Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.") It's part of what makes aviation so exciting for all of us... just when you think you've seen it all, along comes a scenario you've never imagined.

Aero-News has called upon the expertise of Thomas P. Turner, master CFI and all-around-good-guy, to bring our readers -- and us -- daily tips to improve our skills as aviators, and as representatives of the flying community. Some of them, you may have heard before... but for each of us, there will also be something we might never have considered before, or something that didn't "stick" the way it should have the first time we memorized it for the practical test.

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Look for our daily Aero-Tips segments, coming each day to you through the Aero-News Network. Suggestions for future Aero-Tips are always welcome, as are additions or discussion of each day's tips. Remember... when it comes to being good pilots, we're all in this together.

Aero-Tips 03.27.06

It’s always good when readers raise questions -- either I clarify something so readers learn more, or they give me new information that makes me learn, and provide better instruction. My recent Aero-Tips article Non-Precision Descent: Dive or Glide is a good example.

Two readers commented on what they saw as a criticism of what I call the “glide” procedure -- flying a constant-rate descent from the Final Approach Fix (FAF) to the Missed Approach Point (MAP), instead of descending rapidly to the Minimum Descent Altitude (MDA) and then flying level to the MAP... the “dive and drive” method.

One reader wrote:

The goal of a stabilized approach (the glide approach) must be to reach the MDA in a position to land on the runway, not at the MDA. The Instrument Procedures Handbook states “a descent rate should be used that will ensure that the airplane reaches the MDA at a distance from the threshold that will allow landing in the touchdown zone.” (p. 5-29) Admittedly, this calculation can be difficult to make... .. The Vertical Descent Angle (VDA) published on many non-precision approach charts is the angle from the FAF to the touchdown zone, not the missed approach point. (AIM 5-4-5h) Flying this descent angle will allow you to reach the MDA in a position for a normal landing. However, the pilot must figure ground speed to determine the required rate of descent.

My earlier article illustrated the two methods, now modified with a red line that depicts a VDA profile:

If obstacle clearance permits, this is indeed a superior way to fly a non-precision descent, because it exposes pilot and passengers to low altitude for less time in poor conditions, and it permits a stabilized descent from the FAF all the way to the MAP.

Everything’s a tradeoff. The VDP descent can result in higher minima, or (I’ll explain in a moment) higher effective minima.  If an obstacle permits a step-down fix on the dive-and-drive approach, clearance requirements may require the glide descent’s MAP be placed at a higher altitude. More importantly, the minimum altitude in a glide approach will be higher than MDA for the dive-and-drive. This is because, unlike a precision approach, the non-precision provides no clearance below MDA. Whereas in an ILS you can go to Decision Height (DH) and then apply power for the miss, inertia carrying you into (cleared) space below DH as descent turns to climb, in a non-precision approach you’ll have to add power above MDA so you do not descend below MDA in the process of missing the approach -- making minima effectively higher.

Hence, you’ll need to begin the missed approach 50 to 100 feet above MDA if you fly the “glide”-style approach. Adding power to begin the missed, then catching sight of the runway just as you hit MDA is a recipe for disaster if you then chop the throttle and try to land as the nose comes up and you re-enter the clouds. You need to add power about 100 feet high to arrest descent using the dive-and-drive technique also, but you end up at MDA long enough to see the runway if the clouds are right at minimums.

Conclusion: You get a stabilized approach using the glide method, but you get lower minima flying dive-and-drive.

Aero-tip of the day: I’ll repeat the tip from my earlier article: Recognize the advantages and disadvantages of opposing philosophies toward non-precision approach descents -- and choose the method that best fits conditions.

FMI: Aero-Tips