Even though decompression sickness is more alarming and, occurs at a worst cases of flight emergency… Still, Hyopxia is the trickiest and the most dangerous one. According to SkyBrary, “Hypoxia is a state of oxygen deficiency in the body sufficient to impair functions of the brain and other organs. Because of the nature of flight, flight crews are much more likely to suffer from hypoxia than ‘normal’ people.”
Last Update : 2037 UTC, Sep 6, 2022
The unfortunate Cessna 551 Citation 2SP (OE-FGR), has reached the FL360 (36,000ft) after approximately 33 minutes of departure. It’s interesting enough that the plane hasn’t ascended or descended until the unfortunate heavy dive hence crash occurred. The plane was descending at a nominal rate of 2142ft per minute. It shouldn’t be misleading that the aircraft fell into 8100 feet per minute descent, that was only at the final phase, where the Cessna entered what’s known as Spiral Instability…
What is Spiral Instability ?
A spirally stable aircraft tends to go back to its level flight after a turn. Although, a spirally unstable aircraft tends to fall further into each turn with a nose down towards the same direction, due to the rudders redirecting the plane and allowing for more, every time. The bank increases further, is self repeating and proportional if not corrected by the surface controls. Cessna 551 is prone to such instability for having a Tailplane. Once a bank angle exceeds an amount of a turn (e, g 30 degrees), the nose falls into the turn, the speed increases as the roll increases, hence the aircraft enters a spiral dive.
You can know more by going to McGraw Hill’s ACE The Technical Pilot Interview, page 32-33, [Second Edition].
Further more, emergencies that are due to stall (stability stall) and instabilities, causes planes to fall at much higher rate, due to them being nothing more than an object and no longer aerodynamic. This descent primarily and at most of its stages couldn’t have been that. Therefore meteorological conditions of wind-sheer, clear air turbulence and storms goes off the chart.
Such a descent can be a glide one, where either engines fail or run out of fuel, in those cases either the pilot or the FMS (Flight Management System), adjusts for a glide angle that descends at a rate that guarantees the most distance. Those rates are different for each aircraft performance envelope, but they are as maintained as in the case of this flight. According to FlightWare the speed was almost stable during the last descent. Check the image below:
The speed in the above log varied, but to an extent that does happen in non-powered flight, when the adjustments are no longer thrust based but rather AOA (Angle Of Incidence), to obtain as much distance without a major focus on speed. That’s why in most POH (Pilot Operating Handbooks), it’s referred to as Glide Angle and not Glide Speed. To understand glide speed please refer to L/D Max by visiting this link.
The flight direction was fixed all the way from departure till crash, with only an exception overhead the German-French borders, where the heading changed northerly but went back to what it was. One could argue that both flight data from FlightRadar24 and FlightWare weren’t the same but rather curved… That is explained by the earth’s curvature and convergence angle of navigation, the input before the German-French border was no different than the input after it, which could have been one flight plan. It could have been argued that things were supposed to change, of course we’re speaking without looking into the data of the FMS (Flight Management System), where a flight plan could have been implemented but, in non-decisive flights, where the crew is changing destination and non-flight related plans, the FMS is only used to short navigation putting incomplete legs, to change further, without a descent procedure, some FMS’s can continue with the old one or disconnect back to auto-thrust and auto-heading & altitude
German newspaper Bild says “The plane reported cabin pressure problems after take-off and contact was lost after it cleared the Iberian peninsula.”
The nonsensical flight path, fixed altitude and a slight change in speed. makes an assumption that nothing was ever changed or no input has ever been put during the last 3 quarters of the flight, and a case of Hyopxia can be evident.
Hyopxia happens when you lose oxygen levels gradually, something that is a byproduct of a plane decompression or failure in the pressurization system that guarantees oxygen, and air pressure modulation to an amount close to ground. Symptoms of Hyopxia can be late to notice, they include the following:
- Confusion.
- Fast heart rate.
- Slow heart rate.
- Euphoria.
- Impairment of performing recently learnt task.
- Impairment of mental task (learnt tasks).
- Altered sensorium, including loss of consciousness.
There is a Human Performance case that is caused by the opposite of oxygen lack, Decompression Sickness. You now have more oxygen than your body needs. Decompression Sickness is most likely to happen in Explosive decompression, Rapid decompression and Gradual decompression. Symptoms vary but they can be hard to differentiate from Hyopxia’s. Nevertheless, the first happens faster than the latter, which allows for it to be recognized and dealt with. In both cases, a sense of relief sets down, and thoughtfulness and confidence that everything will be alright becomes the state of the pilot’s mind.
Figure 2
It can be noticed in Figure 2 that, the speed is quite opposite in its beginning than the end. Because this flight went to upper latitudes further into the northern hemisphere, it underwent Veering of wind direction which effected its ground speed, the TAS (True Air Speed), could have differed drastically. It can’t be measured how exactly the wind circled the plane, without weather analysis, but it certainly did. In manhandled flight, such speed change should not happen, this alone makes one inclined to think, that the aircraft was uncontrolled starting from its beginning segment. Otherwise, a speed compensation would have been implemented and, the yellow line would have been constant. This also goes to show that the trip further north wasn’t something planned and neither added to the FMS. FMS accounts for cross wind speed automatically if a complete flight plan legs were put.
What is wind Veering ?
Wind is said to be Veering when it changes its direction clockwise, it’s said to be Backing when vice versa.
These wind direction changes differ with height and hemisphere due to earth’s Coriolis Force and change in pressure gradient. The wind either veers or backs overall when climbing up and down latitudes.
You can know more by going to McGraw Hill’s ACE The Technical Pilot Interview, page 239, [Second Edition].
Could have the flight crew encountered the mentioned scenario ? Would have the flight taken a different route or altitude if it did ? Or could it have endured exactly what took place ? ..
If you have any information about the flight crash please, contact management@ushby.org or report to the NTSB by filling this form.
This analysis was given to us by one of our pilots at Ushby,
License NR. 9543 ICAO J-CARC.
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https://crm.ushby.org/index.php?module=ext/public/form&id=12
Photo by Marina Hinic
