Does it still make sense to book domestic flights with partner miles?

Each of the big three legacy U.S. carriers — American Airlines, United Airlines and Delta Air Lines — belongs to a different airline alliance:

• American belongs to Oneworld.
• United belongs to Star Alliance.
• Delta belongs to SkyTeam.

Within each alliance is a network of airline partners. Often, you can use a variety of airline currencies to book the same flight — each with a unique award price.

This means, for example, that you can book flights on American Airlines even if you have zero AA miles. You can use Alaska Airlines miles, British Airways Avios, Iberia Avios, Qantas miles, etc.

For a long while, this "trick" has been supremely useful for booking domestic award flights for similar (or cheaper) award prices than actually using the proprietary currency of the airline you're flying.

However, that doesn't often seem to be the case right now, thanks to U.S. airlines' adoption of dynamic award pricing.

Related: What are airline alliances, and who's in them?

Dynamic pricing working in favor of domestic flights?

"Dynamic award pricing" is a fancy way of saying that an airline has removed its award chart and reserves the right to charge you whatever award price it wants. The result? Your short flight to grandma's house might cost 10,000 miles one day and 60,000 miles the next.

All three major U.S. airlines now use dynamic pricing — prodding many of us to book our domestic flights with the miles from airlines that do enforce an award chart. For example, a round-trip domestic coach flight on United Airlines will cost:

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• 25,000 Singapore Airlines miles.
• 30,000 ANA miles.
• 15,000 Turkish Airlines miles (if you can find availability).

These are cheaper than the exact same flights on United's website in many cases. But lately, we've noticed heavily discounted awards between an astonishing number of domestic destinations. These are inexplicably low fares — often half the price they once were back when award charts were still implemented.

As these awards become more prominent, partner awards become less practical. The odds are rising that you'll find an absolute steal to just about anywhere if you've got a stash of American, United or Delta miles.

Related: Complete guide to American Airlines Web Special awards

A few quick examples

Here's a look at booking the "Big Three" U.S. airlines with partner miles compared with their own miles.

Delta flights with partner miles

Delta has largely led the charge on dynamic pricing and cheap domestic award flights. Here's a flight from Miami to Dallas for as low as 5,500 Delta SkyMiles.

Booking the exact same flights via Delta's partner Air France-KLM Flying Blue, you'll spend 14,500 miles. The prices of these two websites used to be consistently competitive. Now, I find Delta consistently offers the cheaper price for my desired travel dates.

American flights with partner miles

One surefire way to secure inexpensive domestic award flights on American Airlines has been through British Airways. British Airways prices its awards according to the distance of each segment you fly. For domestic American Airlines awards, you'll pay:

• 7,500 British Airways Avios for flights traveling 650 or fewer miles.
• 9,000 British Airways Avios for flights between 651 and 1,151 miles.
• 11,000 British Airways Avios for flights between 1,152 and 2,000 miles.
• 13,000 British Airways Avios for flights between 2,001 and 3,000 miles.

Related: Maximizing the British Airways distance-based award chart

After many searches, we find that it's quite easy to match or beat the price of short-haul American Airlines flights by using AA miles instead of British Airways Avios. What was once a sweet spot is now only anecdotally cheaper.

And on long-haul domestic flights, American Airlines often blows British Airways out of the water. Here's a flight from Columbus, Ohio, to Ontario, California. This is a nearly 2,000-mile trip — and it costs just 6,000 American Airlines miles.

The same flight booked with British Airways Avios is more than triple that price.

American considers these cheap fares Web Special awards, which is their terminology for award flights priced dynamically and not following its existing award chart. Because of this, you may not find traditional saver award space on these same flights, which means it might not be bookable with partner currencies, like Avios. We've also found that many ultra-cheap American fares have long layovers.

United flights with partner miles

Concerning United Airlines, we're finding that partner awards are still often a better deal than using United Airlines. Many searches on partner sites Avianca, Aeroplan, etc., yielded awards that were consistently 2,000-plus miles cheaper than what United offered. Partner awards appear still viable with Star Alliance, but you should always check both United and its partner to cross-compare the best deals.

One thing we have found with United is that its cheap awards often do not have saver space that's bookable with partner miles. For example, you can book a flight from New York to Houston for 9,200 United miles on this sample date. Here are two sample flights, but there are many more times available.

In theory, it should also be bookable for 10,000 LifeMiles, but only one flight is bookable per day on this sample date. Meanwhile, almost all frequencies are bookable with United miles.

Related: 6 award chart 'sweet spots' that will save you money on domestic flights

Bottom line

Since its inception, dynamic award pricing has meant two things:

1. Those with rigid travel dates and destinations have often had to pay exorbitantly for award flights.
2. Those with flexible travel dates and destinations have benefited from super-cheap (yet scarce) awards.

But lately, it seems that super-discounted domestic awards are popping up everywhere — and for more widespread dates than ever. If this trend continues, the need to use partner airline currencies to book our domestic flights becomes not just unnecessary, it becomes unsavory.

Likewise, you might sometimes find it harder to book with partner miles, as the operating carrier may not open traditional saver award space.

Of course, it doesn't hurt to have transferable points like American Express Membership Rewards points in your wallet. These transfer to the likes of British Airways Avios, Avianca LifeMiles and Flying Blue, giving you more flexibility in how you redeem.

But if you've got a healthy mix of airline miles from the big three U.S. airlines, you shouldn't have a problem booking domestic flights for a reasonable price.