jueves, 16 de marzo de 2023

Can I Descend?

Throughout your instrument training you practice flying instrument approach procedures to minimums from which you either see the runway and land or execute a missed approach. In reality it isn’t always that clear. When weather conditions are right at approach minimums what exactly do you plan to see?

Approach Lights

There are a variety of approach light systems that few general aviation pilots take the time to study. In inclement weather it is critical that we brief the anticipated Approach Lighting System (ALS) because it is likely the first thing we will see approaching the runway. Below is a list of the various systems found at US airports with a brief description and graphic depicting their configuration:

HIRL – High Intensity Runway Light system

MALSR – Medium intensity Approach Light System with Runway alignment indicator lights

TDZ/CL – runway Touchdown Zone and Centerline Lighting system

ALSF 1 – high intensity Approach Light System with Sequenced Flashing lights, system length 2,400 to 3,000 feet

ALSF 2 – high intensity Approach Light System with Sequenced Flashing lights and red side row lights the last 1,000 feet, system length 2,400 to 3,000 feet

SALS/SALSF – Short Approach Lighting System, high intensity (same as inner 1,500 feet of ALSF 1)

SSALF – Simplified Short Approach Lighting system with sequenced Flashing lights and runway alignment indicator lights, system length 2,400 to 3,000 feet

MALD/MASLF – Medium intensity Approach Lighting, with and without Sequenced Flashing lights, system length 1,400 feet

ODALS – OmniDirectional Approach Lighting System with sequenced flashing lights, system length 1,400 feet

RAIL – Runway Alignment Indicator Lighted sequence flashing lights (which are only installed in combination with other light systems)

REIL – Runway End Identifier Lights (threshold strobes)

LDIN – sequenced flashing LeaD-IN lights

VASI – Visual Approach Slope Indicator

PAPI – Precision Approach Path Indicator

When flying in low visibility conditions it is important you recognize the appropriate light system. Lights are usually the first visible object but often street lights, parking lot lights or building lights are mistaken for the correct approach light system. Too many accidents result from a controlled descent below MDA or DA short of the runway. A little mental preparation will ensure you identify the correct lights and land safely.

ALSF 1 and ALSF 2 systems are typically used for runways with Category II and III approach minimums. MALSR systems are much more common at GA airports served by Category I ILS and non-precision approaches. However, at uncontrolled airports only accessed by non-precision approach procedures you may only have REILs to identify the runway threshold. In low visibility conditions these lights may be the only thing you see, especially at night. At an uncontrolled airport it is critical you activate the approach lighting system when handed off to CTAF. Initially, activate the lights to their highest setting. Once visually identified you can turn down the lights as necessary to avoid blinding and disorientation.

Precision Approaches

It is a dark and stormy night. You execute a perfect approach procedure to minimums and the flicker of lights catches your eye. Can you descend? The FARs are quite clear what ALS components must be distinctly visible and identifiable in order to continue below minimums.

FAR 91.175 (b)(3) states: At least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:

(i) The approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable

(ii) The threshold

(iii) The threshold markings

(iv) The threshold lights

(v) The runway end identifier lights

(vi) The visual approach slope indicator

(vii) The touchdown zone or touchdown zone markings

(viii) The touchdown zone lights

(ix) The runway or runway markings

(x) The runway lights

At a standard decision height of 200’ AGL for a Category I approach what exactly do you expect to see? A brief review of trigonometry is in order. On a 3° glidepath at 200’ you are approximately 3,846’ from the runway touchdown zone, or 2,846’ from the runway threshold. If the reported visibility is ½ SM or 2,500’ it is clear you will see very little of the runway environment, if any. Also consider the AWOS may be located at mid-field or the far end of the runway where it could observe different visibility than you encounter at the threshold. Even a 500’ discrepancy in visibility can be very significant under these conditions.

FAR 91.175 (b)(2) provides the PIC with some discretion by specifying flight visibility must be equal or greater than that published in the procedure. In other words, you as PIC decide if you have enough visibility to land or not. Many pilots see the approach lights and continue below minimums believing the runway will be in sight shortly. A quick search of the NTSB database will provide plenty of examples. Clearly the prudent action would be to execute a missed approach if the runway environment is not in sight.

Non-Precision Approaches

What if you are flying a non-precision approach? Most require at least 1 SM of visibility so it should be a piece of cake, right? Instead of a Decision Height you are now plodding along at a Minimum Descent Altitude peering through the rain for the runway. Let’s say you see the runway 1 SM from the threshold at 600’ AGL. You now have to fly a 6° glidepath to the touchdown point!

What do the FARs say about this one? 91.175 (b)(1) states: “The aircraft is continuously in a position from which a descent can be made at a normal rate of descent using normal maneuvers.” In my opinion diving towards a runway, especially if contaminated, isn’t a normal maneuver. Luckily the regulation is written loosely enough so that the PIC can decide if the landing can be safely made with adequate runway length.

Better yet, why don’t we determine exactly how far from the threshold we should descend for a normal landing and make this our ‘mental’ missed approach point! This point is actually calculated and increasingly published on approach procedures. The Visual Descent Point (VDP) is a fix from where a descent can be made for a normal approach to landing. The Missed Approach Point (MAP) is still likely the runway threshold or navigation aid so care must be taken not to confuse the two. However, flight at MDA beyond the VDP will likely result in a missed approach even if the runway environment does become visible prior to the MAP. If a published VDP is not available you can easily create one yourself with some simple mental math.Locate the height above touchdown (HAT) next to the MDA.

Divide the HAT by 300 (assuming a 3° glidepath).
Add this distance from the end of the runway to determine the VDP.

For example, if the HAT is 807’ then the VDP is located 2.7 miles from the runway. On a GPS approach with the MAP at the runway, 2.7 miles from the MAP should put you on a normal glidepath for landing. If using a VOR located halfway down a 5000’ runway, add .5 miles to obtain a VDP of 2.6 DME. You will enjoy many more birthdays by taking a few seconds to consider a VDP during the approach briefing.

Circling Approaches

One of the most hazardous maneuvers in instrument flying is the circle to land approach. This maneuver is so dangerous that most Part 121 operators are specifically prohibited from even attempting the maneuver. Certainly there are some instances where it is perfectly reasonable. One instance may be an approach procedure that efficiently transitions from the enroute phase to the airport instead of flying past the airport 15 miles to get established on the straight-in approach. This is only recommended if you have sufficient ceiling and visibility to fly a normal VFR traffic pattern. If you must fly a circling approach to minimums keep the following in mind:Read the notes section for any restrictions on the direction of the circle. Terrain and obstructions on one side of the airport will certainly ruin your day.

Be very careful to maintain your circling MDA throughout the maneuvers. The MDA only guarantees 300’ of obstacle clearance which means you are likely flying a lower than standard pattern altitude.

If possible, fly a left pattern. You will have much better a view off your side of the airplane to maintain visual contact with the runway at all times.

Use all available resources. Brief your passenger to watch your altitude throughout the circle and alert you if you descend while looking outside.

Consider landing with a light tailwind. As PIC you must balance the risk of the circling maneuver or landing with the wind. If the wind is less than 5 knots and you have sufficient runway length landing straight in may be more prudent.

Be prepared to immediately execute a missed approach procedure if visual reference is lost. Enter a climbing turn towards the missed approach point before continuing on the missed approach course. This is intended to keep you above the airport where there are no obstacles until a sufficient climb gradient is established.


A successful outcome to every flight depends on proper planning and preparation. Know your personal minimums and have an alternate plan ready. Remember – There is no where you need to be that is worth your life!

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