How likely might employees working for Singapore Airlines face retrenchment? Should I be worried?

I would say in the 3-6 months things will look bleak for the aviation industry.

Even if the virusis settled in the coming 3 months, people will still be wary of travelling still as the virus is still around and not totally gone.

And as many industries are affected by this virus, spending money on travelling will be the least priority of many people with cash being tight.

You can take this chance to look into other areas and try something that you have passion in. Depending on your current skillset that you have from your work, can consider joining other industries that need your skill set too.
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All industries could face layoffs. low SES lives are like this. bonuses last to take. retrenchment first to reach.

no use worrying. life has to go on. best is to keep emergency cash to tide over if there is a retrenchment, so that your family lives will not be much affected. put off any major non-essential purchases, review any mortgages, installment plans etc, see which ones can be re-negotiated to lower interests.

and may be a good time to prepare and review your CV, just in case you need to post it out.

hope this helps.