How Low Can I Go?

By Franklin Li
Reprinted with permission from FAA Aviation News

You are in a 1977 Cessna 172N taking your parents to a family reunion at your uncle's place in Coeur D'Alene, Idaho. You are a private pilot who recently received your instrument rating at a local FBO in Salem, Oregon, and have logged fifty hours of simulated instrument time, none in actual conditions.

Then you hear over the radio:

Cessna 1-2-3-4-5, eight miles from LEENY, cleared NDB runway five approach. Maintain maximum forward speed to help me sequence a Gulfstream.

Looking at the approach plate, you notice there are mountains around the airport with little room for error. The AWOS at the airport reports a 1,000-foot ceiling and two-miles visibility making this your first experience in actual instrument meteorological conditions. You decide you can safely keep your speed up at around 100 knots to help air traffic. As you approach your final approach fix, you decide to review your approach minimums. You ask yourself, 'How low can I go?'

How do you determine your minimums? The published minimum descent altitude (MDA) for the non-precision NDB Runway 5 approach is 3,000 feet and 3/4-mile visibility for a Category A aircraft like you are flying. The circling minimums to Runway 5 are 3,000 and one-mile visibility if you had to circle to land.

But since you are flying the approach at 100 knots, which MDA listed should you use? Do you use the published minimums for your Category A aircraft or those for the next higher Category B aircraft since you are flying above the 91 knots threshold listed for approach Category B aircraft in the U.S. Terminal Procedures publication. How do you determine if you are to use Category A or Category B approach minimums? The difference is important. In this example using the Coeur D'Alene NDB or GPS RWY 5 procedure, the MDA for Category A aircraft is 3,000 feet and 3,100 feet for Category B aircraft. Your airplane is a Category A aircraft, but you are flying the approach in excess of 90 knots which is the upper limit for Category A aircraft. The difference between Category A and Category B minimums is 100 feet for this particular approach. Which MDA do you use for this approach? The closest FAA reference to help you decide is in the U.S. Terminal Procedures publication, which states in part on page A-2:

An aircraft shall fit in only one category. If it is necessary to maneuver at speeds in excess of the upper limit of a speed range for a category, the minimums for the next higher category should be used. For example, an aircraft which falls in Category A, but is circling to land at a speed in excess of 91 knots, should use the approach Category B minimums when circling to land.

Based upon this recommendation, since you are flying the approach at 100 knots, you should use the approach Category B minimums. But is it mandatory for you to fly the approach using the Category B minimums? That is the question.

Before continuing any further, I want to clarify several myths about instrument approaches.

First, does maneuver refer to only circling approaches? No. A maneuver is not limited to turns. A descent is a maneuver as well. Also remember, an approach, such as a non-precision VOR approach to a runway, can be offset from the straight in approach course and still be considered a straight in approach. In the case of an NDB approach away from the NDB toward the airport, the NDB approach will take you to the general area of the airport rather than a particular runway. You may have to maneuver to your desired runway once you have the airport in sight.

Secondly, what is the significance of 1.3 V_so? The airspeed of 1.3 V_so is not the recommended speed to fly an instrument approach. To determine your aircraft's recommended airspeed, refer to your aircraft's owner's manual. It should also be noted that 1.3 V_so is the minimum speed that should be used when flying an instrument approach. In everyday operations, however, an aircraft may fly an instrument approach at faster speeds due to other traffic or weather conditions.

When reading the procedures more closely, it states, 'the minimums for the next higher category should be used.' When referring to Title 14 Code of Federal Regulations (14 CFR) '1.3 on the discussion of imperative and permissive verbs, 'should' is not defined. Therefore, hypothetically, as long as the aircraft is a Category A airplane, you can push all 160 horses in the Cessna and fly a circling to land approach at 145 knots to Category A minimums although you should be using Category D minimums. Furthermore, when someone approaches you and asks why were you flying at the lower minimums, you can pull out the paragraph and tell them it is not mandatory that you fly the approach at the respective minimums when flying in Category B speeds, but try telling your passengers it is okay to use the lower minimums while possibly risking their safety by flying at the lower minimums.

The reason an aircraft flying at higher speeds should use the higher minimums is simple: obstruction clearance. The approach minimums were based on the speed of the aircraft. When operating at higher speeds, the operation needs a larger area for obstruction clearance due to the aircraft's increased turning radius; and the altitudes at the lower category may not provide the necessary wider area clearance. Furthermore, at faster speeds, the pilot has less time to react. Higher visibility and altitude minimums are necessary to allow the faster-flying pilot to have more time to see the airport, maneuver the aircraft, and configure the aircraft for landing or a missed approach as appropriate.

Also pilots need to consider the design errors allowed in various instruments. For example, an altimeter can be up to 75 feet off; your VOR receiver may be up to several degrees off course; the heading indicator may not be set to the current compass heading or it could be set a degree or two off; and your timing for this non-precision NDB approach may be a little off. Add in the need for correcting for some gusty winds in mountainous terrain, and you can begin to see the importance of using the approach minimums for the speed you are flying rather than the lower minimums for your specific aircraft if you are a Category A aircraft flying at Category B airspeeds.

So, even though the U.S. Terminal Procedures use the word 'should' rather than 'must' or 'shall' when referring to aircraft operating at speeds above their respective categories, based upon all of the safety considerations involved in the design of an approved terminal procedure, it is definitely recommended when you operate at speeds in excess of your aircraft's category that you use the higher minimums for those airspeeds.

As our new instrument rated pilot may have discovered, instrument meteorological conditions can increase your workload if you are not prepared, so you should plan on giving yourself the extra room and protection the increased minimums give you when operating at the higher airspeeds than those published for your respective aircraft's category. Just remember, the next time you fly an instrument approach, mountains and other obstructions do not care about your aircraft's category. The important thing is that you miss them when flying at the appropriate minimums for your approach speed.

Franklin Li is a student at the University of North Dakota. He was a summer intern with Flight Standards Services General Aviation and Commercial Division. Frank met FAA Administrator Marion Blakey at Ronald Reagan Washington National Airport during the first official appearance of FAA's last DC3, which is the centerpiece of FAA's Centennial of Flight display.