As a general rule that award tickets do not earn frequent flyer miles since they are paid for with miles, the same way you do not earn
With that being said, there are a few exceptions or situations when award tickets might earn you miles after.
Tickets purchased with credit card points: they are not award tickets
The above rule applies to award tickets obtained by redeeming miles from a frequent flyer program. For example, if you redeem 12,500 United miles for a flight between Boston and San Diego via Chicago, you won’t earn miles in any program because your booking class (X – Economy award) is not eligible.
However, if the cost for the flight is $150 and you choose to use your Ultimate Rewards points (or Citi ThankYou points, Amex Membership Rewards points, etc.) directly with UR travel instead of transferring them to United, then it has several implications: first, supposing you have a Sapphire Reserve, you’ll spend less points (10,000); but even better, you will accrue miles for your trip, since it is not, as far as the airline is concerned, an award ticket and it is booked in a regular economy fare (let’s say K, for example). You could then earn about 650 MileagePlus miles (based on the price paid) or better, 2,590 Singapore Krisflyer miles (based on the distance).
In this case, it is definitely a calculation that you should make before deciding how you want to purchase the ticket: a frequent flyer reward, in our example, would cost 12,500 miles and you would earn 0 miles in any frequent flyer program, while using
Bad luck, good luck: irregular operations
Irregular operations like delays, cancellations, etc. can be any traveler’s nightmare, and rightfully so: the fear of missing a connection, not making it to your destination on time–or even on the same day, having to run to your next gate through a crowded terminal aren’t very enjoyable situations. But, if you are on an award ticket, involuntary rerouting can be quite profitable (in the miles sense).
Why? It isn’t guaranteed to work every time, but you stand a very good chance of getting rebooked in a booking class used for revenue tickets, and thus eligible to earn miles. Most irregular operations rebooking is done in full-fare classes (depending on your class of travel, that can be economy–usually Y, Premium Economy W, Business J, or First F) either per policy or because that is the only class available so close to departure (if the policy is to rebook in the lowest available fare class). Good news: not only will you earn miles, but since you will likely end up in a higher fare class, you could earn a substantial amount of them.
This works like a charm if you are rebooked onto another airline, particularly on another alliance: e.g. if Air France (SkyTeam) rebooks you on Swiss (Star Alliance), then you will definitely earn miles, as they don’t have access to any award space on Swiss.
Generally, even within an alliance, airlines can’t force availability on another carrier, so if you are rebooked on a different airline, you are likely to earn miles.
If you are rebooked within the same alliance you could be unfortunate enough to be rebooked on another flight that had award availability at the last minute, which means that if the agent takes the time to rebook you properly, you may remain in an award fare class and not earn miles. An example of this is you are connecting on an award from Qantas to British Airways in London, the Qantas flight is delayed and they rebook you on the next British Airways flight, but said flight has award space: you will likely end up in an award fare class and not earn miles.
Some airlines have procedures to rebook you automatically by forcing the same fare class on their own flights (Delta, for example)–earnings would then not change. Similarly, if your ticket has been issued by an airline that awards miles according to the price paid (American, Delta, United), you are unlikely to receive miles if you credit to their own frequent flyer program, as the computer would still read a zero value for your ticket.
While it is definitely not a guarantee, irregular operations can actually have an upside and top up your mileage balance. And sometimes on an unexpected frequent flyer account. So have your account number ready if you know which program you want to credit the new flights to!