Does Total Income + Less depreciation = Total debt?
Does sales + Gain on sales of Plant Assets = net sales?

Is air traffic liability found on an income statement or balance sheet

No.

Net sales = gross sales - returns/allowances
Total Debt = Short Term Borrowings + Current Portion of Long Term Debt + Current Portion of Capital Lease + Long Term Debt + Long Term Capital Lease + Finance Division Debt Current + Finance Division Debt Non Current

I have no idea where your formulas came from.

For the second post, the liability is located on the balance sheet.