«By eye, the passengers, those who were not sleeping, because it was also night, felt more the large turn to reverse the course than the descent which really took place at a very “gentle” rate», says Gianni Lanza, 60 years old, Roman, former commander on the Airbus 320 and flight instructor. That United Airlines Boeing 777-200, headed from Newark to Fiumicino, dropped from about 37 thousand feet (11 thousand meters) to almost 9 thousand feet (2,700 meters) in 8 minutes according to some versions, in 10 according to others. «Let’s also consider 8 minutes: it’s just over 3,500 feet per minute, a slight descent which indicates that everything on the aircraft was under control apart from that alleged malfunction of the cabin pressurization system».
Light?
«Let’s say a long glide with throttles at minimum and airbrakes out. For the descent to become a little steeper you have to lose at least 5 or 6 thousand feet per minute and still panic certainly doesn’t break out among the passengers. But in an emergency that model can accelerate even much more without problems. Well, if anything, those who were awake will have wondered why after that turn the plane descended instead of leveling off at cruising altitude at around 12 kilometers above sea level. But at this point the captain had certainly already notified the passengers of the change of plan. You can imagine their disappointment.”
Emergency masks did not arrive on flight UA510, at least that is what the company declared without being contradicted by the passengers.
«A sign that, despite not working properly, the pressurization system maintained an acceptable pressure in the cabin by continuing to pump the air sucked in by the engines. The pilot has indicators that tell him the situation perfectly and so he decided that as a precaution it was better to interrupt the flight across the Atlantic and return to Newark.”
However, was it essential to lose altitude and perhaps get rid of some fuel since the tanks were still almost full after 49 minutes of flight?
«Yes, it is possible to hypothesize that one of the valves in the system was not working well. It is clear that at 11 thousand meters above sea level one cannot survive without the pressurization system which maintains a situation in the cabin similar to that of 3 thousand meters above sea level. As you go up the valves close more and more, while the system forces air to enter. Downhill the opposite happens: the valves open more and more while it is less and less necessary to pump air.”
What if this system malfunctions?
«If the fault is really serious and there is a sudden loss of pressurization, the masks come down automatically. Action that can also be controlled by the pilot. From that moment on, the autonomy of the system which allows passengers to survive at high altitudes allows the pilot to calmly descend to 3,000 metres, to an altitude where there is no need for aid to breathe”.
United says there has never been a loss of pressure in the cabin.
«There will be the instrument records as proof, but that descent which was slightly more accentuated than the standard one, the one which causes the aircraft to tilt by 3 degrees, testifies that everything was under control. Evidently, however, the indicators linked to the pressurization system indicated that something was not going right and the pilot was right to be cautious. While returning to Newark he must have constantly monitored the delta (the difference) between the internal pressure (of the cabin, ed.) and the external one. It should also be considered that these systems are “redundant”, that is, the “vital” parts are double and even “triple” because if one fails there is at least another to replace it.”
Is this a frequent malfunction?
«I would say no, on today’s airplanes these are rare circumstances and above all they can be faced without consequences for passengers. Furthermore, the plane was over the sea and therefore getting off was not difficult. It’s different if perhaps you are crossing the Alps and to descend towards 3 thousand meters you have to “throw” your nose towards Monaco or towards the Po Valley. Then it may happen that you have to descend even if the pressurization system works well.”
When?
«It occurs when all or part of the power of an engine is lost. Quad jets are an extreme minority and therefore in twin-engine aircraft at that point you only have one engine left which provides the thrust necessary to fly in maximum safety, but which must also power the pressurisation system on its own. So as a rule we go down in altitude anyway.”
Read the full article
on Il Messaggero
2023-09-15 22:03:00
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