MDA or DA 1

28.10.2004

Submission:

There are really two questions here on busting MDA on a go around. Is it legal, and is it safe? The submitter is generally correct on the legal part. In the US, descent below MDA requires that the airport/lights be in sight, a normal landing can be made on the intended runway, and that the visibility exceeds the published minima. So if you don't see the airport, you are not, in general, authorized to descend below MDA. If you descend below, you are in violation of FAR 91.175. However, in airline operations under FAR 121, the approved airlines operations manual provides additional regulatory guidance to the FAR, and a case can be made that if the airline pilot is operating in accordance with his/her FAA approved manual, then there will be no violation.

Enough of the legal stuff. Is this safe? If the runway to which the approach is being flown (assuming a straight in approach here), is 34:1 clear, then it is almost certainly OK to bust MDA a bit on the go around. The types of aircraft we are talking about have huge excess performance and will not bust MDA by more than a few feet. Now if the procedure is being flown heavy weight and engine out, then I don't think that MDA should be busted. If the procedure is not 34:1 clear, or at least 20:1 clear, then you are somewhat at risk by busting MDA. You only have a theoretical max of 250 ft obstacle clearance, and that does not include temperature or other altimeter errors, etc. Is there an obstacle right there at the MDA? Who knows, but there is one somewhere. You just don't know. Remember the American Airlines accident at Stewart--a stale altimeter, strong winds, poor terrain, and a small bust of MDA resulted in tree limbs in the leading edge flaps. So if you know the runway is 34:1 clear under the non-precision approach, you are probably OK. How would you know this? You won't unless there is an ILS to that runway with a DA below the NPA DA. Of course, if the ILS were in service, you would not be shooting the non-precision approach anyway, right?

So in summary. If the airline operations manual allows descent below the MDA on the go around from a non-precision approach, the pilot can do it legally. Non-airline pilots or pilots who's manuals don't authorize it cannot, however, legally do it. It is probably a save procedure if the runway has an underlying ILS. Otherwise, it could be questionable.

Vince Massimini

The MITRE Corporation
McLean VA USA