There is another thread on this subject, which has not attracted many posts. So I'll repost here what I posted there, with a few modifications:
Aircrew are often exempted under various double taxation treaties. For example, the US-UK double taxation treaty exempts UK-based aircrew from US tax.
This is covered in Article 14, paragraph 3 of the US-UK treaty.
https://www.gov.uk/government/public...a-tax-treaties
For some reason, Hong Kong has no double taxation treaty with the USA, even though it has treaties with many other countries.
You can be taxed pro-rata on how much working time you spend in country, as a non-resident.
See Chapter 3 of The US Tax Guide for Aliens, Publication No. 519 of the IRS (" Services Performed
for Foreign Employer"). Income up to $3,000 per annum is exempted but thereafter income earned while in the US is taxable.
https://www.irs.gov/pub/irs-pdf/p519.pdf