Stabilized Approach

“One thousand, stable” - Student descending at 110 kts and 2,000 FPM

Approach to KATL in a Cirrus SR20

In flight training, we often call for “stabilized approaches”. This term came to be with a rising amount of accidents occurring during landing. The General Aviation Joint Steering Committee’s (GAJSC) Loss of Control Group identified causal factors for accidents on landing:

• Un-stabilized approaches

• Inappropriate go-around procedures

Historically, student pilots are introduced to the concept of “stabilized approach” during instrument training. CFIs often forget, however, that a stabilized approach is both a VFR and IFR thing. It matters all the time!

Let’s get right to the core definition, and then consider other factors that affect the safety of a landing.

Definition

Per the FAA, A stabilized approach is one in which the pilot establishes and maintains a constant angle glidepath towards a predetermined point on the landing runway.

Recommended Characteristics of a Stabilized Approach

The Flight Safety Foundation has done a lot of work to provide us with tangible guidance as to “how” a stabilized approach should be conducted. Their guidance is reported below. This is verbatim from their document (Flight Safety Foundation Approach-and-landing Accident Reduction (ALAR) Task Force, v1.1 November 2000).

All flights must be stabilized by 1,000 ft above airport elevation in instrument meteorological conditions (IMC) and by 500 ft in visual meteorological conditions (VMC).

An approach is stabilized when all of the following criteria are met:

1. The aircraft is on the correct flight path;

2. Only small changes in heading/pitch are required to maintain the correct flight path;

3. The aircraft speed is not more than Vref+20 kts indicated airspeed and not less than Vref;

4. The aircraft is in the correct landing configuration;

5. Sink rate is no greater than 1,000 ft/min; if an approach requires a sink rate greater than 1,000 ft/min, a special briefing should be conducted;

6. Power wetting is appropriate for the aircraft configuration and is not below the minimum power for approach as defined by the aircraft operating manual;

7. All briefings and checklists have been conducted;

8. Specific types of approaches are stabilized if they also fulfill the following: instrument landing system (ILS) approaches use be flown within one dot of the glide slope and localizer; a Category II or Category III ILS approach must be flown within the expanded localizer brand; during a circling approach, wings should be level on final when the aircraft reaches 300 ft above airport elevation;

9. Unique approach procedures or abnormal conditions requiring a deviation from the above elements of a stabilized approach require a special briefing.

An approach that becomes unstabilized below 1,000 ft above airport elevation in IMC or below 500 ft above airport elevation in VMC requires an immediate go-around.

Quick Reference Summary

The above guidance is phenomenally good to provide a big picture of what an approach should look like. We can go further and use other FAA guidance. An approach is stabilized when the following conditions are met:

1. Aircraft is on the correct flight path with minor control inputs

2. Airspeed is +10/-5 kts of chosen approach speed

3. On glide slope or VASI or appropriate glide path

4. Constant rate of descent not to exceed 1,000 FPM

5. Aircraft is in landing configuration, depending on type

Reference Speed and Approach Speed

How fast you descent to the runway is rather critical. Under VFR, we will be on the back side of the power curve, so in a descending slow flight! Remember, in this stage of the power curve we invert our controls for speed and altitude control: pitch for airspeed, power for altitude.

Under IFR, however, we stay on the positive side of the power curve to be able to input appropriate controls with effective aircraft reaction. Essentially, we don’t want to get sloppy in slow flight on final descent when trying to fly an ILS.

Approach speeds are based on a very critical engineering speed: “reference speed” or Vref. That is Stall speed (for landing configuration) multiplied by 1.3.

For a Piper Cherokee 180 (PA28-180), stall speed with flaps down is 49.5 kts, which we’ll round off to 50 kts (that’s from the POH on a 1969 model). Therefore, the reference speed, which is your final approach speed, is 50*1.3 = 65 knots. You should consider this the target speed when crossing the runway threshold as you descent to begin the flare.

Approach speeds, “what you fly on final”, should be adapted for wind conditions at the field. The rule of thumb, used by many operators worldwide, is to add the following speeds to your reference speed (not to exceed Vref+20). This is where the Vref+20 mentioned by the Flight Safety Foundation comes from. Airplanes provide performance data only up to Vref+20. Anything above that is outside the published certification data of the manufacturer... Don’t go there):

1. Half of the steady state wind (above 5 knots) e.g., winds are 10 knots, so add 5

2. The entire gust factor e.g., wind gusts are at 15 knots, so add 5 (gust-steady state wind= 15-10=5).

In the above example, the new approach speed would be Vref+10 which is 75 kts.

As a  sanity factor, if there are no winds, I will still ad 5 knots to my approach speed. So, at a minimum, I will fly at 70 kts.

Let's clarify one thing. The airplane's POH may provide you speeds to fly. Cirrus is great at that and provides you with both approach speeds and threshold crossing speeds. In that case, ignore the 1.3Vs0 and fly what is published. Other general aviation airplanes may not provide guidance at all, and that's where you can use these rules of thumb.

Let's clarify a second thing. What I call Vref in this article is "threshold crossing speed". I then add at least +5 for my approach speeds (or the various additives for winds). Approach speed is what I fly on final. So in the case of a brand new PA28-180, I will descend on final at Vref+5, which is 70 knots, and then slow down on short final to cross the threshold at 65 knots (Vref or 1.3Vs0). As always, please know your airplane. An old airplane with a lot of paintwork may have slightly different aerodynamcis compared to a brand new one. So understand how your airplane flies best and modify speeds as needed.

Category Speeds under IFR

The calculation of approach speed is indeed applicable to both VFR and IFR. However, we need to remember that for our instrument-rated friends, instrument approach procedures (IAPs) provide minimums based on aircraft categories. These relate to the threshold crossing speed.

• Category A: <91 kts

• Category B: 91-120 kts

• Category C: 121-140 kts

• Category D: 141-165 kts

These categories are actually based on Vref, which is an engineering calculated value. Therefore, aircraft certification plays a role in these values. In essence, it means that although you can fly “faster” than your airplane is designed for, you cannot fly “slower”. So, as an IFR pilot, you can fly a higher category minimum, but not a lower category minimum (this question actually came up during my IFR checkride in Kansas).