User avatar
Faro
Topic Author
Posts: 1894
Joined: Sun Aug 12, 2007 1:08 am

Out of Curiosity: Two Different Engines on Same Airframe Safer?

Thu Mar 07, 2019 3:07 pm

In safety-critical nuclear power plant installations, they not only use multiple, redundant equipment installations, but also equipment from different manufacturers. Critical water pumps for reactor core cooling circuits, for example, are sourced from different manufacturers. IIRC computer chips used in Flight Management Systems are likewise sourced from different manufacturers for safety.

Theoretically --and only theoretically-- if different engines with substantially the same weight, thrust, spool-up/down times, etc were used on the same airframe, would it lead to a significantly safer aircraft? Barring of course operational issues which i) would affect both engine installations like flying through volcanic ash and ii) would not arise from engine-specific design/reliability characteristics like bird ingestion.

Just out of curiosity, I wonder whether it would make a material difference in theory. For example for ultra-long twin-jet operations over oceanic sectors with little to no en-route diversions. Could this make a material difference? My preliminary impression is no...


Faro
The chalice not my son
 
User avatar
tb727
Posts: 2140
Joined: Thu Jun 30, 2005 1:40 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Thu Mar 07, 2019 3:25 pm

I think that's an answer to a problem that doesn't exist. Modern jet engines are extremely reliable and well designed. While there have been teething problems with new jet engines, in recent times the Trent 1000 and PW1100G's, that is just part of the process. Just about every engine has had it's problems. Take a look at the bathtub curve in engineering reliability. In the long run engines get more and more reliable and efficient until the next leap in technology and it starts all over again.
Too lazy to work, too scared to steal!
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Thu Mar 07, 2019 3:28 pm

Faro wrote:
Theoretically --and only theoretically-- if different engines with substantially the same weight, thrust, spool-up/down times, etc were used on the same airframe, would it lead to a significantly safer aircraft?


Absolutely not.

While a nuclear powerplant does not need to lift its own weight off the ground and fly, an aircraft does. Minimum weight is crucial, and attempting to mix and match numerous parts, integrating systems from one manufacturer and another will add complexity, weight, and create more procedures to deal with in flight, ultimately decreasing safety.

Aircraft operators know that if their fleet uses CF6 engines, they have one engine to maintain, which makes it easier to manage parts, engine replacement, service, and to ensure a uniform standard of repair and maintenance. Having two different engines on the aircraft would add levels of complexity far beyond simple plumbing and fitting of fuel lines and electrical, hydraulic, pneumatic, etc. They aircraft would require unique systems capability on each side of the aircraft, separate support, separate maintenance tracking, separate interface, procedures, parts, etc. Every one of those comes with a plethora of complexities, each of which is another layer of "swiss cheese." Another opportunity for a problem.

On a given engine, there may be two engine driven hydraulic pumps, each identical, each capable of doing the same job. An accurate understanding of the longevity of these pumps, necessary preventative maintenance intervals or replacement intervals, etc, come with fleet use and observations. Component failures are rare. Engine failures rare. Where multiple engine failures occur at the same time, invariably the issue is lack of fuel, ingestion of foreign material (birds, volcanic ash, etc) ice, or a catastrophic uncontained failure of one engine that damages another. In any of those events, there would be nothing gained by having multiple manufacturers.

Where multiple manufacturers do exist on an airframe for a given component or accessory, the parts are compatible and made to a particular standard. It's not uncommon to have a particular part with several manufacturers, but each must be built to the same standard, whether it's a bleed valve or electrical relay, pin connector, or so on.

Can you imagine in an emergency calling for the quick reference handbook or checklist, and having to decide which manufacturers procedures are needed because the equipment is different? 80 knots...V1...Rotate, V2...engine failure...positive rate, gear up...autoflight, no, wait, it's number one, or number two? Yeah, that's the pratt, uh, lets go with...

The engines are identical; they're trimmed the same, operate the same; the parameters should match up fairly closely in operation; same N1, same N2, same fuel flow, same temperatures, etc...because they're all the same engine, all operated the same, and only minor differences will be apparent. Part of a scan of engine instrumentation or indications is looking for something different. If you're running two different manufacturers and models of engines, you'll always have different indications between them; now each must be individually monitored and simultaneously compared against memorized numbers, independently. That can only reduce safety.

It's one thing for the captain to have the fish while the first officer eats the chicken; taking a shotgun approach to aircraft safety by trying to spread the odds of failures between a smattering of different accessories, equipment, etc, is more like guesswork, than science. A great deal of effort goes into tracking component performance on board an aircraft, and in knowing what we can expect from a given item in terms of reliability, life, maintenance needs and intervals, and so forth. Operators and manufacturers are very good at predicting and writing maintenance intervals that allow servicing, replacement, overhaul, inspection and so on to optimize cost and safety. Consistency and standardization is very, very important in aviation; attempting to distort that by using chaos theory to spin the odds not only will upset the apple cart, but compromise safety on many levels.
 
