Hey there!
I am fairly new to the game and was wondering if anyone knows what happens if Staff pay is reduced below the national average? Although -10% were removed from the pay there has been no change in happiness?
(What does happiness effect anyway?)
Thanks for your help! 
Welcome!
After removal of 10% from the payments the mood of the staff will get worse over time. The effect of unhappy staff - would you like to be on a flight with angry flight attendants? 
The assessment of your flights will also decrease.
And of course the effect will only be visible on every payday, since your workers only care about their actual salary and not the "predicted" one. So you have to wait until your week closes.
I don’t know if it’s still the case, but when it happened to me, getting staff mood back up again once it went in the red was an extremely slow and expensive business 