What exactly is "Speed Reference System (SRS)"

[z.khalid]

Can somebody just please briefly explain it to me?
I seem to find a lot of information and I think it is unnecessarily confusing me.

Thanks!

[safetypee]

There may be several interpretations of ‘Speed Reference System (SRS)’:
One relating to Airbus aircraft is in this thread, http://www.pprune.org/questions/243995-airbus-srs-explanation.html

Also see the patent at
Speed reference system for piloting ... - Google Patents (http://www.google.com/patents?id=4T8dAAAAEBAJ&printsec=abstract&zoom=4)

See more from Google.

IIRC there was an old (pre EFIS/FMS) system which used lights or an indexer based on AoA or ‘airflow detection’ to provide a reference speed, usually for approach and landing.

[B737NG_Pilot]

Hi safetypee Can you please provide any link or documents in relation the old (Pre EFIS/FMS)

[Alex Whittingham]

I remember ADIs from 40 years ago with a fast/slow bug on the left side like this (https://upload.wikimedia.org/wikipedia/commons/thumb/c/c5/VMS_Artificial_Horizon.jpg/368px-VMS_Artificial_Horizon.jpg) . Is that what you were referring to, safetypee? The reference speed was set by the pilot from the bug on the ASI with alpha protection at low speed on the approach. Khalid's reference is probably from a new EASA syllabus LO, though, which says "Explain the following vertical modes: vertical speed (V/S); flight path angle (FPA); level change (LVL CHG)/open climb (OP CLB) or open descent (OP DES); speed reference system (SRS); altitude (ALT) hold; vertical navigation (VNAV)/managed climb (CLB) or descent (DES); glideslope (G/S)." and therefore unlikely to be an old system.

[rudestuff]

Never heard of it. Nice to see the EASA talking about flight modes though, I'd never heard of any of those until I was doing my first type rating.

[PFD]

Greetings. The Speed Reference system is a TOGA (Take off and go around mode) on the Airbus. I'll describe what I know about the A320 one as it's the SIM we have.

If you understand Level Change or in Airbus Open Climb/Descent modes, you will know that the autopilot controls speed using pitch and the autothrottle just delivers climb thrust or idle.

With SRS, during takeoff or go around (almost a takeoff) the Flight Director and then the Autopilot will control speed in pitch (or get you to) so that it doesn't fall below V2 plus 10 kts. The throttle at the time will be in the TOGA position delivering loads of thrust. This ensures a safe rapid climb out up to acceleration altitude. Once you reach acceleration altitude, the system will go into normal NAV or Op Climb mode and pitch will be reduced to bring you up to generally 250 kts below FL100.

Hope that helps.

[Alex Whittingham]

Cool, thank you

[lederhosen]

Just completed an LPC and so recently had my head in the books. To add a bit of detail: in the engine failure case the SRS actually targets V2 or the current speed if it is higher but no more than V2 +15. As a long time Boeing captain I have to say that the Airbus engine failure is a much easier scenario....the main step is autopilot on! However the manuals are not so straightforward. Indeed I can remember vividly during my initial type rating spending far too long looking for this information, Airbus acronyms can be very irritating.

[Smooth Airperator]

lederhosen, Boeing elects to withhold a lot of information from their manuals. In the case of Airbus, the manuals can be a case of information overload.