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Why do airline manufacturers not prefer using the best engines(max thrust: weight ratio and best efficiency) for their airplanes? Doesn't this just not lead to lower sales and losses for the company?

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I wish to know if all the planes currently in production could be replaced with the best engines available like the GeNX, Ge9X, or the Trent 1000. Would it not lead to better efficiency?

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    $\begingroup$ Boeing put a new engine on the 737 and called it the 737 MAX. It didn't end well. $\endgroup$ May 13 at 12:56

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Why doesn't Airbus certify airplanes with the best engine in the market? For example, the A330 had a General Electric CF6-80E1 engine which was very small for it

Because when the A330 was first developed during the '70s, the General Electric CF-6 was the best engine then available. As easy as that. Newer versions of the A330 are equipped with more recent PW4000 or Trent 700. The latest iteration of the A330 is the "neo" version which is powered by RR Trent 7000, an engine which "received its EASA type certification on 20 July 2018".

EDIT TO THE EDIT IN THE QUESTION

I wish to know if all the planes currently in production could be replaced with the best engines available like the GeNX, Ge9X, or the Trent 1000

Updating something in the aerospace world takes time, money and a lot of paperwork. So much time, money and paperwork that deciding if it's better to update an old airframe or come up with a clean sheet design can be a difficult choice. Market-trend and competitors analyses help with this choice. And the A330 is actually a very good example of how difficult this choice can be: when the B787 was first revealed in 2004, Airbus decided that:

  1. no, it was definitely no threat for the A330.
  2. maybe yes, we can actually propose an improved A330 with some better aerodynamic and engines.
  3. wait! What about updating the engines of the A380 instead?
  4. yes ok, let's redesign the wing, the tailplane, the cockpit and use newer engines and call it A350.
  5. yep, the B787 is definitely going to kill the A330 and the A350 is so far just a joke that no airline is going to buy... let's do a clean sheet design and call it A350xwb.

Bingo! Or maybe not? The airlines actually liked and pushed for both the solution 2. and 5. and Airbus pursued both. The first one has given rise to the A330neo while the second one has resulted in the A350. Given the poor market results of the A330neo it might have been better to pursue the 3. instead of the 2. or maybe start sooner with just the 5., who knows...

Anyway, it is not always that difficult: the choice between continually updating (also with newer and newer engines) the bestselling A320 (last year 571 were produced, i.e. one every 15 hours) or replacing it with a new maybe-better clean sheet design, is a no-brainer.

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  • $\begingroup$ Oh, I guess my question kinda diverted from what I wanted to ask... $\endgroup$ May 12 at 9:08
  • $\begingroup$ @SambhavKhandelwal the math basically is quite simple. As Sophit described upgrading engines is a tedious process requiring planning, new parts for the plane, testing, certification etc. This all adds up to a huge amount of money, and the new engines are simply not efficient enough to make the swap profitable. $\endgroup$
    – Jpe61
    May 12 at 10:23
  • $\begingroup$ @Bianfable the A330neo has been so disappointing in terms of selling that I didn't even considered it as being its successor 😄 I'll look for some more pertinent example $\endgroup$
    – sophit
    May 12 at 18:22
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    $\begingroup$ Note that the "neo" in Airbus' naming literally stands for "new engine option". $\endgroup$ May 13 at 7:28
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All commercial aircraft use the best engine technology available at the time when they were being designed. Once the design is settled (after years of work!) an engine change would reset the whole design process and delay the plane's introduction.

This means that airframe designers must work hand-in-glove with engine manufacturers (at times, more than one) to make certain that a new engine design will be ready for installation in a new airframe- to ensure the most up-to-date engine possible for the application. Note that the history of aviation is full of examples where the release to manufacturing of a new plane design was delayed by the unavailability of a promised engine.

There is a natural temptation to upgrade older airframes by re-engining them with the latest & greatest in engine technology, but this involves retrofitting a plane with 10,000 hours on it with brand new engines that won't keep the plane generating revenue long enough to pay off the investment.

So when re-engining occurs, it is mostly done on airframes that are still in production so a new engine goes into a new plane. Note also that the customer buying the plane can specify at the time of purchase which type of engine to install, that best suits their use model.

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    $\begingroup$ Quite a few airframes give the buyer a choice of engine, even for the initial production run. It can depend on the requirements of the customer. $\endgroup$ May 13 at 0:15
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Define "best". Best for what? If there were a single universal "best", nothing else would exist.

As already said, aircraft use engines that are available when the type gets designed (or which are designed in parallel with them). Reengining, while possible, is extremely involved and expensive and tends to not be worth the effort. In fact it may well cost more than replacing the aircraft.

And then there are many operators who will want all aircraft in their fleet to have as much parts commonality as possible. Which is why major manufacturers tend to offer their aircraft with a choice of engines. Say a major blue airline has a long history of operating aircraft with GE engines. They're more likely to buy your aircraft if you offer it with engines of that brand, even if engines made by RR would have slightly better performance (and define that, speed? fuel burn? hours between overhaul?) because in their infrastructure it is cheaper to operate those slightly less efficient engines.

Or you are operating an airline in a country that's under an embargo that prevents you from importing parts for a PW engine, but you can import parts for an RR engine. If your favourite Airbus is only offered with PW engines you now can't buy it.

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In additional to the points raise in the other correct answers, 'best' cost wise does not mean most fuel efficient.

If you have a fleet that you have operated for a decade and know that after every x hours you need to service part y, part z can be done every other service and part q you just measure and replace as required then swapping to a 5% more fuel efficient engine that you need to change all of y,z and q four times as often while you learn how things wear may not gain anything. Labor and parts costs can easily overwhelm fuel savings.

Or you can work to the old processes and kill people. There is also a substantial safety risk from frequent change- The 737 Max crashes were in part driven by desire to update the 737 without changing training and processes. Regular engine updates will mean a long tail of incidents and near misses as previously highly skilled teams have to unlearn old information and learn the new, across maintenance, fueling/planning and actual flying, especially where efficiency has been gained at cost of complexity/performance.

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