what_next wrote:
Not under the rules (EASA OPS) that are in effect around here. Would probably be the end of transatlantic flying.
Hmm interesting. Under JAR OPS we did have several options for that.
We planned all planes with enroute alternates so that you would have alternates available within a certain time limit after an incident.
For 2 engined jets, which are the majority today, we planned either Non Etops, where you would need an alternate every hour, or ETOS where, depending on the certification, you could get up to 3 hours at the time. If I remember right, STOPS (Non ETOPS), the diversion time would be calculated at cruise level with 1 engine out, for ETOPS it would be calculated at FL100 (depressurised) and one engine out.
For 3 and 4 engined jets, the time to diversion was 2 hours (internal rules) with one engine shut down and depressurized.
With depressurisation, under most conditions you will not reach your destination but plan to one of the intermediate alternates. Short range, that is not an issue most of the time as plenty are available. Long haul, most of the time you will need to divert to your nearest stops or erops alternate.
With GA planes, I doubt that such calculations are done, I would not know if any flight planner can actually do it, even simple ETP calculations between alternates I have not seen. It would be a neat feature however for autorouter for instance.
172driver wrote:
Interesting to see how this will pan out with the new ETOPS 330 for which Air New Zealand just got approval for flights between Auckland and Buenos Aires using 777s. Not an awful lot of diversion airfields on that run…..
Pacific operations these days have up to 4 hours etops, there are some airlines which have applied for more. In my time, we had 180 minutes for the 330. But up to 240 is no rare thing these days, 777 and 330 are operating like that for ages, probably dreamliner too.
There are other malfunctions that can also cause havoc. There was a story about a Westwind II enroute to Hawaii where a hydraulic line burst (that’s at the limit of the Westwins range of 2500nm). Now, the uplock for the gear is hydraulic so the gear came out. No big deal, except the added drag and reduction in speed put the plane in serious jeopardy. I think they made it, but on fumes. Same thing could happen on an airliner, I assume.
Peter wrote:
OK; a 5cm hole would be fine, with a ~13psi pressure difference. A window say 30cm across is 30-40 times bigger a hole. I can’t believe the pressurisation of a 747 would hold up four of such holes. There are online calculators for this and you are looking at a massive outflow.
Pressure differential on a 747 is nowhere near 13 psi. Frankly, I could remember it wrong (it was a long time ago and just by the way). It might have been that the aircraft survives, not that it keeps pressurization (perhaps with one; the amount of air that can be pumped in is huge, I would have to do some ballpark calculations). Anyway, I wouldn’t want to sit next to that hole (very noisy, etc.).
You are partly right in that a 747 will be pressurised not to SL but about 8000ft.
8000ft = 11.25psi
FL350 = 3.75psi
Differential = 7.5psi
OK, not 13psi, but still a massive loss of air.
Peter wrote:
OK, not 13psi, but still a massive loss of air.
Definitely. But if I remember it wrong, it would imply that loss of more windows would present a problem – other than loss of pressurization, you would lose that anyway. What would it be? Structural failure? That doesn’t seem right. PS: Of course, there is a possibility that neither version is correct.
No; I don’t think I suggested that losing any number of windows would break the airframe. What I think is that losing even a single window would make it awfully difficult to pressurise a 747 to anything survivable if they stayed at say FL350.
I don’t know how to calculate it. One would need to know the maximum available mass flow from the compressors, and one could get a rough idea of the mass flow rate from one of the online calculators for this sort of thing. I just “feel” that it would not work.
I know someone who lost a door seal (only the rubber seal; the door itself didn’t come out) on a bizjet. It was apparently a very explosive event, with a total and virtually immediate loss of cabin pressure.
Peter wrote:
No; I don’t think I suggested that losing any number of windows would break the airframe.
I’m not saying you did. I’m sure there was a limit of just a few windows in what I was told. That implies that something (bad) is going to happen if you lose significantly more. I still think it was that loss of an window would just result in a lot of noise and air rushing out but the cabin would stay pressurized. Maybe not fully, the pressure differential could be lower, even after switching to emergency, hi or whatever mode 747’s environmental system has.
AdamFrisch wrote:
Same thing could happen on an airliner, I assume.
I suppose but I’m not sure it’s allowed. How many jets keep gear up in this fashion anyway? You are right, it’s a possibility, generally speaking. I think I learned that some systems are like this when I was looking into a Commander (the single). But I got the impression this is something you might find on something like a light twin. All the jets I saw that up close used mechanical uplocks and AIUI loss of hydraulics wouldn’t release the gear.
Mooney_Driver wrote:
Pacific operations these days have up to 4 hours etops, there are some airlines which have applied for more. In my time, we had 180 minutes for the 330. But up to 240 is no rare thing these days, 777 and 330 are operating like that for ages, probably dreamliner too.
You may have misunderstood me here, no AB 330 involved. ANZ will be flying a 777 with an ETOPS of 330 minutes (i.e. 5h30’) between ACK and EZE on a track that goes largely over the southern Pacific and skirts Antarctica. Diversion fields? None. Hence the 330 mins……
Our ETP’s are calculated threefold for ETOPS planning
1 Engine Out
2 Decompression
3 Engine out and decompression
taking into account driftdown and diversion speed, increased fuel burn, winds aloft at the lower levels and a penalty for icing.
As said, this is ETOPS planning, which albeit very important is quite theoretical. During actual flight you go to the nearest place with good weather and infrastructure and will have plenty of fuel.
More interesting inmho is flying routes over high terrain and the required escape routes to get down safely while evading grid moras up to the 300’s. The flight deck crew could stay on oxygen for a long time after depressurization and stay high but the chemical generators for the pax only last a few minutes.
