I see a whole lot of positive and negative talk on this board, and lots of re-posting of various filings, but nothing around the VALUE folks are ascribing AIR CANADA.   So here is a little stab at it.   
How is an airline valued?    Typically EV/EBITDA or EV/EBITDAR (where R is aircraft rent). What does this mean?  EV is Enterprise Value (Equity market value + net debt) and EBITDAR is an airline specific tool because it adds back aircraft rent (R), but also adjusts for leases in the EV.  The EBITDA is earnings before interest, depreciation, taxes and amortization. 
So as at December 31, 2019, AC was valued at 4.5x EV/EBITDA, with total EV of $16.5 billion less net debt of $3.7 billion equaled the equity value of $12.8 billion (or $48.50 per share year end value).   The EBITDA was $3,636 million.  This 4.5x premium multiple (for an airline) was deserved given the many positive steps management had taken.   
Let’s move forward.  Free cash flow was negative $393 million in the first quarter, so net debt climbed to $4.17 billion.   
The EV has now fallen to $8.9 billion (assume $18 share price), given net debt of $4.17 billion.   National Bank assumes that AC burns $3.6 billion in 2020, so that would add another $3.2 billion ($3.6 billion less what they burned in Q1) to the net debt, which would end 2020 at $7.3 billion.  
The 4.5x EBITDA multiple they received at year end 2019 differed in prior years.  The year-end trailing EV/EBITDA multiple for AC from 2014-2018 was 3.8x, 2.6x, 2.7x, 3.2x and 4.2x, respectively.   But the company had earned the respect of the market due to its performance.   
The airline is expected to shrink its capacity thru fleet reductions and its expense budget, so if we assume revenue can get back to $15 billion (about 2015 levels) on a run rate basis by mid-2021 (perhaps  optimistic) and they generate an EBITDA margin just 200 basis points below 2019 levels of 19% –what would that imply for EBITDA?    About $2.6 billion.  As a side note, full year EBITDA margins have varied from 12.6% to 19.0% from 2014-2019 for Air Canada.   
What multiple does one put on that figure?   The low of 2.6x from 2015?  Or 4.5x from 2019?   Let’s assume 4x, closer to the peak than the trough.   That means an EV of $10.4 billion.    
If you assume that cash burn just ends at year end 2020, and cash usage is flat to mid-2021,  the net debt of $7.3 billion would be deducted from the EV of $10.4 billion to arrive at the EQUITY VALUATION, based on 4x EBITDA of $2.6 billion.    THAT FIGURE IS $3.1 billion, or $11.48 per share, or 34% below current levels.  
Ultimately, to buy AC stock, one must think these assumptions are ridiculously conservative, but recognize that Air Canada’s EBITDA in 2018 (another good year) was $2.851 billion, just $200 million above that mid-2021 run rate figure of $2.6 billion.   If you think that Air Canada can get back to within 10% of its prior peak EBITDA ($3.27 billion), despite a smaller fleet and fewer employees, and put 4.5x on that, less net debt of $7.3 billion, then the EQUITY VALUE would be $7.4 billion, or $27.46 per share.
Isn’t it interesting that the BAY STREET analysts have targets around $20-28, and the share price has been oscillating between $14-20.    The market is relatively efficient and had already figured this out. Personally, the scenario for the $11.48 target is likely more realistic, when you read the IATA, BOEING and other recovery forecasts.   BAY STREET analysts want banking business from AC, so they can't be too pragmatic or pessimistic. Keep this in mind when you read the reports. But most are competent.  
For those that made it thru this long-winded analysis,  you can see the role the increase in net debt made to the EV.   It swallowed $13.33 per share ($3.6 billion) in ENTERPRISE VALUE.   ALWAYS RECOGNIZE THAT EQUITY IS ONLY ONE PART OF THE CAPITAL STRUCTURE – THE OTHER PART IS DEBT.  
I don’t have a dog in this hunt, but as a retired old guy, trying to do my part to ensure retail investors have at least some perspective on what they are buying when they part with their hard earned dollars.