Registered to an aeronautical ACARS service you will never meet; in practice it is VNC display :11.
IANA assigns TCP/UDP 5911 to ats-acars, "Air Traffic Services applications using ACARS" (the SCTP side of the same number is cpdlc, Controller Pilot Data Link Communication). Those are ICAO datalink registrations for aviation ground/air networks and they do not appear on the public internet. What actually answers here is a VNC server: RFB maps display :N to port 5900+N, so 5911 is display :11, handed out by QEMU/KVM, VirtualBox VRDE, or a multi-user Linux host running one Xvnc per user.
Do not expose
If it is VNC it inherits VNC's problems — a plaintext RFB version banner before auth, and a legacy DES challenge-response capped at 8 password characters. Reach it over SSH or a VPN, not directly.
$ nc -w 3 TARGET 5911 | head -c 12; echoSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.