actual IMC?

On every departure in challenging weather, there are lots of things to consider. As with most decisions, the Devil is in the details. On this occasion they added up to a "Go" decision with which I was comfortable.


Keith Smith said...

Very interesting post! Here's a quiz question for ya, you're heading to Caldwell (KCDW), planning to shoot the LOC RWY 22. You check the METAR, OVC004. If you can identify the KOLLI intersection, the minimums are 540ft. What are the chances we're going to be able to make this approach? Pretty dreadful, right?

Actually, no...you're in good shape! It's _vital_ to remember that ceilings in METARs are AGL, whereas your eye is drawn to the MSL minimums on a typical NACO chart. At CDW, that 540' MSL is actually 367' MSL.

Always look at the smaller numbers on the approach plates when contemplating approaches to minimums.

Flying IFR in any real weather does take on some measure of risk with regard to emergency situations. If you have an engine failure in IMC, it's less likely to be about whether ceilings are 100ft above or below minimums, and much more about the amount of time you have between breaking out and contacting the ground, in terms of finding a spot to put it down.

Outside of an emergency situation, just make sure you have a rock solid option in terms of somewhere to land, even if it's a little further away.

I've departed from my home airfield, N07, the lowest approach for which requires > 1000' ceilings on days with 500ft ceilings. I just accept that I won't be going back there if there's a problem, but will instead shoot for nearby KCDW (small wonder, then, that I picked that example above!)

Frank Van Haste said...


Thanks for your comment. You're the second person to draw my attention to my sloppiness about the MSL vs AGL minima. I admit it's an occasional blind spot of mine, that I attribute at least in part to having "grown up" flying from an essentially "sea level" airport (KBDR).

In any case, one of the reasons I write these things is to learn from others, so I thank you. I hope you'll keep dropping by.



Keith Smith said...


That's completely understandable, particularly since there was probably little value in noting the difference when flying out of an airport during your training that had an elevation of 7ft.

It's always fun and interesting to read about other pilots' styles, techniques and thoughts. Thank you for taking the time to post yours!

If you're interested, you can keep up with my flights at the KS flight log.

LarryPetro said...

Keith, I think you have a typo in your comment. I think that you meant 540' MSL (the published minimum for the CDW LOC RWY 22 with KOLLI intersection) is the same as 367' *AGL* which compares with the METAR ceilings, or more accurately 367' HAA.