Decision time

The charted DA is 216 feet, but the video shows that I sank to about 180 feet before the airplane began to climb on the missed approach. That commentor’s question highlights an important distinction between DAs and minimum descent altitudes (MDA).

The key regulation that discusses DAs and MDAs is FAR 91.175 Takeoff and landing under IFR. Most of us can recite the key elements of that rule: “no pilot may operate an aircraft... below the authorized MDA or continue an approach below the authorized DA/DH unless—(1) The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers…[and] (2) The flight visibility is not less than the [charted] visibility.” You also must be able to see the so-called paint, pavement, or lights specified in paragraph (3). As is usually the case, however, understanding the practical application of a regulation requires further study.

During the approach in the video, I initiated a coupled Takeoff/Go-around (TOGA) missed approach with my Garmin GFC 600 autopilot slightly above the published DA. As the Aeronautical Information Manual explains: “DA indicates to the pilot that the published descent profile is flown to the DA (msl), where a missed approach will be initiated if visual references for landing are not established. Obstacle clearance is provided to allow a momentary descent below DA while transitioning from the final approach to the missed approach.” A close reading of the regulation also subtly implies the meaning of “decision altitude.” The rule says that you can’t “continue an approach below the authorized DA.”

Those details emphasize a key difference between a DA and an MDA. A DA is the point in space along a glideslope or glidepath where you must decide to continue to a landing or begin a missed approach. But an aircraft has momentum (mass times velocity) and can’t instantly transition from a descent into a climb. A “momentary descent below DA” is an inevitable consequence of the momentum of an aircraft tracking a glideslope or glidepath to the DA.

I’ve never seen a definitive number in FAA documents for how much dip is allowed. FAA Order 8260.3 (aka TERPS), a handbook procedure designers use when creating approaches, notes only that, “In the case of a missed approach procedure, the climbing flight path starts at the height of the minimum descent altitude or decision altitude minus height loss.”

International Civil Aviation Organization (ICAO) Document 8168, “Aircraft Operations,” includes Table III-3-4-2. Height loss margins, which lists values for different aircraft categories and when using either a radio altimeter or barometric altimeter. For category A aircraft with a barometric altimeter, the margin is 130 feet. For category B aircraft, the same buffer is 143 feet.

I suspect there isn’t an absolute value for the height loss allowed with a DA, because so much depends on the aircraft’s momentum. I checked with pilots of Boeing 747 and 777 airliners, and their operations typically include a dip of 50 feet for a miss started at 200 feet. When flying a CAT III approach to lower DAs, the wheels on those aircraft might even touch the runway as the pilots transition to a missed approach.A ‘momentary descent below DA’ is an inevitable consequence of the momentum of an aircraft tracking a glideslope or glidepath to the DA.

When you fly a 2D (nonprecision) approach, however, you can’t go below the published MDA until the runway environment is in sight. If you’re flying a 2D approach correctly, you’re at or above the MDA when you reach the visual descent point (VDP) or the missed approach point if the runway isn’t in view. If you follow +V advisory vertical guidance when flying to LNAV (2D) minimums, you must anticipate leveling at the MDA if you can’t see the runway. There’s no buffer below the MDA as when descending with approved vertical guidance along a glideslope or glidepath on a 3D (precision) approach to a DA.

The distinction between DA and MDA is also noted in the Airmen Certification Standards for the instrument rating. According to the Skills section of the Non-Precision Approach task, along the final approach segment, you must “maintain [the] MDA +100/-0 feet to the [VDP] or missed approach point (MAP).”

The Skills section of Task B: Precision Approach applies a different standard. You must “Immediately initiate the missed approach procedure when at the DA/DH [if] the required visual references for the runway are not unmistakably visible and identifiable.” Again, per the AIM and Instrument Flying Handbook, you’re allowed a brief dip as you initiate a climb. But you must “immediately” start the missed approach at the DA. No dawdling or continuing the descent hoping that dropping just a little lower will reveal the runway environment.

Bruce Williams is a CFI. Find him at youtube.com/@BruceAirFlying and bruceair.wordpress.com