Overspeed protection & low altitude levels off

[alamwuhk2a]

Hi JL,

Great job on the new version B47.1! Here is just my oberservation on the overall software, not trying to report a bug but rather a wish list of the following function.

1. Overspeed protection should not be activated when flaps is > 0
2. When the initial climb altitude is less than 1500AGL, climb phase cannot be activated and the MCDU is locked at take off phase. Quite often in Japanese airports when military traffic is around, ATC will only clear a low altitude as low as 1000 after take off.

Again thank you for the great work!

Jacklyn

[spiliot]

Jacklyn, regarding point 2, do you set the lower thrust redaction/acceleration altitude in the MCDU before takeoff?

[alamwuhk2a]

No. In the real aircraft even the thrust red/acc is above the FCU altitude, the MCDU should activate climb phase once at speed alt*

[spiliot]

Regarding point 2, you're right, I found this reference in the FCOM:

LOW ALTITUDE LEVEL-OFF
If the aircraft is required to level off below the acceleration altitude, ALT* engages
and target speed goes to initial climb speed. The “LVR CLB” message flashes on
the FMA. In this case, the crew should expect a faster than normal acceleration,
and be prepared to retract the flaps and slats promptly

Regarding point 1, I couldn't find a reference to back it up. Plus there's no "overspeed protection", it is either "High-speed protection" or "overspeed warning", two different systems (you may be having an overspeed warning but no high-speed protection).

May I suggest that since the A320 is very complex and we're not all aware of the behaviour in marginal situations, if you think something is wrong, please find a reference to FCOM to back it up. And stick to the correct terminology to not confuse things any more

[spiliot]

[accidental double reply, disregard]

[alamwuhk2a]

Hi spiliot,
You are right on the terminology. Should be High speed protection instead.
Would like to ask where in the FCOM did you find about the High speed protection on VFE, VLE, etc? In the FCOM I can only find protection on VMO/MMO, not any other speed. And remember VMO doesn't not go down to VFE when flaps/slats are deployed.
Also according to my own personal experience on the 320, the Overspeed warning will sound when IAS is greater than VFE + 4 when under G/S LOC mode. The only speed protection I can find in the FCOM regarding VFE is AUTOMATIC SPEED PROTECTION IN V/S (OR FPA) MODE IN DESCENT.

DSC 27-20-10-20
The aircraft automatically recovers, following a high speed upset. Depending on the flight
conditions (high acceleration, low pitch attitude), High Speed Protection is activated at/or above
VMO/MMO.

DSC-22_30-75
AUTOMATIC SPEED PROTECTION IN V/S (OR FPA) MODE IN DESCENT
When descending with V/S mode engaged: If the selected V/S value is excessive (with regards to
thrust and speed), the FMGS maintains the V/S target, but the airspeed increases. When reaching
VMAX (VMO or VLE in clean, or VFE+4 kt), the AP temporarily abandons the V/S target, and
automatically decreases the vertical speed to maintain VMAX.
The same applies if FPA mode is used with an excessive FPA target.

[spiliot]

Would like to ask where in the FCOM did you find about the High speed protection on VFE, VLE, etc?

I didn't, and that's exactly my point Your original claim was:

1. Overspeed(actually High-speed) protection should not be activated when flaps is > 0

I didn't remember reading anything about that protection being lost when flaps are deployed and asked for FCOM reference to back it up.

What you probably mean is that high-speed protection (automatic nose-up) shouldn't occur on exceeding VFE/VLE and before VMO/MMO, assuming this is the case since I never personally run on this situation and can't test now. This is another case where wording makes huge different when making a point

Out of curiosity I did some research on the subject:
It looks like VFE/VLE are soft limits. Exceeding these could cause strain/damage to flaps/slat/landing gear and therefore is an operational limit that should be avoided, contrary to VMD/VNE which are hard limits after which the wings might rip off. The same stands for VMO/MMO, they are soft limits and pilot input can override them up to a safe margin before VMD/VNE even with the high-speed protection active but the aircraft will otherwise limit speed to max VMO/MMO without input. This is like what happens with bank angle protection, the soft limit is 33 but the pilot can exceed it up to the hard limit.

So while exceeding VFE/VLE would trigger the overspeed warning, it won't trigger a nose up from the high-speed protection, except during descent in the conditions mentioned in your excerpts.

I'm planning to ask a pilot friend of mine to confirm this, although it will take some time.

[alamwuhk2a]

I don't see any words about "soft limit" in the FCOM. As quoted above, High speed protection is only activated when speed is VMO + 4 or MMO + 0.006. VFE/VLE have nothing to do with it.
Probably your pilot friend won't be as stupid as me to have the experience of running into VFE...

[spiliot]

I use the terms soft/hard limits to explain my interpretation of the behaviour, I didn't claim it is airbus introduced terminology.

To be perfectly clear, high-speed protection can be pilot overridden, quoting the FCOM:

[...]Therefore, in a dive situation:
· If there is no sidestick input on the sidestick, the aircraft will slightly
overshoot VMO/MMO and fly back towards the envelope.
· If the sidestick is maintained full forward, the aircraft will significantly
overshoot VMO/MMO without reaching VD/MD. At approximately VMO +
16 / MMO + 0.04, the pitch nose-down authority smoothly reduces to zero
(which does not mean that the aircraft stabilizes at that speed).

And as you found, VFE/VLE are also taken into account during descend in a selected VS/FPA mode, so they are just as "soft" limits as VMO/MMO is (under those conditions). The "hard" limit that will always be enforced is VD/MD.

You haven't confirmed that exceeding VFE/VLE after takeoff in JH causes a pitch-up command but I guess if it does it is wrong.

[alamwuhk2a]

I get what you mean by "soft" or "hard" limit. But please don't confuse other people with these non-standard terms. As you said the airbus is a fairly complicated machine, sticking to FCOM terms would aid people's understanding.
Regarding the high speed protection with slats/flaps>0, I can confirm that in JH B47.1 it's incorrectly modelled, in contrary to real aircraft(my personal experience) and as stated in the FCOM.
Would be glad to see improvement on the software. Again good job JL!