Aviation In Canada

Wednesday, December 29, 2004

Odd Hold

Another thing that happened during the recent winter storm was an odd holding pattern. One airplane, inbound but not ready to land (who eventually chose to go elsewhere when the report he was waiting for turned out to be ugly) was issued a holding clearance. The hold went something like, "cleared to the Sumspot VOR via direct, hold west, inbound on the 270 raidal, expect further clearance at 1600". The idea was to get the airplane established for a race-track hold with the top of the oval heading inbond to the VOR on a 090 track (the 270 radial on the west side of the facility) and make a right turn of about 180° to go outbound. On the outbound leg, the aircraft adjusts his heading and outbound timing to provide an inbound leg, on the described radial, of approximately one minute.

The aircraft's solution was interesting. He flew inbound on the 270 radial, the appropriate track of 090. Over the VOR, he turned right. The right turn, which should have been 180°, was very wide and ended up being a turn of 225°. He flew this heading to intercept a track outbound from the VOR and parallel to the 270 radial on the south side, offset by about 4nm. Then, after approximately one minute, he made another wide right turn of about 225° which carried him through the 270 radial to the north side and the heading brought him back to reintercept the 270 radial for his inbound leg. This pattern was repeated with precision 4 times before he finally left. This implies that the FMS was driving the holding pattern, and that it was programmed inappropriately. Going so far through the 270 radial to the north side means that he was actually well outside the airspace that ATC would protect for his hold. In the radar environment, it is less critical, since we can see what he's up to, but it's still a surprise. In a non-radar environment, ATC could put someone nearby, provided the protected airspaces don't overlap. Given just how far this jet went from where he was supposed to be, it provided for a bad opportunity for a problem. Someone should be looking into why that hold went so far awry. Anyone know any more about why this sort of thing might happen? Please leave me a comment so we can learn.