The UK was hit by a pretty big revolving storm yesterday (9th Feb 2020) which posed significant challenges to the travel world and most notably aviation.
Today’s media websites are full of video clips of airliners making approaches in the strong winds, rocking and rolling a bit and often going around when the crew were not happy with everything; and it is this that is the point of this post.
Each aircraft type and airline with have clear and specific limits to be observed and providing a landing can be made within those limits and the captain is happy then a successful arrival should be assured. Making a decision to abandon an approach and landing can be made at any point up to the roll out after touchdown.
As demonstrated by a BA Boeing 777, that decision may come after touchdown and it is all perfectly safe to that providing you are within your published limits. In gusty conditions you may make it all the way to the tarmac and then a gust puts you in an uncomfortable position and off you go again for another go or a diversion depending on circumstances.
This type of manoeuvre is practiced in the simulator and a professional crew will slip into the process without having to think about it. In addition they will have briefed one another beforehand on what they intend to do and what the process will be in the vent that a go around is needed.
Aircraft types have published crosswind limits, often in the region of 30 knots (35mph) and without getting into the trigonometry involved in what percentage of a 50kt wind is a cross wind if it is at an angle to the runway; suffice to say the crew will do that and so know what wind speed is the maximum they can accept for any given runway; and Heathrow for example, the wind was close to straight down the runway, which made it more likely to get a successful landing.
It is possible to be almost there, say half mile to landing and the wind given to you by the controller will be consistently outside your limits, in which case you must go around and consider options. Will you hold if you expect and improvement or will you divert? If a diversion is the answer, then where to?
Some of these decisions will have been made before takeoff as this was a forecast event and whenever possible – subject to weight limits – the captain will carry extra fuel over and above the usual amount for that route; thus providing more options and so reducing the inconvenience of a diversion.
The amount of fuel carried for any flight is laid down in regulations and so there would normally be a good safe margin over what is required, as it is never assumed that holding or diversion will not happen; that is always provided for.
What we witnessed yesterday was professionalism from the crews involved in both departure and arrivals into the UK. Some made several approaches before making a landing at the planned destination, others diverted after one or two attempts. In part, company requirements may influence how long the captain will keep trying for as there is the bigger picture of the airline operation, location of passengers and replacement crew and so forth.
Yesterday’s operations were an interesting contrast to the performance of the crew in the Pegasus Airlines crash in Turkey only a few days before (see my post on that below). It seems that they created a fatal accident and in less demanding weather – thought is wasn’t nice – by not following the procedures I have described above. It is a cardinal rule that an approach must be stable and the aircraft settled on the correct approach slope and at the correct speed by a specific point, say by 5 miles from touchdown.
The data showing the approach of the Pegasus aircraft indicates an unstable approach. They were both above and below the correct slope at several points in the approach and, as we now know, they were too high over the threshold and landed well past a survivable touchdown point, with the result that we saw in those tragic pictures. Why not abandon the approach? It is far better to go around and come back for another go than try to push the limits.
There is a saying in the aviation world that goes something like this. “There are rules and there are laws. You may be able to bend the rules but the laws are the laws of physics and you can’t bend those.”