08/24/2012

easyJet takeoff trajectory: two conflicting stories

I recently wrote that an easyJet Switzerland flight did not appear to follow the correct flight path. The airport begs to differ!


The path that the easyJet Switzerland aircraft took when it left (empty) on 9 August at about 10am was, according to the GPS (latitude and longitude) positions broadcast by its transceiver, not compatible with the official Swiss requirements for the particular flight path which was assigned to it. I therefore wrote a blog about it, as well as informing the Geneva airport authorities. The technical details of the data on which my blog was based are given at the end of this blog.

I have now had the reply from the airport, which states that after verification, this flight conformed to the instructions received from Geneva Air Traffic Control. This reply therefore states that what I wrote was not true, and thus casts aspersions on easyJet Switzerland, and hence on Geneva Airport. I am asked to withdraw this previous blog.

So who is correct? The letter from the airport does not give any evidence to contradict what I wrote, other than a disagreement on altitudes. It does not specify at what exact place the aircraft started to turn left, and is not accompanied by a trajectory radar plot showing their version of the trajectory. Unless such a trajectory can be shown to me, plus (if it differs significantly from that indicated by the aircraft's transponder messages) a precise explanation from easyJet as to why the position coordinates (which were extremely accurate just prior to the takeoff) were suddenly unreliable, I see nothing to cause me to withdraw the previous blog.

The subject is important for many people living just off the axis of the runway, since aircraft which can turn very soon after takeoff then cause noise in regions which are not accustomed to such noise. This is particularly important for aircraft taking off over Vernier and then very quickly turning 180° to the right: a trajectory particularly disagreeable for Satigny and neighbouring France. It is also annoying when, as in this case, aircraft take off over Genthod and then turn 180° left over Versoix, Mies, Tannay or Coppet.

A correspondent wrote that when constructing Standard Instrument Departure (SID) routes, everything is done to minimise noise. However, the two departure routes mentioned above specify that before any turn the aircraft must have attained a certain altitude and gone past a specific point. In practice, when these SIDs were constructed, the performance of aircraft very probably meant that the aircraft were already way beyond the specific point before reaching the required altitude. Thus, for a takeoff over Genthod, the aircraft were well over the middle of the lake before turning. Today, with the improved jet engines, the greater number of flights (to the Iberian Peninsula) and the desire to minimise flight times and fuel usage, the aircraft turn much sooner, thus crossing the Mies/Tannay/Coppet regions at a lower altitude.

It is thus arguable that the SIDs should be revised to oblige departing aircraft to travel further in a straight line (which should allow them to be quieter!) before being permitted to turn.

Now, for the technically-minded, the method that I used to establish the trajectory.

I work on the transponder messages, which include in them the date, time, altitude above sea level, GPS position, speed and compass heading of the aircraft. These show me when the aircraft ceased to fly (more or less) in the runway direction. A selection of these, beginning with the position at the start of runway 05, the takeoff moment and the coordinates where the aircraft started to turn are as follows :-

2012/08/09,10:29:28,EZS9003,0,46.22612,6.09140,4.5,36.6                    Turning onto the start of runway 05 (altitude 0 implies on the ground)
2012/08/09,10:30:07,EZS9003,1225,46.23238,6.10066,130.0,45.0           In the air (just) at a speed of 130 knots, heading 45°
2012/08/09,10:31:24,EZS9003,4900,46.27071,6.15643,136.5,44.1           Up to 4900 feet, still parallel to the runway (44.1°)
2012/08/09,10:31:27,EZS9003,4975,46.27209,6.15784,138.2,35.9           Has now turned 10° left.
2012/08/09,10:31:29,EZS9003,5050,46.27367,6.15887,145.8,26.9           Has now turned 18° left
2012/08/09,10:31:31,EZS9003,5125,46.27505,6.15944,139.2,17.1           Has now turned 28° left

20120809_EZS9003_dec2.jpgAccordingly, the ground message just prior to the start of the takeoff had altitude 0, speed 4.5 knots in the direction 36.6°. You can check this position with Google Earth!
The first airborne message was at 1225 feet altitude, 130 knots exactly on the runway axis 45°.
The first significant heading change of direction was at 4975 feet, 138.2 knots and 35.9° heading. This, and the two following positions (heading 26.9° and 17.1°) are the blue aircraft on my plot. These positions are well short of Versoix Mairie, which is 2 nautical miles from GVA.

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Comments

Monsieur Gerard,
La direction de l'AIG vous confirme que le virage a été exécuté suite à un ordre de Skyguide. (je le précise car on pourrait croire que vous reprochez une faute au pilote) Or vous le savez un ordre d'un contrôleur aérien prime sur les standards aéronautiques, il n'y a donc pas violation des règles.

Sur le fond, dans votre premier article, sauf erreur vu que l'anglais n'est pas ma langue maternelle, vous parlez d'angle de montée de l'avion, où avec les anciens avions pour atteindre les 5000 pieds/sol il fallait arriver sur le lac, alors que maintenant cette altitude est atteinte plus rapidement.

On peut se poser la question s'il est préférable d'avoir un avion bas (et donc plus bruyant) qui survole uniquement Versoix-le-Bourg ou un avion plus haut (donc moins bruyant) qui survole plus d'habitants ?

Actuellement, l'idée dans la plupart des aéroports est de monter le plus rapidement possible (ce qui a un coût économique et polluant en terme de CO2) pour être le plus rapidement hors "de la gêne auditive" des riverains.

Vaste débat.

Posted by: Philippe Calame | 08/24/2012

L'aéroport n'a pas encore fourni l'information quant à la trajectoire de l'avion, ni de l'endroit exact auquel l'avion a commencé le virage à gauche. Il y a aussi un écart de 1500 pieds entre les deux chiffres de l'altitude au moment du virage: c'est beaucoup. Si le pilote a été autorisé par Skyguide, ou a reçu l'ordre de Skyguide, pour faire un virage qui n'est pas compatible avec l'ordonnance Suisse AIP, alors on peut se poser des questions sur les raisons invoquées par Skyguide et l'utilité des ordonnances.
En ce qui concerne l'angle de montée, il serait bien d'avoir une discussion ouverte, y compris les autorités des communes survolées par l'aviation, et de faire des simulations (par l'EMPA) mais je continue à penser qu'un décollage en douceur sur le lac pourrait être la meilleure solution (sauf pour les poissons et les pécheurs!). Je pense que beaucoup de riverains habitant Versoix, Mies, Tannay et Coppet seraient d'accord.

Posted by: Mike Gerard | 08/24/2012

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