Decompression switch there is usually 4 or 5 ways of doing it, either turn the bleeds off, shut the packs down, manually open the outflow valve and there is usually a separate emergency outflow that can be opened using via decompress switch its either attached to that or the main one The A220 has 2 emergency outflow valves powered off different busses. But if you do that your cabin pressure drops to what ever your alt your at, and the cabin follows then the aircraft profile with pressure change. If your above 13k the rubber jungle will come out and above 30k there will be people with decompression sickness onboard and also DVT. I have only ever done it once for fumes in the cockpit and when we did it we were below 10k and it was the forward valve to dump through the nose to clear the fumes. The cabin pressure didn't actually change at all. At that point we were 3 mins to landing so we stopped the colossal checklist and landed the aircraft. It turned out one of the radio transceivers had self destructed and was emitting a burning stink but still working. We were at 25k when it all started and it was pretty much Vmo with masks on directly to a 4 mile final coming back to 245 knts at 10k.
If your above 30k you more than likely will have the cabin go IMC with clouds forming in it.
But all you are doing is taking the control of the cabin pressure away from the cabin pressure controller.
The bit from 10k down to the runway would have me fired. I am not so sure my license could be removed though. The profile above that wouldn't.
In theory all ICAO countries should have a FDR analysis system in place for AoC holders so some 250 data streams will be recorded every 3 seconds for every flight. As per the MAX there are primary and secondary streams. The primary flight conditions and variables such as we are looking at here are set in stone to be recorded. The secondary streams are sometimes a bit woolly. And the company should download the QDR (quick data recorder) regularly and monitor compliance to regulations, SOP's and aircraft limitations.
Fundamentally the real screw up started when the gear was selected down above the aircraft protection window of 260knts and although the handle moved the gear didn't. As 3Ddave rightly said if there was an airspeed interlock on the handle it wouldn't have happened.
I haven't seen anything yet to say where the EGPWS gets its gear configuration from. If its microswitches on the handle then that's one major safety system gone.
The performance during decent must have been extremely strange without gear drag. Although for the vast majority of what you deem quiet rightly as a cowboy approach they were well above flap limiting speeds.
Anyway I don't think you will find any commercial pilot that will say that the decent profile is acceptable for a commercial pax carrying jet. From the parachute (meat bombers) pilots you will get black humour along the lines of I did 20 of them yesterday or simply "hold ma beer" they will quite happily come down faster than the meat bombs they have just delivered.
The Captain was a 17000 hour ex mil experienced guy. I really don't understand what his thought process was to attempt this. There are huge cultural differences between myself and an Asian ex mil pilot with double the experience I have. I can sort of understand what they were trying to do, I can understand the performance that they expected and can see that it might have worked if the gear had come down when they expected it. But the rest of it no clue. I wouldn't even attempted it, I would have just asked for more track miles. Once or twice round the hold would have easily sorted it.
The pilot screw up is a given. I am more interested in the safety systems and what information they were giving to the crew. All the machines I have flown so far there would have been warnings starting about 3000ft for sink rate then escalating with no way of muting them down to second stage of flap being selected. Once that was selected all hell would have let loose, gear horns, the EGPWS having a hissy fit. The eventual warning at 160ft would be the EGPWS shouting "too low gear". IF all the gear warnings had gone due to the handle being in the down position then its a concern and a major factor. The old Jetstream prior to EGPWS and TCAS being fitted ( yes I am that old and flew then) the alarms would still have started when the second stage of flap was selected and the gear wasn't down.
The other technical issue is the place meant of the perm mag generator and FEDEC on these engines. The A220 its up the back on the gear box and the underside of the engine is clean even of fuel lines. This is mainly because of the colossal bypass fans. This doesn't in anyway take anything away from the colossal pilot screw up I might add. It just shouldn't be possible for the pods to scrape the runway and it end up with a dual flame out 90 seconds later. This has now happened twice with this aircraft type. The first time in Tallinn they didn't have pax onboard and they managed to get it back on the deck nobody killed but the plane was a write off.
I mentioned it before as a human factor. Pilots are not meant to follow Ramadan while they are working. I have worked in Saudi during it and the local imams are more than happy to issue certificates stating that you shouldn't follow it as a pilot working. And the crew canteen is still open at the airport during the day. I got three certs one for my accommodation door, one for the car and one for the work bag. Now some Muslim pilots do follow it. 22nd of May was the second last day of fasting after it starting on the 23nd of April.
The plane crashed at 14:45 local time. Sunrise was at 05.44 if it was anything like Saudi with people following it the pilots will have been up since 04:00 eating/drinking with the family and then morning prayers. They will have gone to bed at about 22 or 23:00 the night before after Isha at 20:44 . Now its not outside a possibility that the whole of the crew on this aircraft had neither eaten or drunk anything for 10 hours in pretty oppressive high temps over 30 deg C. Personally I feel this could be a major human factor in this accident. I might add all of my Muslim colleagues over the years have been excellent operators and have always ensured they are fit to fly an aircraft and if working during Ramadan have either flown during the night shift or have broken their fast during the day to ensure flight safety. So from them I believe its perfectly possible to respect their religious obligations and be a safe pilot of a commercial aircraft. But we shall have to see if its a factor in the accident. My gut feel is that it might be a major factor. Dhuhr was at 12:30 ish so its not outside the realms of possibility they missed top of decent due prayers. I have landed at lunch time prayer times with no ATC when it should have been active. All the pax have got off and promptly started prayers on the apron and nobody had an issue with it. In the western world there would have been a hissy fit by the regulator and by apron safety ops. First time I went with an experienced in area FO and just landed and then hung around until they went inside. After that I just went with him inside as it was ops normal for the region and absolutely nobody thought I was strange or had an issue with it and I really didn't want to hang around in 40 deg C.