Let us break this down: The concept of the stabilized approach is that at a certain point, otherwise known as a “gate” or “window”, a certain set of parameters should be met. Firstly, we need to determine where the point in space will be.
IATA recommends that the point in space will occur at a certain height along the approach path. Their recommendation is that if the flight is in Instrument Meteorological Condition (IMC) 1000ft above the airfield, elevation is to be used. In Visual Meteorological Condition (VMC) 500ft above airfield elevation is to be used. Note that this is related to a guided approach either using systems like ILS, VOR, NDB or GPS. IATA further goes to recommend that unguided approaches with visual manevouring like the case of a VFR circuit, a window of 300ft above airport elevation can be used.
Now that we know at which point certain parameters are to be met, we need to determine what the parameters are. Note that these are guidelines that are published by IATA. It is therefore necessary for an operator or private pilot to established their own standard operating procedure. The below parameters are based on IATA as well as best practice for flying light piston aircraft. However, the most important and limiting factor would be to ensure that these speeds, configurations, power settings and methods of operation are directly in line with the Pilot Operating Hand book of your specific aircraft.
- Target approach speed should be attained within +5 and -0 kts. This allowance is made due to the dynamic nature of the approach. Our target approach speed is usually Vref (Vso x 1,3. Depending on operator, allowance for wind is to be made by adding half the wind gust factor, up to a maximum of 20kts. In light piston aircraft this speed is usually published in POH
- Rate of descent corresponding with the approach angle and approach speed (generally around 300–500ft/min in light piston aircraft. This is considering that you are using a 3-degree descent angle. 70Kts = 372ft/min 80kts = 425ft/min 90kts = 478ft/min
- Landing configuration of gear and flap extended in landing configuration as per POH
- Stable aircraft attitude in all 3 axes; (Wings level, Correct approach attitude and aircraft nose correctly aligned with centreline unless compensating for a crosswind.)
- Engine power stable above idle. (Example is P28A aircraft usually use 1700-1800rpm set on final approach) if major changes need to be made the aircraft is either excessively high or low.
- Landing checks completed
- The final part of the stabilized approach is the firm decision to intitiate and to complete the actions for a go-around. Notice that I mentioned the decision to initiate and not only the actioning. The reason is that a high percentage of landing accidents occur because no decision was made to go-around when the set parameters where not met. Here are some factors that could constitute a need to go-around;
- Poor manual handling of the aircraft by flight crews. For example, the pilot is unable to obtain the correct speed and flight path because they are not correctly trimming their aircraft for power and configuration changes. This will lead the aircraft to exceed one or more of the parameters mentioned above.
- Non-adherence to standard operating procedure (from the operator or what you have determined for yourself as a private pilot as your personal limits). This item is two-fold. Intentional and Unintentional.
- Example of intentional non-adherence would be a case where the flight crew/pilot might want to fly the approach faster than normal to compensate for a delay in the planned landing time. Usually the crew/pilot decides they will configure the aircraft later than usually. In poorly judged cases the crew/pilot will realise too late that the speed is too high for the necessary configuration change. For example, the speed is too high for flap extension. The crew/pilot must then wait for the speed to reduce. This can then lead to aircraft not complying with the above-mentioned parameters.
- Secondly the unintentional non-adherence is where the workload or an outside factor has distracted the crew/pilot at the time when they should have, for example, lowered the landing gear. Fatigue and heightened stress levels during the approach are major contributors to this factor.
- The local weather at the time of the approach can also be a big contributing factor. Although in VFR flying, we are not dealing with greatly reduced visibility it can still become very tricky to operate when visibility has dropped to the minimum of 5000 meters, especially in the rain. The biggest reason for this is that our aircraft are not equipped with windshield wipers. Due to our lack of visibility, it may become difficult to judge the approach accurately, leading to parameters being exceeded. The other big consideration and most common one for us flying in Cape Town, is high wind conditions. These can easily lead to the aircraft exceeding on the above-mentioned parameter.
To understand the context of the stabilized approach we need to touch on Threat and Error Management. The Threat and Error Management (TEM) model is a conceptual framework. It was developed to help understand and explain, the interaction between safety and human performance, within an operational context. TEM has three components, and you will now see how these apply to the above-mentioned items.
- Threats – generally defined by others as external events & errors that occur beyond the influence of the crew, it increases operational complexity and therefore must be managed to maintain the margins of safety.
- An example of this we can relate to in our VFR flying in the P28A, is strong and gusting winds in Cape Town. It is not within our ability to change this element. Another example can be increased pressure due to requirement from ATC to fly a short approach because of incoming traffic. Both of these reduce safety margins and require risk mitigation.
- Errors – generally defined as actions or inactions by the crew/pilot that leads to deviations from operational intentions or expectations. An example of this would be to forget to select the correct configuration for the approach/landing.
- Undesirable Aircraft State – generally defined as operational conditions where an unintended situation results in a reduction in margins of safety. An example of this, is when any one of the stabilized approach parameters are not met; the aircraft is too fast or descending too fast etc.
With this in mind, it is to be understood that the concept of the stabilized approach procedure is a tool designed to mitigate threats and errors as well as deal with undesirable aircraft states during the approach phase. Note, that when an undesirable aircraft state is left uncorrected, it can lead to further errors and eventually to an incident or accident. This brings us to the final aspect involved in the stabilized approach. The go-around. This is a critical decision/action to be taken and should be done with great care. At or after the point of the stabilized decision height, if any of the parameters are exceeded, an immediate go-around is to be executed.
The actual go-around procedure (actions performed by the pilot) to be flown, once the decision is made, should be one that is standard to your company or flight school. It should be the one that you have practiced many, many times in the past. The reason for this, is that during this type of scenario the stress level is heightened and can become overwhelming for certain individuals. It is thus best to rely on a set of motor skills that have been ingrained by consistent and professional training. Briefing the go-around procedure even if you are single pilot, can dramatically improve response time in an actual event. When flying with your instructor or another pilot it is also good airmanship to call “Go-around”, so that both crew members are acutely aware of the change of aircraft state to follow.
Part two coming soon!