User avatar
tb727
Posts: 2140
Joined: Thu Jun 30, 2005 1:40 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Thu Mar 07, 2019 3:33 pm

Thinking about it more, it would make the odds of unreliable operation increase.
Too lazy to work, too scared to steal!
 
User avatar
AirKevin
Posts: 483
Joined: Wed Apr 26, 2017 2:18 am

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Thu Mar 07, 2019 6:46 pm

Engine instrumentation for each engine:

PW: EPR, N1, N2
RR: EPR, N1, N2, N3
GE: N1, N2

Not to mention the different fuel flows, engine vibrations, and other differing parameters. Not to mention the EPR values on Pratt & Whitney engines don't appear to be the same as that of the Rolls-Royce engines.
Captain Kevin
 
kalvado
Posts: 1802
Joined: Wed Mar 01, 2006 4:29 am

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Thu Mar 07, 2019 7:04 pm

I can think of one case where outcome would be different with 2 different engines - that is, BA 777 at LHR. Most other two-engine shutdowns would affect all engines regardless - birds, ash, fuel starvation.
Not sure how much complexity it would add to the airplane - but definitely more issues with maintenance and logistics.
ETOPS maintenance requirements - different people working on engines - is probably as far as it makes sense to go...
 
kalvado
Posts: 1802
Joined: Wed Mar 01, 2006 4:29 am

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Thu Mar 07, 2019 7:08 pm

AirKevin wrote:
Engine instrumentation for each engine:

PW: EPR, N1, N2
RR: EPR, N1, N2, N3
GE: N1, N2

Not to mention the different fuel flows, engine vibrations, and other differing parameters. Not to mention the EPR values on Pratt & Whitney engines don't appear to be the same as that of the Rolls-Royce engines.

Do you actually have to remember the EPR or N1/N2 numbers from the top of your head in actual flight (not at the tests), or seeing green numbers on the screen is generally good enough? I can think of many situations where "OK" indication is all what is needed....
 
User avatar
Starlionblue
Posts: 19314
Joined: Fri Feb 27, 2004 9:54 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Thu Mar 07, 2019 10:41 pm

AirKevin wrote:
Engine instrumentation for each engine:

PW: EPR, N1, N2
RR: EPR, N1, N2, N3
GE: N1, N2

Not to mention the different fuel flows, engine vibrations, and other differing parameters. Not to mention the EPR values on Pratt & Whitney engines don't appear to be the same as that of the Rolls-Royce engines.


I'll add that the Trent XWB replaces EPR with a synthetic linear thrust reading.

kalvado wrote:
AirKevin wrote:
Engine instrumentation for each engine:

PW: EPR, N1, N2
RR: EPR, N1, N2, N3
GE: N1, N2

Not to mention the different fuel flows, engine vibrations, and other differing parameters. Not to mention the EPR values on Pratt & Whitney engines don't appear to be the same as that of the Rolls-Royce engines.

Do you actually have to remember the EPR or N1/N2 numbers from the top of your head in actual flight (not at the tests), or seeing green numbers on the screen is generally good enough? I can think of many situations where "OK" indication is all what is needed....


Well, not all the numbers, but you do have to know a smattering of reference settings, in particular for unreliable airspeed situations.

For example, in an AF447 type even in the cruise, knowing 80% N1 and 2.5-3 degrees of pitch is pretty important. On climbout and approach, knowing the approximate pitch and power for various configurations helps cross-check that the aircraft is doing what it is supposed to.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
tepidhalibut
Posts: 201
Joined: Fri Dec 10, 2004 8:19 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Fri Mar 08, 2019 8:00 am

I know that Kalvado mentioedn the LHR 777 event, but can anyone name an actual historic incident where different engine types would have saved lives?
 
User avatar
AirKevin
Posts: 483
Joined: Wed Apr 26, 2017 2:18 am

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Fri Mar 08, 2019 1:11 pm

tepidhalibut wrote:
I know that Kalvado mentioedn the LHR 777 event, but can anyone name an actual historic incident where different engine types would have saved lives?

That one, actually. If I remember correctly, ice crystals formed on the fuel/oil heat exchangers, prompting them to be redesigned. This appears to have only affected 777s with Rolls-Royce engines. Pratt & Whitney and General Electric engines were not affected.
Captain Kevin
 
kalvado
Posts: 1802
Joined: Wed Mar 01, 2006 4:29 am

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Fri Mar 08, 2019 5:15 pm

Starlionblue wrote:
Well, not all the numbers, but you do have to know a smattering of reference settings, in particular for unreliable airspeed situations.

