View Full Version : Airbus SRS, GS-Mini & etc

3rd Oct 2008, 17:07
'Bus experts...

Have come across a lot of mention of SRS and GS-Mini in the context of Airbus speed modes but can't find an explanation anywhere - am quite sure these aren't complex concepts, can anyone assist pls?


Where do you put SID/STAR charts etc for displaying in flt?

What's the audio environment like in the crz, is it headsets off etc, SOP dependent? How does it compare to other types?


3rd Oct 2008, 19:07
Googling for "Speed Reference System" (SRS) doesn't immediately provide a wealth of information, but one of the links I found is an incident report and there is a paragraph that reads as follows:

"The FD pitch mode used for take off is SRS (Speed Reference System) which provides FD pitch commands to acquire and maintain a speed of V2+10 kt with both engines operating and V2 (or the existing speed if it is greater than V2) if one engine fails. The SRS mode normally remains engaged until the aircraft climbs to the thrust reduction altitude. Thereafter the FD pitch mode would usually change to CLB (Climb) or P CLB (Profile Climb)."

Hope this helps.

Chris Scott
4th Oct 2008, 02:00
SRS ends, strictly speaking, at the acceleration altitude, not the thrust-reduction altitude. These are normally the same (by default), but can be different if the crew chooses.

GS-MINI is, as the name implies, based on a minimum ground speed. before the approach, once the crew can predict with reasonable confidence what the surface-wind is likely to be at the landing threshold, they enter the figure into the approach Performance page. The FMS works out the correct Vapp, which is a threshold IAS based on weight and headwind component.

Now: let's take the sea-level ISA case, where IAS=TAS; a Vapp of 130kts; and a predicted headwind of 10kts entered into the Performance page. On a conventional aeroplane stabilised at Vapp (130kts) at a height of 500ft, if the headwind is 30 kts the GS will be 100kts. But when it comes over the threshold, where the headwind is only 10kts, it will need to have accelerated to a GS of 120kts to maintain the required Vapp of 130. This will require a lot of extra energy (from the engines), which may cause problems, particularly if the loss of headwind happens suddenly (like at night).

It makes sense, therefore, to ensure that the GS remains at or above 120kts throughout the approach, even though this initially results in a higher IAS (150kts at 500ft in this case). The "managed" speed (IAS) target on the ASI (used by the pilot and the autothrottle) goes up and down with the headwind, but never below Vapp. Reaching the threshold, provided the actual headwind equals the predicted figure, the speed target will be Vapp. If the wind is higher, the speed target will be above Vapp. This should not be a problem for stopping in the runway length, because the GS will be no higher than originally planned.

In practice? Works very well, particularly using manual throttle, provided the pilot uses the "managed" speed as a target speed; not a minimum speed. The power changes required in windsheer are far less than on a conventional aeroplane, because the GS (kinetic energy) is steady.

CHARTS can be clipped to the pilot's table, but this is normally stowed for take-off and landing. There's another clip by the DV window, under the chart light.

COMMS are conventional, varying with airline SOPs.

4th Oct 2008, 06:30
Like any modern Auto Pilot, you have an outer loop and an inner loop.
Both loops are feed by the FMS when in managed mode, by the Pilot using the FCU when in selected mode.
The outer loop handles a request to change the aircraft C of G position in space, such as turning, climbing, descending
The inner loop will provide how much rotation is required around the aircraft axis to make good the changes requested by the outer loop. In most cases the inner loop is fully managed by the FMS, however as the FMS is dormant between preflight flight phase and THR RED, the FG is then ruling the show and computes and displays through the FD ,the pitch requirements in order to follow the SRS elaborated by one of the FG. functions