# Why don't 747s use unreliable, but cheap, jet engines instead?

I'm a neophyte who was reading a little bit about jet engines, and the economics behind them. In particular, I was curious about what makes jet engines so expensive compared to piston engines.

A recurring theme that came up, again and again, was the cost of quality control, and related factors like product liability. Jet engines are designed to be ultra-reliable under conditions of high stress, and understandably that drastically adds to the cost of manufacture and design.

However, what doesn't make sense is why these standards are so universally high, even in the case of jets on multi-engine craft with redundancy. It would seem that you definitely want reliability on a single-engine craft, a multi-engine that can't fly when it loses an engine, or even a two engine craft.

But for a four-engine aircraft like a 747, why pay so much for ultra-reliability? A 747 can stay at cruising altitude with only 2 engines. Modern jet engines have a failure rate of about 0.01 per 1000 hours. A 747 could use engines with a failure rate of 1 per 1000 hours, and still achieve the same safety profile as an aircraft with only 1 engine of redundancy. (Assuming engine failure rates are independent)

I'd assume that relaxing the jet manufacturing process to allowing 100X less reliability would drastically reduce costs. It seems like we could be manufacturing very cheap four-engine aircraft by allowing for high-failure rate engines. What am I missing here?

• Engines have heavy, rotating parts. If these crack, they cause a lot of damage. That can damage the plane, and kill people. Read about this crash in Sioux city: en.m.wikipedia.org/wiki/United_Airlines_Flight_232. Or this very recent event: avherald.com/h?article=4b7725fb&opt=0 An other incident that was a very near miss was QF32: en.m.wikipedia.org/wiki/Qantas_Flight_32 – Penguin Apr 25 '18 at 12:18
• How much more would you pay for a ticket on the more reliable aircraft? The cost savings would have to be enough that a competing airline using the most reliable engines they can get would not be able to take all your business with an ad that says something like, "Our engines are 100 times more reliable than some of our competitors" while showing a loving family with young children on their way to Disney World. – Todd Wilcox Apr 25 '18 at 17:56
• You wrote, "A 747 can stay at cruising altitude with only 2 engines." But what about engines failing during take-off on a hot day at maximum weight? The desirability of jet engines is not just their fuel economy but their reliability; they are much more reliable than powerful piston engines. – Flynn Apr 25 '18 at 18:24
• I am very interested in where you got the information that a 747 can stay at cruising altitude with only 2 engines. This is definitely NOT true for 747-100/200 aircraft, and though I cannot from personal experience say that it is not true for -300, -400, and -800 variants, I would be very surprised if it was. Typically multi-engine jet aircraft try to operate at or near their optimum altitude, and an engine failure will make it impossible to maintain the optimum all-engines-working altitude. – Terry Apr 25 '18 at 19:01
• Yes, I'm all set to go for a flight on that airline, You don't buy tickets, you buy chances. – tj1000 Apr 25 '18 at 22:19

The cost of a jet engine is mainly dictated by the operating conditions: high temperatures (sometimes above the melting point of the metals used) and high rotation speeds. This means you need high-performance materials even for a low-reliability engine: if you compromise material performance too much, it'll fail on the first start.

Failures are risky. Having to shut down an engine mid-flight is one thing, but we've had some recent failures where chunks of metal were launched at high speed into the wing and fuselage.

The industry has gone the other way: by making the engines more reliable, it's now possible to build a long-range airliner that uses only 2 engines instead of four, reducing the purchase, maintenance and fuel cost. It makes little sense to set up a special B-quality production line for a single aircraft type that's being phased out anyway.

Failures are costly. An in-flight failure means taking the aircraft out of service to repair the failure, which means having extra aircraft on standby to replace it.

