United ‘blocks’ award seats that their partners offer when it doesn’t want to pay for those seats. They’re the only Star Alliance airline that does this. A Lufthansa seat to Europe might be available with Continental, US Airways, or Air Canada miles but United might claim the seat is unavailable.

I’ve been beating this drum for quite some time, not just here on this blog but in other media like Inside Flyer and Conde Nast Traveler, and I’ve helped to drive coverage elsewhere such as in the Wall Street Journal.

Now, blocking wasn’t much of an issue from early May through early August. It would creep up occasionally, but nothing like in the past.

Take the end of April, for instance, virtually no Lufthansa space that the airline was offering was bookable with United miles. United agents would simply say that the seats were unavailable. And agents still give customers false information, that Lufthansa isn’t offering those seats to United when in fact Lufthansa is offering the seats to all of their partners including United but United had programmed their computers not to ‘see’ the award seats.

United has to pay its partners when members redeem their miles for those seats, at least when United members are redeeming more on the partners than those partners’ members are redeeming for United seats. And Lufhansa availability tends to be quite good, much better than availability on United aircraft, so it winds up costing United Mileage Plus a good bit of money. Which they don’t want to spend (even though Mileage Plus has been very profitable for years despite the ups ad downs of the airline industry as a whole and of course United Airlines itself).

Every other Star carrier’s miles have thus been more valuable than United’s miles, because other Star carriers have had access to the seats that United refuses to book. What’s more, United’s award chart is no longe rthe value that it once was. They’ve increased the price of awards significantly, first in October 2006 and then in January 2009. Now the US Airways chart and the Air Canada Aeroplan chart are both better in most cases. And in several instances international program charts like Asiana and All Nippon and British Midland are better as well (though not in all cases). The United and Continental charts have been made pretty much identical, but Continental doesn’t block partner award space (and Continental has much more generous routing rules) making those Continental Onepass miles far more valuable than United miles.

When United blocking mostly lifted in early May, I thought we might be on the right track given the pending United-Continental merger. I was able to book flights that had always been impossible with United miles, such as first class seats Europe – Bangkok on Thai. There were even reports on Flyetalk of folks booking Air New Zealand’s Los Angeles – London flight in business class, something which hadn’t been allowed by Mileage Plus in years.

But blocking is clearly back, take for instance some recent searches where I found award seats on Lufthansa for January 2 and 3.. four seats in business class were available Dusseldorf – Chicago on the former date, and for Frankfurt – Chicago on both dates. United claimed those seats were unavailable.

So blocking is not dead. And it’ll be an interesting question for the future, and to me the biggest single issue as to whether the United-Continental merger is a boon or disaster for collectors of miles in the Mileage Plus and Onepass programs.

Now, it’s fully expected that the merger will be consumated this year. But that the mileage programs will continue to operate separately for most of next year. And during that time I imagine it’ll be possible to move miles back and forth between the two programs, the way Delta-Northwest allowed it and the way that US Airways-America West did as well.

During that time, and assuming that Continental doesn’t implement blocking, the entire thing will be a non-issue — United won’t book the award? Just move your miles to Onepass.

But what abotu when the merger completes? Will they follow United’s or Continental’s lead on this and any number of other issues.

• Blocking is the biggest one.

• Routing rules. United requires you to fly to India via the Atlantic, to Australia from North American to the South Pacific without transiting Europe or Asia, and will only let you travel either on a published routing or within the published maximum permitted mileage for a city pair. Continental imposes none of those restrictions. And Continental will allow a stopover and an open jaw whereas United allows only one or the other. So Continental is more generous in all of these respects.

• One-way awards. United offers it, so far only on their own metal but indicated they’d roll it out to partners as well. Continental doesn’t have this option. I imagine the combined program will offer one-way awards.

• Award pricing. Continental will let you mix and match classes of service, fly to Europe in first class one-way and back in business class and they’ll price hte award at the average of first and business class. United will take the higher award price.

• Combinability of Star and non-Star partners on one award. Continental allows it now, United does not.

On the whole I expect the more restrictive United position to prevail, including on expiring miles where United expires them and Continental does not. Mileage Plus is the bigger program, no doubt it’ll be the name of the surviving program, and the architecture to be more restrictive already exists so there won’t be an IT issue in implementation. I can only hope that Continental miles don’t devalue by adopting United’s blocking practices.

Now there are of course other issues for frequent flyers. To me, the big aircraft issues are whether the combined carriers keep Economy Plus (my bet is they will) and three-cabin international first class (my bet is that they won’t, which saddens me greatly).

And there are of course elite benefits which are different. The programs have already aligned somewhat with unlimited automatic domestic ugprades. And Continental now even has introduced meaningful international upgrade certificates. And United has introduced international mileage ugprades from the lowest fares with a cash co-pay. But there are still significant differences and of course the Onepass top mileage-earned tier is at 75,000 and the United top tier at 100,000. Certainly there will still be a 100,000 mile tier, and I’d be surprised if they followed the US Airways model of having 25/50/75/100.. simply too many tiers even though they don’t want to offend their 75,000 mile flyers. And there will remain a revenue-based true top tier along the lines of GLobal Services, no doubt.

But to me, it all comes back to award blocking. And for lower tiered members, economy plus. That will determine whether the combination is good for members or not. And we’ll have to wait, nervously, to find out.