“Stabilized Approach” is a term that most pilots have heard but most likely have not thoroughly reviewed. The term itself conveys the idea that stable is preferable to unstable. This seems OK if for no other reason than “unstable,” intuitively, is not a condition that we want to associate with an airplane in any phase of flight—particularly the phases closest to the ground.
While we might agree on that assessment and simply end the discussion here, in the interest of elevating aviation safety, taking a few minutes to review the term is a better idea.
A look at some Stabilized Approach background tells us that the genesis of the formalization of the concept is rooted in an effort to reduce the number of Controlled Flight Into Terrain (CFIT) accidents. To that end, the Flight Safety Foundation established a task force to study the topic. Much of the information in this article is included in the ALAR (Approach-and-Landing Accident Reduction) Tool Kit published in 2000 by the Flight Safety Foundation.
Information developed by the task force reveals that unstabilized approaches were a causal factor in 66 percent of 76 approach-and-landing accidents and serious incidents worldwide in 1984 through 1997.
Accidents with the aircraft in a low-energy condition (low/slow) tended to be characterized by loss of control or CFIT. Those accidents with the aircraft in a high-energy condition tended to be runway overruns and runway excursions. Included are some accidents due to loss of control and some CFIT due to inadequate situational awareness.
The task force also found that flight handling difficulties occurred in situations that included rushing approaches, attempts to comply with demanding ATC clearances, adverse wind conditions, and improper use of automation.
Those conclusions alone imply what defines a stabilized approach, but let’s look at what has been developed as the accepted definition.
[Note: The task force research focused on aircraft certified under Part 25. The concept, however, is no less valid for Part 23 aircraft including Turbo Commanders.]
Straight from the ALAR Tool Kit are the RECOMMENDED ELEMENTS OF a STABILIZED APPROACH:
All flights must be stabilized by 1,000 feet above airport elevation in instrument meteorological (IMC) conditions, and by 500 feet above airport elevation in visual meteorological conditions (VMC). An approach is stabilized when all of the following conditions are met:
- The aircraft is on the correct flight path;
- Only small changes in heading/pitch are required to maintain the correct flight path;
- The aircraft speed is not more than VREF +20 knots indicated airspeed and not less than VREF;
- The aircraft is in the correct landing configuration;
- Sink rate is no greater than 1,000 feet per minute; if an approach requires a sink rate greater than 1,000 feet per minute, a special briefing should be conducted;
- Power setting is appropriate for the aircraft configuration and is not below the minimum power for approach as defined by the aircraft operating manual;
- All briefings and checklists have been conducted;
- Specific types of approaches are stabilized if they also fulfill the following: instrument landing (ILS) approaches must be flown within one dot of the glideslope and localizer; a Category II or Category III ILS approach must be flown within the expanded localizer band; during a circling approach, wings should be level on final when the aircraft reaches 300 feet above airport elevation; and
- 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 feet above the airport elevation in IMC or below 500 feet above airport elevation in VMC requires an immediate go-around.
What is defined, more generally, is an aircraft going the correct direction, configured to land, flying at a speed that results in a descent rate that will allow transition to the landing phase, and landing and stopping on the desired runway.
That seems obvious enough, but the statistics indicate that, in practice, too many pilots do not routinely adhere to the formal elements of the stabilized approach or even the intuitive picture of an airplane that is “stable” during in the approach profile.
Among the SIMCOM Dallas instructor group in a feedback session discussing the development of this article, one near-unanimous observation was that one of the most often-observed pilot habits in simulator sessions is excess speed crossing the threshold. A second observation was that there is a tendency for pilots to be behind the approach.
As excess speed (a high-energy condition) crops up as a possible causal factor in some of the accidents that were examined, it will be the focal point of this article. In subsequent articles, we will delve into the other elements that comprise and compromise the Stabilized Approach definition.
Item 3 in the Stabilized Approach list of elements defines the acceptable speed range as VREF to VREF +20 knots. Part 23 category aircraft operating manuals do not publish VREF speeds, but you can calculate your own by borrowing some information from the engineers, Commander test pilots, and the FAA.
