I’m sure you heard this term (or its opposite) many times, but unstable approaches became more and more popular in 2020, leading to an overall decrease in safety when it comes to the final stages of the flight.
To see how worrying the situation regarding unstable approaches looks like, the International Federation of Air Line Pilots’ Associations (IFALPA) decided to release a Safety Bulletin showing the increasing trend of unstable approaches during “Reduced Operations“.
The bulletin is based upon datas collected by IATA’s Flight Data Exchange (FDX) database, showing how, back in May, the we had between 35 and 40 unstable approaches each 1000 sectors.
What is a stable approach?
In a few words, a stable approach is what every pilot aims to execute, keeping a smooth and controlled flight path right until the wheel touch the runway, but that’s not always the case, and there are some conditions to be met in order to define “stable” an approach:
- Target approach speed a few knots faster than the desired touchdown speed and on the ‘right’ side of the total drag curve (corrected for wind if necessary);
- Rate of descent commensurate with the approach angle and approach speed (generally around 600–700 feet per minute for jet aircraft on a 3 ̊ approach);
- Landing configuration of gear and flap extended;
- Stable aircraft attitude in all 3 axes;
- Engine thrust stable above idle.
An approach can be too fast in its initial stages, sometimes not because it’s wrongly executed, but because ATC may push for high-speed approach to increase the number of movements, especially in major airports.
Thus, those conditions, published in this document by IATA, are to be respected below a certain altitude which can be defined by the company’s SOPs between 500 feet and 1500 feet.
If, below that “gate” the approach is not stable, the pilot must execute a go-around or whichever procedure prescribed by the operating manual.
Usually the SOPs are more restrictive than the general documents and rules, trying to increase the safety margins.
If you ever heard of the “Swiss cheese model“, the restrictions applied by operators aim to decrease the likelihood of accident happening from an error.
Is it possible to recover an unstable approach?
Technically, if all the procedures are followed correctly, the pilots always recover an unstable approach, either correcting it above the gate altitude or executing a missed approach.
The unstable approach, formally speaking, can be defined as “undesired aircraft state” (UAS), which happens if an error has been committed and its more direct consequences ignored or not corrected, and although it can lead to an accident, an UAS is recoverable.
A bad approach, even if it’s immediate to think the opposite, doesn’t even come out from a human error.
A windshear, a sleepy air traffic controller, a busy environment can easily mess up a perfect approach, even below the gate.
As always, it’s not a matter of who/what caused it, but how to ensure that, even in this situation, safety is still guaranteed.
That’s why aviation has so many rules, restrictions and safety measures.
No one could blame a pilot for flying an unstable approach, as long as he/she corrects it or does whatever its necessary to ensure that a low unstable approach ends in a crash landing.
The possible consequences of an unstable approach
Let’s pretend that an unstable approach is continued ignoring any procedure, and the airplane is about to touch the ground.
The consequence of this approach, overall, are bad. But what happens depends on what’s the aircraft state: being too fast is different than being too high, or being misaligned with the runway, but the aircraft might also end up fast, high and off-track, all together (a bit too extreme for any pilot, honestly).
Landing an aircraft after this kind of path, leads often to incidents or accidents. If the speed is too high there is a major risk of over-stressing the landing gear that could fail, being too high can lead to a runway overshoot in the attempt to come to a stop over a short portion of the runway.
Anyway, there are cases in which somehow the aircraft, following a bad approach, successfully lands and finishes its sector in a “normal” way… For example, after being too high on the approach, there’s still enough runway length to execute a safe landing.
That doesn’t change anything. An unstable approach remains a potentially dangerous state, independently on how it ends.
The Air India A320
An interesting example of how lucky one can be when deciding to land after a wrong approach happened on the 4th October of 2017, and the final report has just been published.
According to FlightGlobal, an Air India A320 was left with “just 600 meters” to slow down after touchdown, and this happened due to a cocktail of violations and unfavorable circumstances that you should be surprised by the fact that the aircraft and its occupants didn’t suffer any damage or injury.
Basically the first unfavorable circumstance was the unavailability of the ILS approach, but it must be said that every pilot is trained to execute non-precision approaches without vertical guidance, and it’s not that bad especially in good weather conditions as the ones present at the time of occurrence.
On the localizer approach, the aircraft was “continuously high and fast“.
The crew was supposedly trying to correct the altitude by flying an approach with a 1000 feet per minute rate of descent, but that causes the speed to increase, and the A320 ended up being more than 200 feet too high, with its speed exceeding the target as well.
Most of pilots would just advance the thrust levers and fly back for another attempt, but even below 500 feet (the ultimate possible gate), the crew decided to continue.
Due to speed and height, the airplane touched down 1300 meters past the threshold in a runway which feature a Landing Distance Available of 1905 meters.
In all that chaos, the occurrence hasn’t been reported by the crew, therefore no cockpit registrations were available to the competent authority.
When something like this happens, the crew is ignoring any barrier designed and implemented to avoid unpleasant events.
It’s true that the aircraft was affected by “variable tailwind components“, but no circumstance should allow the pilots to go over the standard procedures.