Saturday, April 16, 2011

One Of The Things That Stays Emblazened In My Mind:Never Bust Minimums

One of the things that stays emblazoned in my mind is: never bust minimums. No not never! Minimums are there because the powers to be have decided that is as low as you can safely descend without ground reference.*

Oh, sorry. I am of course writing about instrument approaches. Particularly about following the prescribed approach procedures to the letter. Why am I writing about this? Because I just reviewed some recent fatal accidents on the NTSB web site. This website is recommended for any serious pilot by the way. A good place to learn from the mistakes of others.

Two recent accidents stand out. Both resulted in pilot and passenger deaths and destroyed airplanes. Both were flown by airline transport rated pilots (ATP’s). So maybe experience isn’t always the best teacher.

In the first accident, a Beech twin tried unsuccessfully to land at weather below minimums. Specifically: ¼ mile viz in light drizzle and a ceiling of 100 ft. The lowest minimums allowed on an ILS are ½ mile viz and a ceiling of 200 ft. Yes, there are lower minimums at some facilities, but these above are standard. To go below these requires special equipment and training (not generally available to the general aviation pilot).

In the second accident, again an ATP rated pilot makes a fatally bad decision in a light twin. Flying an RNAV GPS approach with a minimum descent altitude (MDA) of about 500 ft above ground the pilot continues decent to almost 250 ft above ground and crashes killing all, and destroying the plane. You have to wonder what were those guys thinking? They did not declare an emergency in either case, so conceivably could have continued to a safe alternate. Anyone filing an IFR flight plan must list an alternate airport. To list an alternate, the weather at the ETA must be at or better than 600/2 for a precision approach (ILS), or 800/2 for a non precision approach (GPS or VOR). Sometimes that means cancelling the trip if it becomes unlikely that an alternate can meet these weather conditions and/or if the amount of fuel aboard is less than required to get there.

As an example: If one were going from NYC to BOS, about a 2 hour flight in a small plane, and the weather in BOS is iffy there would have to be enough fuel in the plane available to reach an alternate airport and still have 45 minutes of fuel aboard. If everything along the pilot’s route from NYC to BOS is socked in and there is nothing better within reach of BOS then the pilot might have to go all the way back to where they started, NYC in this case. If they encounter a headwind on the way back they probably won’t have enough fuel to do it. So what to do? CANCEL and rent a car. Been there done that! Read some of my earliest blogs where I recount just such a scenario.

*The minimum descent altitude (MDA) depends on the particular airport, which runway and type procedure. The lowest standard ILS MDA is usually 200 ft. The one caveat about “busting” the MDA is this: if at the MDA and/or decision point as a marker beacon, the high intensity or other lights extending from the runway are visible, the pilot may continue the approach to the runway even though the runway is not yet in sight. That means the pilot may go lower than the published MDA under those conditions.