Why are some A350s missing SSA probes on their nose?

Question

I recently just noticed that some A350s are missing their iconic set of 3 main SSAs (Side Slip Angle Probes) on their nose. I'm not sure if its an issue of newer variants or varied airline options, but I'm sure it has something to do with a less needed sense of redundancy. Any exact answers behind this?

Thanks!

Picture sources: Top:https://www.traveller.com.au/the-inside-story-of-the-a350-airbus-rival-to-the-boeing-787-dreamliner-12c17x Bottom: https://airwaysmag.com/airlines/british-airways-takes-off-a350-1000/

Answer 1

You are right: the three side slip angle probes are redundant because side slip can be calculated by the ADIRS without these probes. Airbus included them in earlier A350 models to crosscheck this calculation, but they are removed now:

Spot the difference 2: SSA (Side Slip Angle) Probes.

This one’s a bit more technical. If you look closely around the nose of the aircraft, you can see an array of sensors. These form part of the Air Data and Inertial Reference System – in other words, they feed flight parameters to the pilots, such as the angle the aircraft is flying, the outside temperature, and the barometric and static pressure.

As the A350 entered service, Airbus still wanted to gather more information to compare the data between different systems that help pilots fly on approach in crosswinds. To do that, a few early models had extra probes added to cross check the data from the other instruments. You can see these three extra vanes on the front of the aircraft just below the ‘Zorro’s Mask’ windscreens. Having now gathered enough side slip information to establish that they’re not required, they’re being removed on all new A350s. They’ll also be taken off our first two aircraft in the coming months, which will save weight and maintenance costs.

^{(Virgin Atlantic Blog: Airbus evolution: Can you spot the difference?, emphasis mine)}

Answer 2

@FreeMan The original post is a bit misleading, the early production aircraft would most likely not have kept the sensors to keep testing if they were needed.
The modification process takes months, if not years. By the time there was enough flight test data from the actual flight test aircraft to confirm that the sensors were not needed, there would have been multiple aircraft already in production. Then, once the data had been confirmed, the process to remove the sensors from the designed configuration could start, taking time (as mentioned). All this time the production of aircraft would have continued with the original design. The modification would have been made “retrofittable” thereby allowing aircraft already been delivered to be updated (through service bulletin). Hope this clears up your question, albeit a couple of years late.