View Full Version : A320 speed in SRS mode - missed approach
Jim510
3rd Mar 2019, 22:53
Hello everybody!
I have a question about A320 behaviour in missed approach. Let's say the aircraft is light, and you have speed limit of 160 in first turn. When you set TOGA, the aircraft will go in SRS mode. For example, Vapp target was 130 kts. Is it possible to have too much energy, giving you speed of more than 160 kts, due to pitch limit of the SRS mode? In DSC 22-30-80-20 FCOM is stating that target speed is limited to V2+15 KT.
On my previous aircraft the procedure in this case was to pull V/S if you see that your speed is increasing above your desired speed and get the A/THR in speed mode. So on the A320, selecting V/S, selecting speed and setting thrust levers in CLB detent would make automation do what we want. The speed and climb gradient would be controlled. Do you agree with this? And is it maybe written somewhere?
Thank you for your replies.
Jim
Fursty Ferret
4th Mar 2019, 08:03
Yes, in this case you'd need to intervene as you describe but have you considered simply reducing to climb thrust more promptly if it looks like your 160kt constraint won't be met?
Open climb is also a possibility if terrain clearance is an issue, but with TOGA set, a light aircraft, and an increasing headwind you will approach the 30 degree pitch limit.
Jim510
4th Mar 2019, 09:10
Yes, in this case you'd need to intervene as you describe but have you considered simply reducing to climb thrust more promptly if it looks like your 160kt constraint won't be met?
Open climb is also a possibility if terrain clearance is an issue, but with TOGA set, a light aircraft, and an increasing headwind you will approach the 30 degree pitch limit.
Thanks for the reply. Open climb has crossed my mind. Would the aircraft really go all the way to pitch 30 trying to keep the speed?
Yes, terrain clearence really is an issue on this missed approach, so I wanted to put higher thrust reduction and acceleration in perf ga page, and then to keep more than required gradient with vertical speed, keeping the speed at the limit with autothrust in speed mode.
I was wondering what would the automation do in this case. Obviously, the constraint would be in the box. Let's say we put lower thr red and accel in perf ga page. And we stay at flaps 2. Would the aircraft follow it? I'm asking because I had some unpleasant experience with A320 in one SID on our network which I have flown many times - aircraft was not able to keep the speed in the limit, always busting the limit for 15-20 knots.
The pitch limitation in SRS is only mentioned in takeoff phase and not in GA. During GA the aircraft maintains Vapp or speed at GA engagement up to Max VLS+25. It doesn't randomly target VLS +25. To maintain under 160kt there should be no need to do anything. A few things about SRS. SRS is only a managed speed mode which changes only to OPCLB mode. Even if you pull speed it will change by itself to OPCLB as GA doesn't have CLB mode. If you recycle THR LVRs back to CLB it will obviously give less ROC and at ACC ALT slower acceleration.
dream747
5th Mar 2019, 12:14
Anyone knows why there is no CLB mode in the Go-Around phase? I would think Airbus can simply include it, but what's the rationale for not doing so?
In a GA the FMGS is in hibernation, doesn't do any predictions. So managed mode is not possible. Perhaps it's just a transient mode to string back the sequenced approach.
Jim510
6th Mar 2019, 09:06
Sorry, just to be clear - newer aircraft will automatically go to NAV (if you dedided to follow published missed approach), and the CLB mode will engage after reaching ga acceleration altitude?
You would only go to OP CLB if the aircraft is still in GA TRK, or in HDG.
pineteam
6th Mar 2019, 11:27
Sorry, just to be clear - newer aircraft will automatically go to NAV (if you dedided to follow published missed approach), and the CLB mode will engage after reaching ga acceleration altitude?
You would only go to OP CLB if the aircraft is still in GA TRK, or in HDG.
No. CLB mode is not available in Go around phase. you will always go to OPN CLB. That’ why it’s really important to monitor the constraint in a go around and to set the first initial altitude.
EGPFlyer
6th Mar 2019, 13:15
No. CLB mode is not available in Go around phase. you will always go to OPN CLB. That’ why it’s really important to monitor the constraint in a go around and to set the first initial altitude.
climb mode is armed if you have NAV in the go around phase. It engages at acceleration altitude and the speed targets green dot. If NAV mode isn’t engaged at acceleration alt then it reverts to Open Climb
climb mode is armed if you have NAV in the go around phase. It engages at acceleration altitude and the speed targets green dot. If NAV mode isn’t engaged at acceleration alt then it reverts to Open Climb Not correct. In GA phase, notwithstanding what is the lateral mode CLB is simply not available. It's designed that way. When armed only CLB is shown even for OPCLB.
EGPFlyer
6th Mar 2019, 14:00
Apologies, you are quite correct. I made the mistake to assume that it was the same as takeoff. The Go around phase remains until you activate the approach/enable ALTN etc
EGPF
Thanks. AAnyway to make it authentic below from FCOM:
The CLB mode can be engaged, if the following conditions are all met:
‐ The aircraft has been in flight for more than 5 s
‐ The selected FCU level is above the present aircraft level
‐ The descent, approach, or go-around phase is not active
‐ NAV mode is engaged
‐ Glideslope (G/S) mode is not engaged.