Preamble
As per The Aviation Herald, on October 8, 2023, an Airbus A330-200 from SereneAir, registration AP-BNE, performing flight ER 1801 from Islamabad (Pakistan) to Jeddah (Saudi Arabia) with more than 309 Umrah pilgrims on board, was en-route at FL360 about 280 NM north of Karachi (Pakistan) when the crew initiated an emergency descent to FL175 at first (terrain limitation), then to FL100 due to the loss of cabin pressure. The aircraft diverted to Karachi for a safe landing on runway 25L about one hour after leaving FL360.
⚠️ At the time this article was written, no preliminary safety report had been issued, so all the analysis here was made using uniquely the (rather incomplete) ADS-B data, so it has the only purpose of illustrating the importance of Escape Routes design due to en-route depressurization and engine-failure scenarios. |
The images below illustrate the ADS-B data obtained from FlightAware and Flightradar24. The second one seems to have collected flight data longer, so it was the one used for the study.
The Aircraft
The flight ER 1801 was carried out with the aircraft AP-BNE, a 17.6 years-old Airbus A330-202 (MSN 733), equipped with CF6-80E1A4 engines.
The A330 can be equipped with a 15-min, 22-min, or up to 36-min oxygen chemical system. The system behavior, when triggered, follows a maximum altitude envelope, as provided by the manufacturer.
Basically, to make sure it will provide sufficient supplemental oxygen to the passengers, the aircraft must fly at or below these altitudes after the given time after its activation, as described in such envelope.
Since we don't have access to which system is installed in the AP-BNE, based on the oxygen profiles above, the aircraft age and history, and considering the report that the crew initiated an emergency descent to FL175 at first, we may guess it has the basic/standard 15-min system.
Independent of that, though, considering it a 15-min would give us a more conservative (and interesting) study than a 22-min or more 😉
The Route
Crosschecking the ADS-B data with en-route navigation data, we can determine what was the planned route, which was via J133 airway (as per AIRAC cycle 2310):
Due to the ADS-B signal loss, we are unable to determine with confidence the escape route adopted by the crew, but considering the ADS-B signal recovered close to Karachi (OPKC), we can suppose that in some point, the flight diverted to join airway J172 and head to OPKC.
We shall analyze if that was the best (and a safe) alternative as follows.
The Terrain
When evaluating terrain for depressurization scenarios, we usually check initially for segments where grid MORA is over FL100, since if the minimum safe altitude for obstacle clearance is lower than 10,000 ft, there is no need for diversion.
A quick verification of the images extracted from Jeppesen FD Pro, one can notice that the segment between SARIT and ATRIS has grid MORA ranging from FL123 up to FL154, which suggests we shall check for Points of No Return (PNR) within this segment.
(interesting enough, have you also noticed that SARIT and ATRIS are anagrams?! 🤓 Anyway, back to the study!)
The Analysis
Considering the 15-min oxygen profile and the emergency descent performance for the A330 at MMO/VMO, we can design the descent profile for the study and, in a conservative approach, consider that, after the failure, the aircraft shall reach/descent to:
- FL180 within 35 NM
- FL140 within 85 NM
- FL100 within 290 NM
With this profile and the grid MORA, it is already enough to conclude that any failure before ZB can safely return to Islamabad (OPIS), while a failure after PARLO can be safely continued along the planned route to the closest en-route alternate. So, we have now narrowed the critical segment to be between ZB and PARLO.
To further narrow the study, we shall now define the PNR between ZB and PARLO, and for that, to use conservative values, we shall consider the MEA (Minimum En-route Altitude) of the planned route: as a good practice, we shall always try to verify safety compliance using the most conservative altitudes and, step by step, reduce those margins, instead of going directly to the minimum stipulated by the regulation! So, from grid MORA, to MEA, to MOCA and, as final resource, terrain survey 😅
In this specific scenario, we have a "special case" where any failure happening within 5 NM before ZB until 20 NM after ZB can be safely continued or returned along the planned route. So we can actually choose our PNR to be any point within this range!
We would usually choose in this situation the further one (20 NM after ZB), but for convenience, as it is easier for crew to monitor, let's consider ZB as our PNR. So, any failure before it, the aircraft must return to OPIS, while any failure after ZB must continue along the planned route to closest en-route alternate.
As much as this decision is at flight crew discretion, based on different factors, such as nature of failure, company policy, weather conditions, etc., the best practice is to give options to the crew. And, considering Serene Air is a Pakistani airline, as are most of their customers, it would make perfectly sense to proceed to a Pakistani en-route alternate (instead of going all the way to Muscat, for instance).
None of the Pakistani airports within the route are really great options, though, especially considering an A330. So, actually diverting to OPMT, OPSK or even OPKC (as the crew opted), are indeed better options here.
To make a diversion to OPKC following J172 airway, as adopted by the crew, we may consider the previously indicated route: J133 KALAT G452 OVTEX J172 KC..OPKC. However, considering MEA values here is not feasible, as keeping FL140 until NIPET (which is the MEA between OVTEX and NIPET) would exceed the oxygen profile limit.
More than that, we can note that actually 10 NM after OVTEX, we shall descent to FL100, so not even considering grid MORA (which is lower than MEA in the segment) wouldn't suffice to check if the route is safe!
So here we come to our last resource: terrain survey 😎
To evaluate the terrain, a 10NM splay (to be strict with regulation, a 5NM would be enough) for the segment between OVTEX and KC was designed and the most significant terrain peaks were gathered.
The evaluation indicates that minimum safe altitude is higher than FL100 until 45 NM from OVTEX (considering a 2,000 ft margin from terrain), which would still be not safe for a depressurization occurring in the most critical point (which we defined as ZB).
Even if go strictly to the requirements by regulation, applying a 5NM splay, there would still be higher terrain for concern within 18NM from OVTEK:
So, we may conclude that, with that scenario, to consider this escape route feasible, we should choose the PNR to be 20NM after ZB, instead of ZB itself!
Alternatively, a good solution would be to have 2 PNRs: ZB and KABRA, for instance. So, any failure after KABRA could follow the adopted escape route via J172, while an emergency between ZB and KABRA would follow a different route to OPKC, via OPKR, as indicated below:
Conclusions
Before any conclusions, it is important to highlight that when we do such studies, our goal is not to point any mistake in the airline operations or pilots decisions, especially because we don't have enough data even to opine about it! Most of all, we use cases such as this one to improve our knowledge and our view is that pilots always take the best decision based on the information they have in hands and with the time they have to do so.
Saying that, this study has shown that a diversion was not necessary, but indeed the best operational approach. The escape route via J172, though, as the shortest way to OPKC, may had required a previous study to be able to provide guidance to the pilots, showing it would safely keep the minimum required obstacle clearance and hence giving enough confidence for the crew to adopt such a procedure. Hopefully, this was the case here and crew had simply followed the Escape Routes provided by the airline!
As shown by the analysis, if the failure would have happened earlier in the route (between ZB and KABRA), probably adopting a diversion to OPRK and then proceeding to OPKC, would be the safest option.
We are glad that this incident didn't turn into anything more serious, and that the crew managed to bring the flight safely to land. This certainly demonstrated the importance of a previous Escape Route study and how safe aviation is! 🙃