• An in-flight failure that merely results in the aircraft being taken out of service for repairs is your best-case scenario. "Substandard plane crashes over Atlantic, killing 100 passengers" is the kind of news article that could very easily kill an entire airline... – Shadur Apr 26 '18 at 9:49
• Ask the good folks on Southwest 1380 how important reliability is. Sure, most of them survived, but how many people would fly an airline where you're lucky to survive every flight? – Machavity Apr 26 '18 at 13:59
• Passed out sounds rash? There not selling well currently, but the B747 and A380 likely stay very long with us. Both jets work well and the freighter model of B747 is demanded, old models are re-used. Sadly the average-cost and no-hub concept are working against them. – Peter Jul 10 '18 at 12:43

Your assumption that failure rates of identical engines in identical service are independent is probably not true. If it was true it would take a lot of proving, and the cost of that would end up adding to the costs of the engines. Failure is not random but depends on age/wear -- the classic bathtub curve demonstrates that. Then the load on the other engines increases if one or two fail, increasing the probability of additional failures.

On top of that think of the bad press for the airline which keeps having engine failures. Even if the passengers on that flight didn't notice, the ones waiting for it to be fixed before they could take off would. The knock-on effects on airline and airport operations of even a minor fault can get very expensive very fast too.

• Also note a minor fault causing a delay in departure can cause lots of expenses on airport taxes and (almost ever) will cause a domino effect because that airplane is expected to keep going from hub to hub on a tight scheduled time – jean Apr 25 '18 at 19:10
• It's worth noting this has happened in real life: bad maintenance caused all 3 engines to fail during the course of the flight. One of the flight crew even radioed: "We believe it to be faulty indications since the chance of all three engines having zero oil pressure and zero quantity is almost nil." – TemporalWolf Apr 25 '18 at 19:56
• @TemporalWolf: "O-ring" reminds me of... Challenger?? – user541686 Apr 26 '18 at 4:49
• – a CVn Apr 26 '18 at 8:25
• @Harper but even in reliable engines failures can't be assumed to be uncorrelated, which is what TemporalWolf's comment demonstrates – Chris H Apr 26 '18 at 15:12

"What am I missing here?" is a good question to ask. Cost of manufacturing is not the only cost involved, as other answers mention. Other costs include:

• How does the engine fail? Does it simply drop in performance or stop operating, or does it throw parts or blow up, damaging the aircraft? This may result in requirements for additional engines, kinetic shielding, etc. with impacts on payload, fuel consumption, etc.
• Regulatory risk. The FAA (or other national and/or local regulatory authorities) may, to protect passengers, aircrew and the people and property that airliners fly over:
• Deny type certification
• Deny permission to operate these aircraft
• Restrict the allowed usage:
• Routes, e.g. over-land only, not allowed over major cities (think 9/11)
• Cargo-only vs. passengers
• Business risks (reputation, lawsuits, marketing problems after a failure)
• Advertising costs (to persuade passengers to use an airline after a string of engine failures)
• Difficulties in recruiting good mechanics and aircrew
• Increased maintenance
• Passenger risk. IIRC when a jet engine breaks free of a mounting pylon, it will tend to accelerate hard in a curved path and may 'shoot down' the aircraft it came from
• Liability due to parts of the engines falling on people
• In relation to the point about being restricted on certain routes, you could mention that certain A320neo's with PW1000G engines have had their ETOPS certification reduced because of poor engine reliability. Instead of being allowed to fly 120 or 140 minutes from a diversion airfield, the route cannot be more than 60 minutes away. That could costs airlines money, if they need to lease other aircraft, or swap engines, meaning more maintenance costs, and lost revenue with aircraft on the ground, not flying. See: avherald.com/h?comment=4b4c4509&opt=0 – Penguin Apr 25 '18 at 18:50
• Regarding your point on passenger risk and engines breaking free, you might want to compare El Al Flight 1862. – a CVn Apr 26 '18 at 8:34
• Often engines do not breakaway at the intended shear points, and tear away other parts critical to flight, like the slat hydraulics. When the flight crew has a heaping serving of "deal with broken engine and changed flight behavior" on their plate, they simply don't have time/ bandwidth to deal with unforeseen side effects. – Harper - Reinstate Monica Apr 26 '18 at 15:14

Besides all the very good reasons in the other answers, one that has not been mentioned is that if you increase the failure rate by 100x, you increase the rate of replacing engines by 100x.