VREF is defined as the LANDING THRESHOLD CROSSING speed for aircraft certified under Part 25. The substitute for Part 23 certified aircraft is 1.3 VSO. (That’s in title 14 of the United States Code of Federal Regulations, otherwise known as the Federal Aviation Regulations or FARs.) If you check the LANDING DISTANCE charts in the performance section (Section 5) of your Turbo Commander POH, “SCHEDULED SPEEDS” are shown in a small box above the chart. Those Scheduled Speeds for a 695A are listed below and look very much like 1.3 VSO. The 1.3 VSO column was added to the chart below to save you the trouble of doing the calculations. For the sake of expedition, we will refer to the SCHEDULED SPEEDS as VREF.
|WEIGHT (lbs)||50 FT HT SPEED (KIAS||1.3 VSO|
[For the purpose of calculating 1.3 VSO, stall speeds were charted using the FLIGHT IDLE INDICATED STALL BUFFET SPEEDS chart in Section 5 of a 695A Pilot Operating Handbook.]
The Flight Safety Foundation concluded that there were “some” flight handling difficulties associated with “rushing approaches” and “attempts to comply with demanding ATC clearances.” Is it possible that what the SIMCOM instructors see as “being behind the approach” might be what the Flight Safety Foundation is describing as “rushing approaches?” Is it possible that complying with a demanding ATC clearance might be more manageable at VREF +20 that at some higher airspeed?
Element 7 of a Stabilized Approach is having all briefings and checklists conducted. Might a slower speed allow a busy single pilot just enough extra time to complete those tasks?
The questions are rhetorical, of course. Excess speed crossing the threshold presents obvious possible problems. Excess speed earlier in the approach environment may result in much more subtle outcomes that are equally dangerous.
You get the concept: Less speed translates into more time while covering a given distance. Becoming comfortable with VREF +20 to VREF is a cornerstone of being able to consistently execute low-stress approaches and landings.
If you are not comfortable flying down a glide path at VREF +20 and slowing to VREF as you cross the runway threshold, consider taking the first steps to improving your airmanship and confidence at these speeds.
If you are due or near due for recurrent training, we will be happy to have you spend two or three days with us at SIMCOM Dallas. If you prefer to pass on recurrent training but wish to utilize a simulator to practice constant-speed descents at VREF and work on cockpit procedures in the approach environment, we invite you to call and book “wet sim” time. You will get the sim and an instructor ready to focus on what you want to accomplish. If you prefer to practice in your aircraft, we will be happy to accommodate.
Two additional notes:
When does a Stabilized approach begin? The definition is clear in its statement referencing 1000 feet above airport elevation in IMC and 500 feet above airport elevation in VMC.
More practically, managing the aircraft to meet the conditions of a Stabilized Approach well before reaching either the IMC or VMC altitude limit requires attention well in advance of reaching the “ceiling” of a Stabilized Approach. One-thousand feet above airport elevation is approximately two minutes to touchdown. So perhaps the more useful question is, When does YOUR Stabilized Approach begin?
When is the Stabilized Approach complete?
It is a splitting hairs a bit, but the answer is 50 feet above the landing threshold. That factoid is somewhere in the certification standards and, of course, is implied in the landing distance charts that begin at 50 feet.
We will take a closer look at the approach and landing segments in subsequent articles. We will examine all nine elements of a Stabilized Approach and the impact on the flight profile leading us to 1000 feet (IMC) or 500 feet (VMC) above the airport elevation.
In the meantime, you can begin to add the Stabilized Approach elements in your normal flying. Make a copy of the list of elements and integrate them into your approach self-briefing.
If you can’t fit in training time for the speed control work, practice on your own. The next time you fly, take a very small step and reduce your “normal” approach mode power by 50 hp or 50 ft-lbs of torque. Note the airspeed and how the airplane “feels.” Then work from there.
Adopting the Stabilized Approach concept is a process, not an event! It involves developing a set of cockpit procedures that are most likely a variation on your current procedures rather than ones that are new. Adjusting speed to VREF can be done in small steps that you take literally on every flight.
Operating under Part 91, making the Stabilized Approach your personal “SOP” is purely optional. Doing so and developing the cockpit routine needed to implement the concept, then maintaining the proficiency to execute consistently, will add an undeniable measure of safety.
Until next month, SIMCOM Dallas wishes you blue skies and reminds you to not be shy about going around when an approach is something other than routine! (When did you last practice a go-around?)
SimCom Aviaiton Training’s Dallas campus is the exclusive provider of simulator-based Twin Commander pilot training. For more see http://www.simulator.com/turboprop/twin-commander.