Putting precision in the non-precision approach

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If you have the technology to fly an NDB approach, you can fly a CDFA (Continuous Descent Final Approach), which is a smooth continuous descent from the final approach fix down to 50 feet above the landing threshold. You don't need any fancy avionics for this.

We all feel a sense of worry when spotting the lights after flying along a minimum descent altitude. How far away is the runway? When do we start down? Are there any unlit obstacles between here and there? There are more than a few problems with how we've always done it in the past, and leaving the final approach fix with a 1000 fpm rate of descent means we have to remember to start the level-off before the MDA (Minimum Descent Altitude). Even with an autopilot we have to make sure it is programmed correctly

Even if we succeed at levelling off at the MDA, we need to make sure we don't start down after spotting the runway until it is safe to do so. At night in poor visibility, we are terrible judges of distance. DME (Distance Measuring Equipment) helps, to be sure, but after spotting the runway the temptation to nose over is too much for some. Fog and low-level clouds are far from uniform, and just because you've spotted the runway from the MDA doesn't mean you will keep sight of it.

When flying along the MDA you don't technically have to go around until the missed approach point, which typically sits at the approach end of the runway. Spotting the runway this late in the approach is too late to land the airplane. In 1994, the crew of Korean Air Flight 2033 tried to do just this at Cheju Airport in Seoul. They were unable to get the airplane stopped on the runway. While the airplane was destroyed in the overrun, everyone survived.

Pilots are mission-oriented and if they don't get in on the first try there is a strong urge to try again, just a little lower. Reaching a decision altitude along a glidepath is fairly cut and dried; driving along a MDA leaves time to explore the regions of a little lower. Any instrument pilot with several hundred approaches to minimums will tell you that when you're at minimums, you should never point the airplane earthward unless there is a runway directly ahead. Any precision approach with vertical guidance meets this criterion. A non-precision approach flown under dive and drive techniques does not.

It has become clear to most aviation authorities that it makes better sense to approach the runway with some type of vertical guidance, even if the instrument approach doesn't have a glideslope of some sort. While a CDFA makes sense and in some cases is mandatory, you can't use the technique on all approaches. In the U.S., the approach has to have a published glideslope (GS) or vertical descent angle (VDA) from the final approach fix (FAF) or in some cases a step-down fix, to the threshold crossing height (TCH). An FAA chart will spell out as well as the angle right on the profile view of an approach that includes an ILS or RNAV (GPS) glideslope. A Jeppesen chart will show glideslope along the glidepath. If a non-RNAV or RNAV approach without a glideslope has a validated vertical descent angle, it will be shown with an angle symbol on the profile view of a FAA chart. On a Jeppesen chart it will be a dotted gray line below the DA/MDA.

What is Your DDA?
In most cases MDA is lowest you can go without seeing the runway. If you are aiming the airplane to the touchdown zone, you will need an altitude increment to give you and your autopilot the necessary space to go around. This increment is added to the MDA to determine a Derived Decision Altitude (DDA). Some countries specify a height increment, but most leave it to the operator. Your aircraft manuals may specify the increment or give you a maximum certificated height loss.

Basic Procedures
The objective of a CDFA is to leave the final approach fix fully configured, on speed and ready to land. You should not have to destabilize the aircraft by making thrust, airspeed or trim adjustments when spotting the runway.

Low-Tech Procedures
Even if you don't have an autopilot or flight director, you can fly a CFDA. The FAA Instrument Flying Handbook (FAA-H-8083-15B) includes a descent table that converts the angle of descent into the feet/nm to lose and a descent rate for given ground speeds. If, for example, your ground speed once fully configured is 120Kt, flying a vertical descent angle of 3.0 degrees, your vertical velocity indicator (VVI) should be 637 fpm. You can check your progress along the way by subtracting the feet/nm every mile. If the final approach fix (FAF) is at 2000 ft, after 1 mile you should be 2000-318 = 1682 ft. It would be a good idea to work out target altitudes when briefing the approach and write them on the approach plate.

Flying a CDFA is the safer way to fly almost all non-precision approaches. In some countries it is a mandatory procedure; in others it is highly recommended. After one of its DC-9s ran into some trees short of a runway in 1996, American Airlines felt compelled to remind its crews, Despite its name, a non-precision approach must be flown with exacting precision.

Excerpts from: Don't Dive and Drive by James Albright, Business & Commercial Aviation July 2014