You can see in this example how an engine change takes several hours by several mechanics, with specialized tools.

Let's take your figures (I didn't check them) that a jet engine fails at around 100,000 hours. That's more than the usual lifespan of most airliners. Now you want to park your plane for, say, 8 hours, not once in 100,000, but once every 1,000 hours. On a regular lifespan of 100,000 hours, now engine changes account for 800 hours of downtime. The cost to the airline for each day the aircraft is parked is enormous.

And then you have to account for all the aircraft shuffling every time a new engine fails. And you have to account for all the logistics to have 100x more engines available for replacement, and 100x times more crew. In the end, it's hard to think there will be any savings.

• 100x more crew is an overstatement. With current engines, you still need crew, and they don't spend all of their time doing engine replacements. Maybe you mean 100x more crew time spent on this task that (I think) currently accounts for only a small fraction of total maintenance/repair time. The rest of the answer is good, though, working through the numbers that other answers only hinted at. Good point that current engine lifetimes are more than the lifetime of an airliner, so this quantitative change in replacement rate could have a qualitative effect. – Peter Cordes Apr 26 '18 at 0:17

# You are missing downtime and unpredictability

An air liner typically spends about 16 hours of every 24 hours per day in the air... for its entire lifetime. Every minute that an airliner spends on the ground, it is incurring cost, not revenue.

An engine failure means you need to pull the airliner out of service, get a replacement to take its slot, and put the failed plane in the shop to put in a new engine. Not only does this mean your regular plane goes out of service, you also need to have replacement planes standing by to take their place.

This — by the way — is why energy from renewable sources are not becoming any cheaper despite the hardware becoming cheap. The unreliable nature of wind and solar means you 1) need to have gas or other fossil energy available to jump in when the wind dies down, or you have poor solar availability 2) when everything is working fine you have too much energy and need to — in extreme cases — pay to be rid of it.

In the end it makes much more economic sense to have something that is slightly more expensive but reliable and predictable as opposed to something that is cheaper, but unreliable and random and that disrupts normal operation. Keeping things running smoothly is almost always preferable to being subjected to random interruptions.

• On alternative power, that is being greatly mitigated by both energy storage and load dispatching, e.g. "Your tank water heater can put off its upkeep cycle for 5 minutes". Hydro plants are retooling to get better at storing power, e.g. Oroville only had 3 reversible turbines but they are making all 6 reversible, they already have a massive lower pool for that purpose. We'll figure it out. – Harper - Reinstate Monica Apr 26 '18 at 15:27
• @Harper That was word salad, and not of relevance to this post. "We'll figure it out". That promise has been made since the 1970's. for over 40 years people have said "Tomorrow renewables will deal with all our needs". That is not a bankable statement any more... – MichaelK Apr 26 '18 at 15:34
• It's only word salad to people who don't know the field, i.e. who should not be talking about the field. – Harper - Reinstate Monica Apr 26 '18 at 17:10
• @Harper Get back to me after it has been figured out and is in operation. Words and hot air does not make energy. – MichaelK Apr 26 '18 at 17:12
• Not without a turbine anyway. – Ruadhan2300 Apr 27 '18 at 8:28

One way to look at current reliability metrics is ETOPS. Twin engine airliners are licensed to fly remote routes by meeting ETOPS standards. This dictate how many minutes away from a diverting airport they can fly, and it's based on the percentage likelihood they'll have an in-flight engine shut-down in 1,000 hours of flight. They need to achieve:

5% per 1000 engine hours for EDTO 120 minutes.

2% per 1000 engine hours for EDTO 180 minutes.

1% per 1000 engine hours for EDTO beyond 180 minutes.

