Excessive deviation Go Around in Cat 2 on ATR 72?
 

Hi!

When you fly a cat 2 appr. in an ATR-72 and go around due to Excess Deviation (i.e 1/3 of localizer and 3/4 o glide), why do the AFM tell us that we shall go around on the standby horison only?
I know what the Afm tells me but I cant understand why?
What

regards

Bluesys..

They want you go back to the standby incase there was a problem with the "electronics" so the idea is you always go around from the standby on a CATII.

On your next sim, get them to set up the scenario... Don't use the standby for the to around and you could have some fun (I'm sure your sim instructor will know the setup...:hmm:)

Well I know this. But in theory what is the logic behind going around on standby
horison if there is a excess deviation? Go around due to guidance (on stby horison)I understand because its logical.

regards
Bluesys

Any ATR pilots maybe?

Here is my guess, I haven't flown the ATR for a while, so I'll try to recall the best I can.

You could have a drift in the AHRS 1 versus 2. In general case there would be a master caution when AHRS 1 and 2 differ by 6 degrees or more in pitch or roll (pls check the numbers in the book). If it was to occur during a CAT 2 approach, the AP may try to make corrections based on roll/roll rate or pitch/pitch rate, that would not match real life deviations, thefefore the corrections would be inadequate, and an excessive deviation may occur. During a go around you would have to go SBY first, until you can crosscheck both attitude indicators vs SBY, just like you would on AHRS drift during take off.

Flex