Why bother having different engine options on one aircraft?

Question

When an airline purchases a new passenger jet, they often have a choice of engines (from different manufacturers). Why is this?

I would expect that one of the engine options is "best" and that all customers would buy that option. Thus it wouldn't be worthwhile to certify and offer the other (unwanted) options. Evidently this is not the case, so the question is why.

I can think of two possible reasons for this:

1. Different customers may have different requirements (e.g. shorthaul versus longhaul routes, operating in hot or cold environments) and different engine options have differing performance (e.g. better cruise efficiency versus climb efficiency or better performance in hot/cold temperatures). But I can't imagine that the requirements would be significantly different and, if they were very different, they would buy a totally different plane, not just a different engine.
2. Business/legal considerations: Perhaps airlines have contracts with engine manufacturers separate from aircraft manufacturers. e.g. British Airways may have a special relationship with Rolls Royce (possibly due to trade tariffs). But, again, I would imagine that airlines would always seek the most efficient engines, rather than signing such contracts.

Are either (or both) of these reasons right or is there a totally different reason?

Answer 1

When Lockheed built their L-1011 TriStar, they had one engine option, the Rolls-Royce RB211. RR did not deliver in time, and that has been attributed to the airplane's demise.

Although the TriStar's design schedule closely followed that of its competitor, Douglas beat Lockheed to market by a year due to delays in powerplant development.

So, having more than one engine option lowers the risk of a new airplane launch, and this has the benefit of allowing the airlines to pick their favorite.

Having one engine option on the other hand, like with the Boeing 777-300ER and the 777-200LR/F, allows cheaper certification, but it frustrates the customers:

Even if the 777X programme is successful, the sole-source deal has left some customers frustrated. Boeing argues that it chose the GE90 as the best engine for the 777X programme after its own rigorous competition. But as one airline executive says: "Our response is we'd rather run our own competition."

He adds the suspicion that Boeing decided to take the benefits of the engine choice competition for itself rather than let it accrue to the customer. Also, by eliminating a choice of engines, the company saved the costs of certifying more than one of them, he points out.

Back to your points: Your final conclusion is right, the performance difference is not a factor. For the second point, airlines get maintenance deals, so they like to stick with the manufacturer they already have business with. Getting discounts is easier when there is competition.

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Related and interesting: How much would it cost for a large airline to switch engine manufacturer for their planes?

Answer 2

Engines require a significant amount of specialist maintenence through their life. This means that an airline must train technicians to work on each kind of engine it runs, and maintain stocks of spares for these engines. Engines must also be periodically returned to the manufacturer for rebuilding and refurbishment.

Engines from the same manufacturer will have a certain commonality in terms of parts and maintenance techniques, which means that inventory and training costs will be reduced. Also, because engine maintenance requires significant interaction with the manufacturer (for refurbishing but also for parts and training), airlines wish to be able to chose which company that will be and also to have the opportunity to build up a business relationship with them. Airlines are likely to feel that they'll get a better deal from a manufacturer that supplies them with a large number of engines.

Even if an airline chooses to buy a small number of engines from a supplier, there's still hope of getting a good deal because they can threaten to take their business elsewhere. But if only one engine is available for a particular plane, the choice is "Buy from supplier X or good luck with that gigantic glider", and the customer has very little negotiating power.

Answer 3

I would imagine that there are factors like airline maintenance crew familiarity, national identity (think British Airways switching away from Rolls-Royce engines entirely to only GENX), and perceived vs. real engine reliability and long-term operational costs. There are carriers (and people, the current US president's personal plane being an example of this) who won't fly any engine except the RR. I would imagine that when the next VC-XX plane comes along to replace the 747-200's (which I believe have CFMs in them), the choice will be for either CFM or GE over Rolls. To chose engines which are designed, manufactured, and (primarily) maintained by a non-US company would not exactly look good on the jet which carries the country's chief executive.

The other issues are around very real things like the Trent 1000's problems on the 787. RR has now stated they don't expect to make any money on the deals as the engines are wearing out much faster than expected. GE, on the other hand, seems to be OK at this point. The 787 has had its issues, but they aren't completely germane to the question asked and are more around the fact that the 787 is a completely new way to build an airplane for Boeing.

Now, if you want a book answer (there are many), here's a 6 page long, but meaty paper which covers it: http://ieomsociety.org/ieom2011/pdfs/IEOM109.pdf. In short, short-haul or "budget airlines" tend to have the same engine across their fleet to save on maintenance whereas the "flag" carriers are much more diverse. All this goes to my first point of "airline maintenance crew familiarity". The flag carriers can afford to have many people trained on many types, the budget carriers, not so much. I'll leave you with the prime example of this -- Southwest Airlines. They are so budget conscious that they fly the exact same engine model and type on ALL of their 737s and all 737s are identical with-in the extent made possible by Boeing. This means things like the new HUD display that most 737s have is not allowed to be used in any SWA jet, even when it's equipped. This makes the crew able to go from plane 1 to 2 to 3 to 974 and they will all look the same and will all be type-rated the same. The maintenance departments train on only one finite set of parts for one type of engine with the same specs.

Answer 4

Airlines don't want to buy "engines" at all. From the airlines' point of view, they are just horribly complicated things that they don't understand, and they don't want to spend their own money learning anything more than the information in the Pilot's Handbook!

What airlines really want is "something to power their planes". These days, the engines are often leased, not bought, as part of a "power generation package" which includes long term performance guarantees, scheduled maintenance, etc.

Packages like that are often tailored to very specific requirements. For example in the middle east, some airlines prefer to use higher power settings for the climb after takeoff, so they can get above the airspace that is controlled by a neighbouring country's ATC and avoid delays, extra fuel burn while restricted to low altitude operation, etc.

The only physical difference between two "different versions" of a particular engine type, with different advertised power ratings, may the electronic engine control system. In that situation, the airline might negotiate a "total power generation" deal where they only pay for the "higher rated" engine when they actually use it.

In that sort of business environment, the idea of a "best" engine for every customer is much too simplistic.

Answer 5

ymb1 answer is good. I would add that often there are political reasons: for example currently you cannot sell GE engine to any Russian company for the trade sanction beetween USA and Russia. If you sell an helicopter or airplane with your GE engine... you simply don't. The solution is to use another engine that is not made in USA.

Answer 6

I would expect that one of the engine options is "best" and that all customers would buy that option.

"Best" is a subjective concept.

What is best for one airline might not be evaluated the same way by another airline. One airline might define "best" as whichever viable option is cheaper at the time they make the purchase. Another airline might define "best" based on fuel efficiency, total power output, total life, or some other metric or set of metrics that feed into a total cost of ownership formula. Another airline might define "best" as matching the long-term supplier they happen to have standardized with. Or the engine vendor that does the best job schmoozing the airline executives (really).

Having multiple engine options allows the airplane to work with whichever option the airline customer deems best, and thus increases the value and market for the plane vs only one possible engine. It also increases the utility of the place in case of a supply problem with one engine type.

The idea that "best is subjective" applies to many areas of life beyond just airline economics. It speaks to value choices that all of us make, often subconsciously.

Answer 7

Engines for an aircraft are more consumables than proper constituent. They will be maintained, overhauled and replaced many times during the lifetime of the airframe, all of which require great number of deals and purchases from the engine maker. The airline may want to either strengthen the bond with one engine manufacturer to benefit from synergies, or the exact opposite, diversify engine sources in order to strenghten negotiating position.

So choice of "GE vs RR" for a plane is more akin to "diesel vs gasoline" for a car.

It's not #1, it's #2. But less to do with tariffs and more with pre-existing business deals.