Tonight I flew from Munich to Dortmund on an A319 and I happened to sit just above the wings and turbine on the right side. Upon landing, the turbine opened a window or panel which exposed some grid like structure inside it and just a few seconds later, after touchdown, closed it again. Could someone explain to me what that is, how it is called and what its purpose is?
What you saw is the thrust reverser of the engine, which redirects some of the airflow forwards and therefore helps slowing the aircraft down. The grid like structure are the cascade vanes. This is what it looks like inside the engine:
(Airbus A380 FCOM - Engines - Thrust Reverser System)
The blue arrows indicate the flow of the so called bypass air, which is air that goes through the fan (and is therefore accelerated), but does not go into the core engine. With the thrust reverser deployed, this air is redirected forwards. The core air going through the actual turbine engine will still exit at the back. Since there is more bypass air than core air1, there is a net thrust generated in the forwards direction.
If you are wondering what the pilots are doing to engage reverse thrust, this is a picture of the A320 family thrust levers:
The main thrust levers are pulled all the way back during the flare to the position shown in the image (labelled 0). To open the reverser doors, you pull up on the smaller handles at the front of the thrust levers. Once the reverser doors have opened, the thrust levers can be pulled backwards to add more reverse thrust (range labelled REV in image). To close the reverser doors, the thrust levers are moved back forwards and the smaller handles are pushed back down.
Note that on a dry runway reverse thrust does not slow the aircraft down faster (assuming auto brakes are used). The brakes on the main landing gear are capable of achieving the same deceleration without reverse thrust. Engaging reverse thrust therefore reduces brake temperatures and wear.
The thrust reverser doors can look different depending on the engine type. Based on your image, the aircraft you were on was equipped with IAE V2500 engines2. The reverser doors on the CFM56 engine, as used on some A320 family aircraft, look a bit different3:
(image source: Wikimedia)
1 The IAE V2522-A5 engines used on the Airbus A319 have a bypass ratio of 4.9 : 1 (source).
2 FlightAware shows a EWG4063 flight tonight from Munich to Dortmund and according to planespotters.net Eurowings uses IAE engines on their A319s.
3 The CFM56 engine is also used on the 737, but uses the cascade thrust reversers there (thanks to DeepSpace for pointing this out). See also Why is the CFM56 produced with two different thrust reverse options?
3$\begingroup$ Warning: Do not engage reverse thrust just after rotation on takeoff ! :-(. | Long ago an Air NZ in a training situation had one engine shut down on takeoff to simulate engine failure, unexpected by the pilot in control. Unfortunately the lever was able to be inadvertently pushed through the gate and into reverse thrust, resulting in loss of aircraft and AFAIR 2 killed, 3 injured. 1966. Numerous photos $\endgroup$ Oct 17, 2020 at 14:31
1$\begingroup$ The last paragraph about the CFM56 is a bit misleading. The CFM56 variants that are installed on the 737 (-3, -7) use the cascade thrust reversers. Only those that are installed on Airbuses (-5) have the pivoting-doors thrust reversers. $\endgroup$ Oct 17, 2020 at 16:26
1$\begingroup$ @DeepSpace Thanks, I updated the answer. $\endgroup$ Oct 17, 2020 at 16:32
1$\begingroup$ @PeterCordes I think they're saying that reverse thrust could make the plane stop faster but is actually mostly just used to reduce the amount of braking required to stop in the same time as if no reverse thrust was used $\endgroup$ Oct 19, 2020 at 7:15
1$\begingroup$ @PeterCordes No, I'm not mixing anything up. Note that I said "assuming auto brakes are used" (and auto brakes are almost always used on airliners that have them). The auto brakes always target a certain total deceleration. When adding more reverse thrust, the auto brake system will just use less wheel brakes to achieve the same total deceleration. Only on a slippery surface, where the anti-skid reduces wheel braking, will reverse thrust actually reduce stopping distance. $\endgroup$ Oct 19, 2020 at 7:15
It's the reverse thrust door opening. I will get back to you with a picture.
Courtesy By Pieter van Marion from Netherlands - PH-BVC KLM, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=29588152
For a brief explanation, once the aircraft has touched down (and in some cases even before it touches down) there are a set of doors and vanes which deflect the jet efflux forward which are deployed to help slow the airplane. However, these are generally NOT taken into account when certifying the airplane's landing distance. They are therefore a bonus when landing. There are two main types,
Cascade vanes and
Clamshell doors aka
Buckets. The A319 has cascade vanes, similar to seen in the image.
Some more trivia, the reverse thrust on some airplanes can only be activated when the nosewheel is on the ground, due to the possibility of the buckets scraping the runway with a high nose-up attitude.
4$\begingroup$ The cascade reversers on the CRJ-200 were operated by a screwjack and flex drive cable deal and were really slow to extend. By the time you had max reverse, a second later you got the speed call and it was time to go back to idle. $\endgroup$– John KOct 16, 2020 at 20:39
$\begingroup$ Apply reverse thrust before touchdown...? $\endgroup$– Jpe61Oct 16, 2020 at 22:43
5$\begingroup$ @Jpe61 Yes, the Trident was an example. $\endgroup$– RafflesOct 17, 2020 at 8:32
2$\begingroup$ @Jpe61 -- it's not seen on newer types, but the DC-8 made extensive use of in-flight reversing to make up for its lack of proper speedbrakes. (In-flight reverse might be a thing on newer military transports, but I don't have the documentation to say that for sure) $\endgroup$ Oct 18, 2020 at 1:01
2$\begingroup$ @BobJarvis-ReinstateMonica -- there's some fairly complicated aerodynamics that determines whether in-flight reversing is safe or not (basically, it boils down to how badly reverse thrust messes up airflow over the wing -- DC-8s only use idle reverse on the inboards while in flight, for instance, that generates all the sink they need) $\endgroup$ Oct 18, 2020 at 18:02