A320 ALERT HEIGHT : from theory to practice
 

After having read all the definitions and explanations on Alert height as given in the relavent Airbus Manuals ( not really pilot friendly ) I am trying to decide whether or not I fully understand the precise concept .

As far as I understand, Alert height is nothing more than a Airbus defined cut-off value . The is nothing system related , when we talk of AH. The value chosen by Airbus ( in the case of A320F being 100ft RA) it is just a value that Airbus accessed as being safe to ignore any single failure (relating to systems that affect the auto land capability) and continue for a CAT 3B auto-land. The 100ft RA being a value that from a probability point of view it’s almost impossible to have another failure till touchdown affecting the auto land system .

Assuming I gave the explanation of the concept of AH correctly , then why is it that from the moment the AH is set at 100 ft , Airbus system logic freezes the landing capability at 200ft? ( that is to say if you had a single failure below 200ft RA , CAT 3B would still be indicated . )

Shouldn’t that be occurring at or below 100ft RA?

Alert Height in itself is not OEM related, just the specific value is. On the 737 we used 200ft AH, on the minibus 100ft.

I did search of the FCOM for "200 ft" as well as "freeze". 80 hits, but no trace of what you say. Any reference from your side?

E.g. if the RA2 collapses at 170 ft, is the suggestion that there'd be no mode reversion to CAT 2?

That statement came from a A320 LVO CBT . It was produced by my former employer. Since posting this query I realized that the statement is wrong but I’m not going to bother contacting them to change it. In actuality the freezing of the capability for the 320 family is 100ft. The statement is found in the new Airbus FCOM in section
PRO-NOR-SRP-01-70 . I am pasting the statement here : “ Below 100 ft (radio altimeter), the FMGS freezes the landing capability until LAND mode is disengaged or both APs are off. Therefore a failure occurring below 100 ft does not change the category of the system.“
Having said that the issue is resolved and everything falls into place .

It's probability based. Higher values indicate more robust systems. Theoretically a fail op system could have no alert height but it would require a go around all the way down to the touchdown point.

As for system freezing, I'm not an AB guy but you don't want the pilots thinking about anything other than the runway in the later stages so system downgrades at low altitudes are just a distraction.

You will probably find that below the "freeze height" the list of failures that will give a NO AUTOLAND indication increases.

There is no point reverting to "APPR 1" or the AB equivalent. Better just go-around.

FlightDetent, are your aircraft Fail passive or fail operational?

As I understand it, mode reversions etc aren't really relevant to alert height.

Hypothetically, if you get a mode reversion (from CAT3 DUAL to SINGLE or CAT 2, for example) below alert height you will get a triple click and should go around. However, the aircraft is still capable of completing an autoland and the autoland light will not illuminate.

The autoland warning light is there, unsurprisingly, to warn you that the aircraft is no longer capable of completing an autoland (loss of ILS signal, multiple failures etc) and will only flash in that scenario.

You’ll get a triple click but can continue. You only go around for a red autoland light below alert height.

Ah yes, good point.

Have a look at the diagram called "Failures and Associated Actions Below 1000' for Cat111 Approach without DH" in PRO-NOR-SRP-01-70 which makes it clear that from 200'RA to 100'RA you will GA for Triple Clicks, Amber Caution, Eng Fail, Alpha Floor, AP OFF and/or the Autoland Warning Light (unless visual). From AH (100'RA in the minibus) you will only GA for the Big Red Light. You will still hear triple clicks and single chimes, but you can ignore them.
This calls for a mind set change at AH and careful briefing.
I'm sorry I don't have the nouse to be able to post a pic of the diagram - perhaps someone cleverer than me will.....

Whilst on the subject, what would you all do if you had something minor like a datalink fault ECAM at 900’, before the landing inhibit kicks in? I’ve often wondered why they didn’t make it start at 1000’ in line with the autoland procedure.

I had a SIM instructor pop-up one of the DOOR open failures just below 1000'.
- he wanted us to continue, I was happy - he was the chief-pilot
- he's retired and I am gone.

Below AH, no warnings are presented aurally and on the lights in from of you (MC/MW except the AUTOLAND light). Eg. if you have an engine fire, it will be on the EW/D but you will not have the CRC and the master warning light until 80kts. There is also no triple click below alert height.

Could the answer to that be that such a failure is only a Level 1 and can thus be ignored because no Single Chime?

Engine fire you would not go around for anyway (at least I wouldnt) and the Fcom does not say either it is a go around reason.

The alert height is purely based on statistics. Statistically the chance to loose the other redundant system is so small that you continue.
Yes you still so get the Ecam warning on 40 ft if the engine fails at 90. But you continue.

Pretty sure it gives a chime on our aircraft... it doesn’t really matter, you could replace it with any number of level 2 faults that would have no bearing on the approach and landing.

Personally, if I could quickly assess it as inconsequential then I’d ignore it and land but was just curious as to other’s thoughts.

I’d be interested as to a reference for that info as it’s new to me... recently in the sim they gave us an engine fail below 100’ and we definitely had a chime, just before touchdown

The landing capability freezes at Alert height.
Below alert height you only need to go-around for the red autoland warning light.
Relevant Master warning and caution aurals are NOT inhibited below alert height (those not already inhibited).
The only time you will get the crickets/triple clic below alert height will be loss of both autopilots which will also bring on the red autoland warning light.
The system was designed that below 100'you only go-around for the autoland red warning light/ No thinking / decision making below alert height.
Failures below 100' tend to present on ECAM at about the start of the flare.

Correct. Probably due to the generator coming off line.

But did your mode annunciator change from Cat3 Dual?
If it still says Cat3 Dual - then simply autoland and deal with the new problem when stopped on the runway.

No, below 100’ the landing capablity is frozen. The chime was for the engine fail ECAM which I caught out the corner of my eye.

Major Cleve Saville. You won’t get the triple click with dual AP drop out, it’ll be the cavalry charge. The way I think about is

Red light on it’s own- auto pilot go around
Red light in conjunction with a noise (cavalry charge)- manually flown GA

WARNING ASSOCIATED WITH ILS "LANDING CAPABILITY"
Any downgrading in the aircraft’s capability for automatic approach and landing sounds a triple-click aural warning.

Yes you will get a cavalry charge but the point of the post was about the downgrade in landing capability below 100' There is also a downgrade in landing capability and therefore a triple click whether you hear it or not due to the cavalry charge.