Wednesday, March 01, 2006

Rock & A Hard Place

I just got back from a four day trip that had quite a few legs in and out of Sun Valley, the first time I've been back since my unexpected Boise diversion. The poor weather across most of the west coast - strong wind, turbulence, low clouds, rain and snow - guaranteed some headaches for the Sun Valley turns.

When the Sun Valley airport is unusable, we operate out of Twin Falls, located about 90 minutes south. The company has a pretty good procedure to ensure that the flights stay on time when this happens. Outbound passengers call a hotline several hours before departure; if Sun Valley is closed, they are instructed to show up at the terminal some time before the scheduled departure time to ride to Twin Falls on the chartered coach. Ideally, the bus arrives in Twin Falls before the airplane does; the inbound passengers ride back to Sun Valley on the same bus. The decision to operate out of Twin Falls or not is made several hours prior to the arrival time.

On Monday we flew out of TWF twice, going to and coming from Oakland. The weather was getting worse the whole day, so I didn't hold out much hope for making it into Sun Valley on our two turns there on Tuesday. Indeed, as we prepared to depart for SUN on Tuesday morning, the weather began to deteriorate again. Still, it was a little late for the company to switch operations to Twin Falls. We decided to give it a shot.

Approaching Sun Valley, the ATIS was calling the weather a 1700' broken ceiling and 3 miles' visibility in light rain. This posed two problems. First, that's right at the minimums for the RNAV (GPS) W Rwy 31 approach. Secondly, even if we got in, we couldn't depart IFR. Take a look at the takeoff minimums here (the airport is listed under Hailey, ID). Takeoff minimums for runway 13 include a 2700' ceiling. Unlike general aviation, the airlines must comply with takeoff minimums. We called our dispatcher and informed him of the problem; he suggested that we attempt an approach, and if we got in the weather was good enough to depart VFR. We told him we'd try the approach but gave him no guarantees on being able to depart.

The approach was as tight as I've had in Sun Valley without going missed. We broke out right at minimums and were just able to see the runway. The ride in was pretty bumpy thanks to a 40 knot wind over the nearby ridges, but we got down safely. Next step: figure out how to get back out.

At most airports, the IFR takeoff minimums are far below the minimums for visual flight, so being below takeoff minimums means you're not going anywhere. At places like Sun Valley where the departure minimums are very high, you have another few options: departing VFR, or departing IFR with a restriction to maintain VMC (visual conditions) until a certain point. We do this very seldom at the airlines, so it sent the captain and I scurrying for our Flight Operations Manuals (FOMs).

To go VFR or depart with a VMC restriction, we would need at least three miles visibility and a 1000' ceiling; by this time the airport was at 5 miles & 1900'. So far, so good. We would need to maintain VFR cloud clearances (500' below clouds) and stay at least 500' above any terrain. No problem. Our FOM contains an additional restriction on VFR departures: it can only be used from an airport lacking an ATC facility. KSUN has a tower, so that wouldn't work. Our only chance to get out was to depart IFR with a VMC climb restriction.

Here's the catch: With a VMC climb restriction, you must be able to maintain visual conditions all the way to the minimum altitude for IFR in your area, whether that be a minimum enroute altitude (MEA), minimum obstacle clearance altitude (MOCA), or minimum vectoring altitude (MVA). These can be quite high in mountainous areas. We got ground control to ask Salt Lake Center what the minimum altitude was near Hailey NDB, just outside the valley, and it turned out to be 9000'. With the ceiling at 7200', we could maintain VMC up to 6700 feet but then would have no way to legally climb to 9000. We couldn't legally depart - we were stuck.

Well, not quite. While we were burrowing into the FOM, the weather was improving, so by the time we were ready to call our dispatcher and tell him no-go, the ATIS had been updated and it was now officially 10 miles vis, broken ceiling at 3200. Now the airport was above takeoff minimums so we could depart as usual, using the published departure procedure.

These are the kinds of sticky situations that most airline crews encounter pretty infrequently. Most of us are pretty unfamiliar with the rules that apply to these situations, which is why we carry the FOM with us and refer to it as needed. Of course, just because the rulebook says you can do it doesn't always mean you should do it, and that's where experience and judgement have no substitute.


Anonymous said...

Sun Valley is the best little airport in the world and only
the best of the best can Captain
in here.

Hugh said...

Thanks for your description of departure procedures for KSUN. I was trying to figure out what was going on this last Sunday when we were leaving for SEA. The weather was very good but below that IFR departure minimum. The pilot spent some time talking to the tower and we ended up unloading 6 people so we could take off downwind towards the mountains where the weather was clearer. I now understand we could make it to the Minimum safe altitude departing VMC restriction in that direction. Seemed like a poor choice when compared to taking off VFR into the wind and away from the mountains (rwy 13), though I guess your FOM doesn't allow that. Maybe the ops should review the FOM for KSUN. Great blogging.