Plane & Pilot
Tuesday, March 8, 2011

Perfect Your Approaches


Nonprecision suggests a casual approach to IFR procedures, but you’d best fly them with precision


Once the clock is running, make any necessary turn to an on-course, inbound heading. If the approach is a straight-in, that heading will always be within 30 degrees of the runway alignment; otherwise, the approach will be designated as circling, and higher minimums will apply. "Tach" refers to a power adjustment to start down toward the MDA. (This more accurately might be labeled "throttle," since airplanes with constant-speed props may show little change in rpm.)

"Trim" is self-explanatory and may be automatic if you were taught to always minimize elevator pressure. "Track" is merely a double-/triple-check of what's happening to the nav needles. Are you holding course, or is a correction necessary?

"Tune" refers to the need to switch the radio to tower or Unicom frequency, and "talk" is the least important of your priorities, announcing that you're inbound from the FAF.

Defining the proper speed to accomplish all these tasks is a little more difficult. As mentioned above, slower is nearly always better. If it were a simple matter of holding the published airspeed from the FAF to the MAP, the task wouldn't be too difficult. But keep in mind, your time will be based on groundspeed, not indicated airspeed.

This means you may have to correct indicated airspeed for the wind. If you've listened to the ATIS, you'll have a rough idea of what that is on the ground. Since most approaches are flown into the wind, you often can assume you'll be slightly short of the MDA when the time expires unless you make a groundspeed correction. Also, remember that the wind at 1,500 feet may be different from that on the ground. If you have DME or GPS, you can make a mental note of the headwind and plan to fly with a reasonable correction, say five knots faster. (Without a glideslope, some form of distance measuring may be mandatory. Narsarsuaq, Greenland, has an unusual NDB/DME approach with a mandatory 5.6 degree descent, compared to 3.0 degrees on the normal ILS.)

There are two methods of descending from the final approach fix to the missed approach point—drift down slowly to arrive at the MAP when the time expires or descend to the MDA as quickly as possible so you can be established on track and ready to look for the airport when time runs out. The drift-down is perhaps the more favored method, as it's a stabilized procedure. The zip-down is more difficult to time as it requires additional power adjustments when you stabilize at the minimum descent altitude.

Either way, the goal is to fly right up to the MAP at an altitude no lower than the minimum descent height, look for the runway, and if it's not there, proceed to plan B, the missed approach. Don't fly beyond the MAP at low altitude, hoping something will show up. Even if it does, you may be too far along and too high to effect a successful landing.

A safe landing is, after all, the bottom line of any instrument approach. Nonprecision approaches can be nearly as safe as the precision variety if the pilot follows the rules to the letter. Remember, however, that NPAs provide less electronic guidance and have a reduced margin for error. If the weather is really "grundgy" and you're rusty on your approach techniques, you might be wise to divert to an alternate with a full ILS, or better still, look for an airport that's still VFR.



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