Pension scheme
Apologies if this is not strictly on-topic, but this might explain a bit about how pension scheme obligations are reflected in an airline's market capitalisation.
If it is generally known that a company owes a large amount of money to a pension scheme this has to come out of future profits and the market cap will be lower to reflect this.
Consider a company that might normally be worth 4 bn pounds based on expected profits.
If it's public knowledge that it's behind in its pension payments to the tune of 3 bn pounds, the share price and thus market cap will be knocked down to somewhere *below* 1 bn. It'll be *below* because a big future liability makes it difficult for a company to run its normal business. In the case of BA in the past the pension debt meant it wasn't able to arrange financing to purchase new planes and grow / renew its fleet of planes.
Of course, the amount that a company is deemed to owe its pension scheme is a complex calculation - and makes a large number of assumptions about stock market returns. The key is that pensions were traditionally defined benefit - meaning your employer promised to put enough cash into the pension scheme so that you were guaranteed e.g. 20,000 pounds income per year - i.e. the employer takes the risk over stock market returns between now and your retirement date.
If the stock market takes a big tumble as has happened recently, the value of the pension pot takes a fall and thus the company is considered liable to cover the shortfall and so owe a lot more money into the pension pot. There are of course techniques to look at long term averages, but this should give the rough idea.
Some of the large legacy carriers have been described as pension schemes that happen to run an airline