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Last month, I shared the rationale for doing all non-precision approaches as a constant angle / constant rate descent also known as CANPA or CARD approach. Instead of "dive [to the MDA] and drive [to the MAP]", you would determine a visual descent point (VDP) and select a constant rate of descent from the final approach fix (FAF) to the missed approach point (MAP). CANPA just allows you to descend to the MDA or final step-down fix in a manner that you see the runway and you're in a position to continue the approach with minimal adjustments to the flight path.
Consider this approach. The final segment begins at WOLQU at or above 1700MSL which is 5.0 NM from the runway. The missed approach point is designated with an arrow at 1.2 NM from the runway. At this point, you should be at or above 460MSL and land or go missed approach.
1. Determine the altitude to be lost. It means between WOLQU and the MAP you have 3.8 NM to descend 1240 feet (1700 - 460 = 1240).
2. Determine the Feet/NM. 1240 feet / 3.8 NM = 326 Feet/NM. This means that for every nautical mile you travel past WOLQU, you’ll need to descend 326 feet in order to have a stabilized descent towards the missed approach point. The problem is that we do not think in Feet/NM nor does the aircraft have a gauge which helps you with this calculation.
3. Determine the descent rate (Feet/Minute). We need to use the “rule of 60” to determine feet per minute so we can use the Vertical Speed Indicator. This is dependent on your approach ground speed. Every unit of 60 knots can be used as a multiplier to convert Feet/NM to Feet/Minute. The table below should give you an idea how easy it is:
At 90 knots, your required descent rate would be 326 Feet/Minute multiplied by 1.5 to give you a descent rate of 489 Feet/Minute required to maintain a stabilized approach. That’s something you can see that on the VSI. This calculation, however, could add unnecessary busywork while you're trying to conduct an approach. As such, I've created downloadable CANPA descent rate tables for 90 and 105 knot approach speeds. This two approach speeds cover about 90% of the general aviation aircraft out there. If it doesn't cover your aircraft, at least you can use the formula described above to determine the appropriate numbers for your aircraft.
Notice that each table is specific to an approach speed. For calculations greater than 1000 Feet/Minute, the figures are shaded as a warning to you. Descent rates of greater than 1000 Feet/Minute while on the final segment are consider excessive and are therefore unstable. Avoid descent rates of 1000 Feet/Minute or greater past the Final Approach Fix (FAF).
Calculating a Visual Descent Point
Sometimes the missed approach point is the VOR at the center of the airport or a DME distance which marks the runway threshold. Pilots use DME and time to provide an indication that a missed approach is imminent. A visual descent point (VDP) allows the pilot to determine at a given distance from the runway threshold, a point at which the pilot can begin or continue a normal, stabilized descent. If the runway is not in sight at the VDP, the pilot has a few miles to prepare for the missed approach. The first step to computing a VDP is to divide the Height Above Touchdown (HAT) from your approach procedure by your descent gradient of 3° (300 feet/NM). The formula is:
HAT / 300 = VDP in NM from end of runway
597 feet / 300 = 2NM
In this example, 2NM from the missed approach point, a normal 3º descent should begin. If the runway is not at sight at this point, the pilot is prepared 2NM (1.5 minutes at 90 knots) prior to the missed approach point. That can help you to mentally prepare for the missed approach.
Once we start the descent at the FAF from 2000, we have 3NM to descend 760 feet (2000-1240) or 253 Feet/NM. Use the “rule of 60” to convert 253 Feet/NM into Feet/Minute. At 90 knots, you’ll descend at 380 Feet/Minute from the FAF to the VDP.
Using the CANPA descent rate table, I can round both factors down for a more conservative answer or round both factors up for a less conservative number. I get a descent rate of 362 Feet/Minute or 387 Feet/Minute respectively.
The Safest Way To Do It
Using these methods of determining Visual Descent Points and CANPA descent rates will keep you safe as you fly Instrument Approach Procedures. Use the various tools and suggestions presented in this series to reduce your workload and increase your margin of safety.
Use the safest, airline-proven flying technique for non-precision approaches that minimizes aerodynamic surprises and virtually eliminates the possibility of Controlled Flight Into Terrain.
Safer Approaches will teach you how to conduct Instrument Approach Procedures to a higher standard of safety and precision. You will learn:
- Four Fundamentals of Safe Approaches,
- How to virtually eliminate possibility of CFIT of Controlled Flight Into Terrain,
- How to perform a Constant Angle Non-Precision Approach (CANPA),
- How to calculate a Visual Descent Point (VDP),
- How to practice building your flying precision.
What's inside the package?
1. The Safer Approaches publication, 14pp.
2. The Stabilized Approach Descent Rate Table, a plastic 4" x 6" kneeboard sized IFR tool that will eliminate the mental math applying these techniques during your IFR flying.
Price: $7.95 (free shipping to US addresses)