My understanding is that the aircraft drifts down, so the initial TAS at altitude is high, it slowly decreases as the aircraft descends, the likelihood is that the aircraft won't have reached the level off altitude in 60 minutes, so you will still have a slightly higher TAS, but once the aircraft levels off, the TAS for the single engine speed will be at its lowest, so basically 120 minutes is not automatically double the 60 minute distance nor is 180 minutes 3 times the distance.

This is different to the methodology used by Jeppesen flight planning where the aircraft is immediately considered to be at 10,000 feet immediately following the event.

Try not to confuse operations with certification as they are two different fish.

Out to 180 minutes is only required to be a fixed distance (no wind accounted for) based on the declared (certified in the operation manual approved by your regulator). Different regulators can and do have a different certification view though.

Currently my regulator requires wind allowances when operating beyond 180 minutes out to 240 minutes. 240 also has a greater fuel monitoring requirement that required an FMC upgrade in 17.1 on the B777.

I think one of the most relevant things to remember is that it’s just a planning exercise to make reasonably sure that you have somewhere half-decent to go if something happens. You don’t have to divert to your nominated ETOPS alternate(s), as someone was trying to convince me the other day: anywhere sensible and justifiable will do...

A little bit of interesting info....

One of the requirements for granting ETOPS approval at service entry is that the manufacturer will have performed a 3000 cycle test on the engine and then stripped the engine down and carried out a detailed examination.

RR has yet to do that for the Trent 7000 as of late last year resulting in 180 minute ETOPS with a 500 cycle limitation on the engines and an ETOPS expiry date of year end for full compliance.

punkalouver
 

You are a little late with your news, l believe the aircraft has since received 180 minute ETOPS clearance without the 500 cycle limit.
Earlier this week it also received EASA ETOPS (EDTO) clearance for beyond 180 minutes.

Thanks,
I posted the info with the intention of providing interesting info about the certification process, not from a newsworthy point of view. Hope you found it interesting and am hoping for more interesting certification info as a reply.