When flights get cancelled because of heavy winds / bad weather, why is it only e.g. 10% of all flights and not 100%? Isn’t either too dangerous so no plane can take off or it’s safe so they all can take off ?
In: Other
I’m an Airline Pilot. There are a lot of separate factors here.
1) Different aircraft variants have different crosswind limits
2) The same aircraft variants can have different crosswind limits between different operators/airlines
3) different operators/airlines have different stable approach criteria and different restrictions on contaminated runway operations/adverse weather ops
4) Airport facilities. Intense snow and severe icing. Can the deicing provider cope with everyone simultaneously (hint, outside of the hugely experienced airports who cope with snow every year, the answer is no)
5) Strong winds – we take off into headwinds. In the A320 we can accept crosswind up to 38kts and a tailwind up to 10kts. Every flight is different though – although allowed to take off in 10kts of tailwind, we are so heavy on this particular flight that our take off performance calculations show we can’t take off in accordance with the performance requirements. Can’t use that runway end, have to use the other. Can’t take off into aircraft approaching the other runway end. Big delays.
6) As pilots when we say ‘bad weather’ we are generally thinking about
Strong gusty crosswinds
Windshear and microbursts
Thunderstorms (TS) & Cumulonimbus (CB) clouds that can generate moderate to severe turbulence, windshear, icing
Heavy freezing rain
Low visibility (<550m in fog/drizzle/low cloud)
Mountain waves
Very strong gusty headwinds
If there is TS/CB activity in the vicinity of the airport then everyone going in and out is going to need to take avoiding action and be vectored around it. ATC are going to be very, very busy indeed and consequently the flow rate of aircraft in and out will need to be chopped. When this happens most aircraft in and out end up being given what we call a slot/CTOT/CDM TSAT which is a designated time we’re allowed to go. This could be hours and hours after the scheduled departure time.
So your flight may be cancelled because
1) The weather is out of limits
2) The chopped flow rate means your flight has to be cancelled
3) The slot means your flight crew will be ‘out of hours’ – the delay means our duty hours would breach the limits. There is a special procedure called discretion to extend the limits slightly but only to get home on the last flight after an unexpected delay, e.g a diversion due to a passenger medical emergency. In Europe it can’t really be used to leave home base for an expected delay like forecast severe weather. In these circumstances the airline call new crew from standby but if there aren’t any/enough available then the flight simply cannot operate.
I fly the A320. If the crosswind including gusts exceeds 38kts we simply can’t shoot the approach or take off. If the airport is covered in TS and CB’s we’ll just have to divert. If they’re isolated and we can try to pick through we’ll give it a go but if there is a sniff of safety being compromised we’ll have to go around and go off to the alternate.
Bear in mind we will have loaded lots of extra fuel (I’m talking several tons…as much as is necessary but also not so much that it causes landing performance problems) to give us lots of holding time. We try our best to achieve the schedule but if the weather is out of limits or other aircraft are reporting genuine windshear or severe turbulence etc it just can’t be done. Can’t take off into reported genuine windshear. No one is going to take off into a proper embedded thunderstorm.
-edit-
Lots of questions asking me to explain windshear and microbursts and whether they are common.
Read this, it’s an exceptionally good article on what WS actually is.
https://safetyfirst.airbus.com/wind-shear-an-invisible-enemy-to-pilots/
Microbursts aren’t, because we don’t fly through thunderstorms. We also have doppler radar that measures the shear rates of water droplets in the atmosphere ahead to detect and warn of windshear i.e. microbursts and gust fronts.
Watch this from 1:10 onwards
If you guys are still really interested, read this.
https://skybrary.aero/bookshelf/books/164.pdf
-edit again-
Ok the amount of responses to this has gone a bit fucking mental. I’m busy atm but when I get back home in a few hours I’ll follow up on all your questions and messages.
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