Let me rephrase the question: how many engine specific numbers do you actually keep in mind?
Or, same question in a different form: how it works with type rating? E.g. if you fly A330 with RR, and your company adds PW birds for some reason - what kind of training is required for you to get on one? Can you fly both, or you're locked to one fleet? For me such 2 subfleets at least somewhat resemble different engines on one bird scenario. Are there a lot of numbers to memorize from that class; and how it works in general?
 
User avatar
DocLightning
Posts: 21476
Joined: Wed Nov 16, 2005 8:51 am

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Fri Mar 08, 2019 6:30 pm

On top of having different weights, different engines will spool up and down at different rates, so you would risk having an unbalanced aircraft and an unbalanced thrust situation. Nuclear powerplants don't need to be symmetrical. Airplanes do need to be symmetrical.

Could the latter problem be fixed with software? Maybe, but it's nontrivial. The former problem could not be fixed with software.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
ELBOB
Posts: 294
Joined: Sun Jun 21, 2015 6:56 am

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Sat Mar 09, 2019 12:57 pm

DocLightning wrote:
different engines will spool up and down at different rates


So do engines of the 'same' type due to manufacturing variations. But the pilots ( of big jets ) don't care because they leave the FADEC to work that out once they set the thrust levers. Just like it already handles all the minutiae of fan drag in the descent etc
 
aerotech777
Posts: 86
Joined: Sat Aug 15, 2009 3:53 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Sat Mar 09, 2019 1:18 pm

Not really in the same subject of the thread, but different engines are installed for example on a same aircraft for testing new engine (common practice). Even one UDF engine (Propfan) was installed on aircraft (I think it was B727) to test purpose.

I remember when Boeing was saying that in Boeing 787 it is possible to swap between RR and GE engines (using the same mounts and engine to aircraft interfaces if I am not mistaken). Is this possible now or it was just marketing trick?

There is also engine intermix and I seen this when I was working on B737-200 and B727 (JT8D with -9, -15, or -17). There are also few changes on some cockpit instruments like EGT indicator (with a sticker underneath it mentioning the dash number). I don't know if it is required to change the EPR indicator if there is engine intermix.

I think there is also engine intermix on B747 with JT9D. I don't know if there is engine intermix on GE or RR engines (for example between CFM56 -5 and-7).

I am wondering if engine intermix is done on glass cockpit aircraft (may be some change in the software).
 
strfyr51
Posts: 3812
Joined: Tue Apr 10, 2012 5:04 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Sun Mar 10, 2019 8:49 am

747Whale wrote:
Faro wrote:
Theoretically --and only theoretically-- if different engines with substantially the same weight, thrust, spool-up/down times, etc were used on the same airframe, would it lead to a significantly safer aircraft?


Absolutely not.

While a nuclear powerplant does not need to lift its own weight off the ground and fly, an aircraft does. Minimum weight is crucial, and attempting to mix and match numerous parts, integrating systems from one manufacturer and another will add complexity, weight, and create more procedures to deal with in flight, ultimately decreasing safety.

Aircraft operators know that if their fleet uses CF6 engines, they have one engine to maintain, which makes it easier to manage parts, engine replacement, service, and to ensure a uniform standard of repair and maintenance. Having two different engines on the aircraft would add levels of complexity far beyond simple plumbing and fitting of fuel lines and electrical, hydraulic, pneumatic, etc. They aircraft would require unique systems capability on each side of the aircraft, separate support, separate maintenance tracking, separate interface, procedures, parts, etc. Every one of those comes with a plethora of complexities, each of which is another layer of "swiss cheese." Another opportunity for a problem.

On a given engine, there may be two engine driven hydraulic pumps, each identical, each capable of doing the same job. An accurate understanding of the longevity of these pumps, necessary preventative maintenance intervals or replacement intervals, etc, come with fleet use and observations. Component failures are rare. Engine failures rare. Where multiple engine failures occur at the same time, invariably the issue is lack of fuel, ingestion of foreign material (birds, volcanic ash, etc) ice, or a catastrophic uncontained failure of one engine that damages another. In any of those events, there would be nothing gained by having multiple manufacturers.

Where multiple manufacturers do exist on an airframe for a given component or accessory, the parts are compatible and made to a particular standard. It's not uncommon to have a particular part with several manufacturers, but each must be built to the same standard, whether it's a bleed valve or electrical relay, pin connector, or so on.

Can you imagine in an emergency calling for the quick reference handbook or checklist, and having to decide which manufacturers procedures are needed because the equipment is different? 80 knots...V1...Rotate, V2...engine failure...positive rate, gear up...autoflight, no, wait, it's number one, or number two? Yeah, that's the pratt, uh, lets go with...