Now most airliners don't meet these standards, and they aren't required for four engined planes (which I believe have different standards), but for the sake of this analysis lets assume 747 engines are at 5%.

A rough range of pricing for jet engines for commercial airliners is \$12M to \$35M, a 747 is obviously fitted with engines from the high end of the scale, so lets assume \$120M for a set of four. Assuming a 20 year life span, that's$6M a year in depreciation.

Now our new engine company (SuperCheapTurbines!, inc) shows up with our super duper low cost engines that provide the same performance as the \$30M engines for only \$1.5M each. The only trade off is they average a failure once every 1,000 hours instead of 20,000 hours but the airline will save $5.7M a year in depreciation. Note: I'm using 20x cheaper and a 20x higher fail rate for a more realistic comparison. So how many do we sell, and do we decide to go public immediately or sell out to Boeing for billions? The answer is none, no, and no. Every airline declines our engines because paradoxically they say they are too costly! Think about how that 747 is being used. Assume it's averaging 12 hours of flight time per day, and typically for a single long distance flight with an average of 300 passengers at \$500 each in revenue. So it's generating $150,000 per flight/day in revenue, or over \$50M a year. When it flew with the expensive engines, their failure rate meant it had an engine shutdown in flight roughly once every 400 days.

But with our engines fail about 10% of the time every 100 hours, and given there are four of them, the 747 would have a 35% chance of a shutdown engine every 100 hours. Here's the problem, while often airlines have the discretion to continue a flight with one engine out on a four engine airliner given the massive reliability of current engines, it's still a dangerous situation. If another engine goes out it will become a full fledged emergency, but for 5% engines a second engine failure would only be expected once every 1,000 - 2,000 single engine out occurrences.

But given how unreliable our engines are, there is virtually no chance regulators and airlines would permit flights to continue with only 3 engine operation, they would flag it as emergency requiring immediate abort of the flight. If one of our engines fails, there is roughly a 1 in 60 chance a second engine will fail on the same flight. So every engine out for a 747 equipped with SuperCheapTurbines! is likely an aborted flight.

And we'll have a lot of aborted flights. SuperCheapTurbines! equipped 747s will average an engine out once every 300 hours, or once every 25 days, costing over \\$2M in lost revenue a year, and millions more in replacement engines. And that's just the start, every abort creates a huge amount of additional costs:

1. Repairing the engine.

2. Carrying more expensive backup 747s.

3. Lower fares because of the airlines terrible reliability.

4. And occasionally an engine out situation cascades into a triple engine out or other emergency, losing a plane with crew and passengers and costing the airline hundreds of millions of dollars in settlements and in replacing the plane. And further in reputation.

Essentially this is why air travel has advanced so much from the propeller days. Back then to fly long distances safely designers added more engines, but the more engines they added the more likely an engine out situation became, or worse, an engine fire. Powerful, reliable jet engines have allowed airlines to switch almost entirely to twin jets that mathematically have far fewer engine out situations than three and four jet engine airliners, which means they can fly long distances safely even over water.

Note: I skipped over some things here, including normal maintenance costs which I assume are similar, and the higher interest costs for financing more expensive engines because I don't think they change much in the math.

There are several things wrong with this:

1. An unreliable engine will require more maintenance, which is really expensive and will almost certainly more than wipe out any savings over the life of the program.

2. A cheap, unreliable engine is probably also not as fuel-efficient, thus costing more fuel.

3. As Terry (a former 747 pilot) mentioned in the comments, your assumption that a 747 can maintain cruising altitude on 2 engines is not correct (or really anywhere close to correct.) With 2 engines, it would have to fly much lower, which means more drag which means more fuel burn and less range. If an engine is lost, the aircraft would still need to divert, which also means lots of extra cost to the airline and lots of mad passengers stuck somewhere in Canada until a replacement aircraft can be sent.

4. Advertising. "Our engines blow up 100x more often than our competitors'" is likely not an effective advertising slogan.

So, all told, this would be a false economy and would probably actually end up costing more in the long run.