Commercial Fixed Wing
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Situational Awareness
Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Wake Vortex Encounter
Air Traffic Control Issued New Clearance
Procedure; Environment - Non Weather Related
We were being vectored to final for Runway 29. I was the PF [Pilot Flying]. We had been assigned 180 knots by the Approach Controller; I was maintaining that. There was an aircraft about 5 miles ahead as we were on base to final. While intercepting the localizer; the Captain instructed me to slow to 160 knots; as the aircraft was now 4 miles in front of us. I hesitated as we had been assigned a speed; but then followed the Captain's instructions. He told ATC 'we are slowing to 160 knots for separation with the aircraft in front of us.' ATC immediately told us not to slow without permission; then said to maintain 160 knots to 6 DME. He then told us he can go to 2.5 miles separation. We encountered some light wake turbulence and after landing saw that the preceding aircraft was a 787. ATC in Vienna is always challenging to deal with at best. As PF; I was put in the position where I had to choose whether to follow an ATC clearance; or the Captain's instructions. ATC in Vienna is not the best. Approach Controllers will assign speeds on final that are immediately contradicted by the Tower Controller after switching over; for example; 'maintain 180 knots to 6 miles' then after switching to Tower; 'slow to minimum approach speed; aircraft departing ahead.' The Captain made orders to me contradicting an ATC clearance; that didn't make sense at the time; so I just had to randomly follow his instructions. After landing and thinking about it for a while; he explained his reasoning; which then made some sense. Better communication during the event would have helped us be on the same page during it; which I always think is a better way to do things. The Captain made the right call in my opinion; we would have probably encountered more wake turbulence with the 787 as our separation decreased. Tower also departed an aircraft ahead of us on 29; and had we not slowed; it more than likely would have resulted in a go around for us.
Air carrier F/O reported slowing below ATC assigned speed when directed to do so by the Captain; who saw a potential conflict occurring with the aircraft preceding them.