Canada Documentation Question?
 

Can my mother-n-law who is Canadian travel by air to Canada with her expired passport and her current Green permanent resident Card?

I realize it is less a customs quest and more a airline policy question. As Canadian would let her in and the US would clear her with her green card.

So it all boils down to what the airlines would do.

A permanent resident card is not acceptable - only a current passport is - for entry into the US. An airline will require the expiry date of the passport and not issue a boarding pass if it is expired.

You do not need the passport to get back into the US, my wife always puts hers away before leaving Canada and only shows her green Card to immigration. The state departments web site even says green card is OK for air travel. The issue is Airline policy. And I just wonder if anyone has tried that recently. Mother n Law did go back to Canada about 5-6 years ago by air using only her Green card and Birth certificate without issue.

When I return to YVR there has always been a passport check at the gate prior to boarding regardless of what US city I depart from. It is done at the counter and the information is checked with the information in their system, at least that's how UA does it.

Again that is airline policy, you have already cleared US immigration and are in a sterile US environment prior to boarding. I assume the airline only should only care about identifying you at that point not what documents you have? I assume you could use a DL at that point. But the problem isn't any law but the common practice as we all well know.

You should check with the airline that she will be flying. Whenever I travel on Air Canada or United, they check my travel documents (passport or NEXUS card) prior to leaving the US. They don't want travelers to get to the destination and then not be allowed into Canada. If she still has some time before her trip, she should try to renew her passport before leaving. I renewed mine a few months ago and got it back in about 3 weeks.

Canada Documentation Question?
 

You may or may not be correct about the State Dept entry requirements but airlines are a different story. If they want to deny boarding without a valid passport, they can and will. No point arguing with them as they are usually strict about their rules (which again, may not coincide with govt rules).

TSA says that the US Permanent Residence Card (GREEN) is acceptable ID to fly in the USA.

http://www.tsa.gov/traveler-information/acceptable-ids

So getting on the plane in the USA should not be an issue.

While the Green Card might be sufficient to get past the TSA security screening, the airlines that I've flown (Air Canada and United) also check to make sure that passengers have valid documentation for entering Canada. Here are Air Canada's requirements. I still recommend renewing the passport if there is enough time. Even if you call and someone from Customer Service says that it's OK to use an expired passport, a gate agent might still refuse to allow you to board.

The US WHTI requires US airlines and airlines serving the US to do certain things that are independent of what is required of government employees to process someone for immigration/customs admissibility at a US or Canadian port of entry.

Airlines have to first make an effort to comply with the government; then the passengers have to comply with satisfying the airline to get checked in and transported; and (usually) after (at least some of) that mentioned previously has taken place, then the passenger has to satisfy US CBP and/or CBSA.

So three different but related things going on that have to be ordinarily cleared by different or multiple parties.