Charges from an airline that are separate from the base fare. These include fees like baggage charges, seat selection, in-flight food, and lounge access. Crucially, incidental fees typically do not include the cost of the ticket itself, cabin upgrades, or mileage purchases.

💡 THE FLIGHT EXPERT TAKE: THE FREE MONEY LOOPHOLE
Many premium travel credit cards offer annual airline incidental credits ranging from $100 to $300. These credits are designed to offset those annoying extra costs that aren’t included in your ticket price.

Pro Tip: These credits are almost always “use it or lose it” by the end of the calendar year. If you haven’t exhausted your credit by December, you can trigger the reimbursement by pre-paying for checked bags on future flights, purchasing lounge day passes, or buying on-board refreshments.

Be careful with services like Wi-Fi. Because many airlines use third-party providers for internet, those charges might not code as a direct airline expense and may not trigger your credit.

A commercial contract between two airlines that allows them to handle each other’s passengers and baggage on a single itinerary. This agreement acts as the “technical plumbing” of the aviation industry, enabling you to book a single ticket that includes flights on different carriers – such as flying United to London and then Lufthansa to Munich – with your bags automatically checking all the way through to your final destination.

💡 THE FLIGHT EXPERT TAKE: BEWARE THE HACKER FARE
Many travel sites sell “hacker fares” or self-transfer itineraries that combine two airlines that do not have an interline agreement. While these offer amazing prices, they are often booked as separate tickets, which means if your first flight is delayed and you miss the second, the second airline is not legally responsible for rebooking you. You could lose the entire value of your ticket and be stranded.

Pro Tip: Always check if your booking is a single itinerary with a single confirmation number (PNR). While a codeshare is a marketing partnership where one airline sells a seat on another’s plane as its own, a standard interline agreement is strictly about the convenience of bag transfers and ticket acceptance. Avoid self-transfers unless you have a massive layover buffer and no checked luggage.

Irregular operations – often abbreviated as IROPS or IRROPS – occur when a major disruption breaks an airline’s normal flight schedule. This includes significant flight delays, cancellations, diversions, or unexpected equipment swaps. When an airline declares IROPS, it triggers internal contingency plans that give agents more flexibility to rebook passengers and waive certain ticket restrictions.

💡 THE FLIGHT EXPERT TAKE: THE RULE 240 STRATEGY
When IROPS occur, airlines have more flexibility than they often admit. If you end up stranded because of a mechanical issue or crew shortage, ask the agent to “interline” you to a competitor.

For example, if your airline cancels your flight, they can often endorse your ticket to a different carrier to get you home. However, they rarely offer this voluntarily because it costs them money – you have to ask for it specifically.

Pro Tip: If you’re stuck, use the phrase: “Can you endorse my ticket to [Competitor Airline] under Rule 240?” While the original federal Rule 240 was phased out with deregulation, many legacy carriers still have similar “Involuntary Reroute” provisions in their contract of carriage.

This allows them to transfer your ticket to another airline with which they have an interline agreement. If the first agent says no, politely ask for a supervisor or try calling the elite status phone line, as they are often more familiar with these manual overrides.

A deep commercial partnership between two or more airlines that goes beyond a standard codeshare. In a joint venture (such as the transatlantic partnership between American Airlines, British Airways, and Iberia), the airlines coordinate schedules, set pricing jointly, and share the revenue from flights, regardless of which carrier operates the aircraft.

💡 THE FLIGHT EXPERT TAKE: Joint Venture or Not?
Airlines don’t usually put a “Joint Venture” label on your ticket; they use codeshares to hide them in plain sight. You can identify a joint venture by looking for three specific clues:
1) The flight is international (JVs are almost exclusively for transoceanic routes).
2) The airlines are in the same alliance.
3) The flight number has a “marketing” carrier – for example, you book “Delta 5246” but the fine print says “Operated by Virgin Atlantic.”
4) The price of the flight is the same across all partner carrier’s websites.

Pro Tip: If you see the exact same price for the exact same flight time across three different airline websites (like United, Lufthansa, and Air Canada), you’re looking at a joint venture. Because they share profits, they use metal neutrality to ensure you pay the same amount regardless of whose website you use. To get the best deal, don’t look at the ticket price, look at the soft product (food and service) and the baggage fees, which can still vary between the partners even when the fare is identical.