Is leaving airport at a stop before the final destination illegal?

From an airline staff member, as long as you do not have checked-in luggage to be delivered at the final destination and the sectors are within the same country then it is hard for the airlines to figure that out (they don't care actually).

How will the airlines find out? Sometimes, at some airlines, in some sectors there will be an airline representative at the plane's door to check the boarding pass during deplaning to make sure only people who should leave at this airport will leave, this happens rarely and usually happens in international flights with more than one sector. Also head counting could reveal this. Again, for domestic flights usually no one checks. Just grab your bags and leave if you want to. In case a representative stopped you at the door and asked you to show your boarding, this is one of the good times to play dumb and say you thought you reached your destination! one more thing, as long as they assigned a representative to check boarding passes this means they don't want to make problems, they just want people who should leave to leave and people who shouldn't leave to stay :)

Is this legal or not? This depends on the country (or state) and airlines rules (I think it is not allowed everywhere). In my country (and I can confirm this in some other countries as well) if you want to leave the plane before you should leave it, it is a problem! you will be questioned and held at the airport until the plane reaches its final destination. Again, this usually happens for people who come to a flight attendant and ask to be deplaned, such as people with aviophobia who decide to cancel flying at the last moment or people who got a call or something before the departure and ask to leave the airplane.

What to worry about? If you left the plane a sector or two earlier and the plane took off and never landed (crashed)! and you show up alive, you will be the first suspect! Even though this is really unlikely but you have to bear this in mind.

I personally have done this many times and left a sector earlier, but I wouldn't tell you to do it or not, you have to decide this yourself. Off the record, it is really unlikely (rare actually) to be caught. I suggest that you talk to the ground agent who will board the aircraft at the stop, ask him nicely and most likely he/she will help.

You have legal and you have ethics. As already mentioned in earlier answer. It is more a civil case. So then the question is, is it ethical.

I remember reading about these tricks in 2011 in an article in the NY-times while being in the air. The question was not so much whether it was illegal or not, but more on the ethical side of the tricks. It boiled down to the fact that it is as ethical as airlines exploiting monopoly positions on certain routes. In fact by using these tricks you are playing their game more or less.

Did some googling and found both the article and blog I read back then. I would suggest reading both, since the wording is much better then mine in this answer.

It happened to me--or I did once not take the second leg of a two leg trip. I left country A with a transfer in country B for final arrival at country C. My visa for country C was not issued, while I did have a visa for country B. So I just got off the flight in country B. As it turned out, and unbeknown to me, the airline then canceled my return booking since I had failed to arrive in country C. I fussed and argued at the airline office in country B until they finally relented and gave me my return booking.

On another occasion, again involving three countries but a different airline, I again skipped the final leg of the trip, but faced no problems because it was the return portion of the journey.

No.

I believe your question arises from not understanding the distinction between breaching the terms of a contract and violating a national or state law.

A contract is an agreement between any two parties, such as you and the airline. If you violate some term of that contract, then you are in breach of contract. However, in western common law breach of contract is not illegal. It is a state of dispute between two parties which may be resolved in court, but does not have to be.

Laws are quite different in that they are unilaterally proclaimed by the government of a given territory over which that government exercises jurisdiction. Laws are enforced by police and potentially other security apparatus of the state, and in most countries a court is almost always involved in determining whether you have breached and law and what sanction (punishment) to apply.

Therefore, there are two very distinct branches of the law: civil, which deals with disagreements between civil parties, and criminal, which deals with violations of statute law.

Violating the terms of your ticket may lead to a disagreement between and the airline. However, I'm not aware of any country or state which makes it a crime not to complete a ticketed journey.

I would add that there is probably very little your airline can do if you chose not to complete your flight. In common law, the claimant must prove not only existence and breach of contract, but that they have suffered a quantifiable loss as a direct result of that breach. What loss has the airline suffered by you not completing your journey?

Assuming your actions did not cause the airline to suffer a loss, there may be no legal remedy available to them. Of course, that does not stop them from taking other measures, such as cancelling your return journey.

An interesting wrinkle here to consider is if this illegal of the airlines in the first place.

In essence it's the following deal: "I sell you one pound of beef for $10 or I sell you two pounds for $7. However you must eat the beef, you can't just throw it away or give to your neighbor". The key here is that the larger quantity is cheaper than the smaller one, not just in unit price but in absolute dollars.

This really doesn't any business sense at all. It only works because the airlines have quasi monopolies on certain routes and can hence price gauge and operate on a significantly higher margin. The higher margins make this an attractive route for other carriers as well and so there is a clear incentive to get into this market as well.

However, the other carriers don't do this and the question is why? One potential explanation is some sort of agreement: "we stay out of your high margin routes, if you stay out of ours". Such an agreement would likely be illegal.

It's hard to imagine that such a bizarre pricing practice would survive if there were true and open competition and an actual free market.

This only applies for passengers without luggage. For your luggage will continue to the end destination. I had been promised in Peru by a small airline in combination with a KLM flight from Lima to Frankfurt via Amsterdam. They told me I could get off the plane in A'dam but had to be quick to tell KLM there to get my luggage out too. They did not do that...they lost my luggage carrier and suitcase for 3 months-the luggage was retrieved in Frankfurt after 1 month, the carrier never. They wanted to penalise me for it but I refused, although I did not have it in written words from the Peruvian domestic carrier...in the end I could count on a reimbursement for the loss of my luggage carrier...only after 3 months tweets, emails, and on Messenger!!! Now, I think I want to leave in Malayasia in KL, but my end destination is Colombo, also it is a return ticket I am on...Rather than buying a roundtrip from Bali..., as for personal reasons I don't want to go back yet to Colombo.