In the past year or so, investors have soured on shares of Boeing (BA -0.20%), due in part to worries that low fuel prices will discourage airlines from replacing older airplanes.

There is certainly some truth to the idea that airlines can better afford to keep older aircraft running when fuel prices are low. However, even with cheap oil, flying aging planes only makes sense up to a point. That's a lesson that Canadian low-cost carrier WestJet Airlines (WJA) is learning the hard way.

Old planes are great -- sometimes

In recent years, Delta Air Lines (DAL 2.86%) has led the way in keeping older jets flying. In addition to extending the lives of its own planes, it has also bought and leased more than 100 aging planes that other airlines no longer wanted.

In recent years, Delta has added more than 100 used jets to its fleet. Image source: The Motley Fool.

Delta began this strategy when fuel prices were much higher. It found that the cost savings from acquiring older planes cheaply more than offset the higher fuel consumption of those planes. Lower fuel prices just made this strategy even more lucrative.

One of the reasons why Delta has been so successful using older planes is that it has a world-class maintenance operation that is used to handling a wide range of aircraft types. Yet even Delta has its limits when it comes to older planes. Two years ago, it decided to accelerate the retirement of its Boeing 747 jumbo jets. This meant permanently grounding a number of planes that were not even 20 years old.

For smaller airlines with less maintenance expertise, older jets can cause lots of trouble. The high maintenance costs and lower reliability of older planes can wipe out any savings relative to new jets, even when fuel prices are low.

WestJet runs into trouble

WestJet's experience with its fleet of four used Boeing 767s is proving just how tough it is for most airlines to implement a used aircraft strategy. The Canadian discounter added the 767 to its fleet primarily to support new routes to London. However, with an average age of 24 years, these planes are past their prime.

As a result, WestJet has suffered numerous maintenance issues since starting service to London on the 767 back in May, leading to a rash of long delays and flight cancellations. Not surprisingly, WestJet has had to deal with a lot of disgruntled customers. Additionally, under EU regulations, WestJet has to compensate each passenger to the tune of 600 euros for flights that are delayed by more than three hours leaving London.

WestJet has blamed these reliability issues on the length of the flights to London as well as the fact that the Boeing 767 fleet is new to the company. A PR spokesperson noted that WestJet initially faced reliability issues with its fleet of Q400 regional turboprops a couple of years ago but that these were soon resolved.

WestJet's gamble on older planes isn't working so far. Image source: Wikimedia Commons user abdallahh.

Still, this isn't especially comforting. WestJet's oldest Q400s are just three years old, so comparing that fleet to the 24 year old Boeing 767s doesn't make much sense. Meanwhile, the flights from Canada to London aren't getting any shorter!

Time for new planes?

Earlier this year -- even before the recent reliability issues cropped up -- WestJet began talking to Boeing and Airbus about buying new planes to replace the 767s by 2019 or 2020. That reflected the reality that airplanes don't last forever, and the aging 767s were at best a temporary solution.

WestJet CEO Gregg Saretsky must now confront the possibility that the Boeing 767 fleet isn't even a suitable stopgap. Unless the 767s become dramatically more reliable, WestJet risks permanently damaging its brand by continuing to operate them.

It's possible that this calculus will induce WestJet to end its London experiment and refocus on its core North American market. But a more likely option is that it will look to bring in new widebody planes from Boeing or Airbus even sooner than originally planned.

No matter what WestJet chooses to do, one thing is clear: Even when fuel prices are low, used jets aren't necessarily a great substitute for new planes. At best, they are a temporary solution until new planes arrive -- and sometimes they aren't even that.