The engines are identical; they're trimmed the same, operate the same; the parameters should match up fairly closely in operation; same N1, same N2, same fuel flow, same temperatures, etc...because they're all the same engine, all operated the same, and only minor differences will be apparent. Part of a scan of engine instrumentation or indications is looking for something different. If you're running two different manufacturers and models of engines, you'll always have different indications between them; now each must be individually monitored and simultaneously compared against memorized numbers, independently. That can only reduce safety.

It's one thing for the captain to have the fish while the first officer eats the chicken; taking a shotgun approach to aircraft safety by trying to spread the odds of failures between a smattering of different accessories, equipment, etc, is more like guesswork, than science. A great deal of effort goes into tracking component performance on board an aircraft, and in knowing what we can expect from a given item in terms of reliability, life, maintenance needs and intervals, and so forth. Operators and manufacturers are very good at predicting and writing maintenance intervals that allow servicing, replacement, overhaul, inspection and so on to optimize cost and safety. Consistency and standardization is very, very important in aviation; attempting to distort that by using chaos theory to spin the odds not only will upset the apple cart, but compromise safety on many levels.

It's no longer as bad as all that, Managing 2 different engine types on the same model requires one thing. factory support!! the smaller number of the model? Like an Airbus A320-212 or a -232? both can be managed by any airline that flies the CFM or the V2500. United has A320's with both the CFM56 and the V2500. And we also have a shop familiar with both as we also fly the B737 as well. So we've got experience on the CFM-56 -2/3/5 and 7series engines Plus? The V2500's Not to count the Pratt engines of many types with the GE's from the CF-6 thru the GE90 series
 
stratclub
Posts: 1286
Joined: Fri Jan 05, 2018 10:38 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Sun Mar 10, 2019 9:07 am

Short answer, dumb idea..............It seems like it would be like putting snow tires on one side of a car and putting all season tires on the other side of a car.
 
RetiredWeasel
Posts: 704
Joined: Sat Jul 05, 2014 8:16 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Sun Mar 10, 2019 3:26 pm

Back in the days of flying the 747-200 we frequently flew intermixes of the JT9D engines with the mixes being -7R, -7Q , -7Fs and/or -7Js. All mixes were approved and certified by the FAA. Different EPRs for TO and climb were set for the appropriate engines. The Flight Engineer had the Reference Manual which had a number of charts to figure these settings. There were some restrictions on weight and other ops that had to be accounted for when flying with intermixes.
 
CosmicCruiser
Posts: 2319
Joined: Tue Feb 22, 2005 3:01 am

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Sun Mar 10, 2019 3:40 pm

the Valsan 727 is another example
 
User avatar
Starlionblue
Posts: 19314
Joined: Fri Feb 27, 2004 9:54 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Mon Mar 11, 2019 3:39 am

kalvado wrote:
Starlionblue wrote:
Well, not all the numbers, but you do have to know a smattering of reference settings, in particular for unreliable airspeed situations.

Let me rephrase the question: how many engine specific numbers do you actually keep in mind?
Or, same question in a different form: how it works with type rating? E.g. if you fly A330 with RR, and your company adds PW birds for some reason - what kind of training is required for you to get on one? Can you fly both, or you're locked to one fleet? For me such 2 subfleets at least somewhat resemble different engines on one bird scenario. Are there a lot of numbers to memorize from that class; and how it works in general?


At a minimum, you need to memorise the memory procedures, which include a few engine settings in case of unreliable airspeed. You also need to memorise a bunch of limitations.

The same aircraft with two different engine types doesn't require much more than some computer based training. In some places pilots fly both 777 and 787, or A330 and A350. And yes, you memorise slightly different memory procedures and limitations for different aircraft variants and/or types.

It isn't hundreds of numbers so it isn't as hard as it sounds. You study in whatever fashion works for you (I like flashcards) until you know the numbers, and review them periodically. In the beginning, it can be a bit overwhelming but with time it all becomes pretty firmly ingrained.

Knowing approximate pitch and power settings for various flight regimes is very helpful. Even if you don't know every pitch and power setting for every configuration exactly, with experience you eventually gain a good working knowledge.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
747Whale
Posts: 725
Joined: Fri Dec 07, 2018 7:41 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Mon Mar 11, 2019 6:28 pm

Having a fleet with different engines is one thing; having different engines on the same airplane is another matter, and not a particularly good one.

I've done engine-intermix on aircraft like the 747 where we had all J's and one A, but still all JT9D's. Same procedures, same systems, and the procedures were standardized. Having two different makes of engine on the same airframe is ridiculous, with the possible exception of a tertiary mount for a testbed in experimental operations.

Having the same engine in each position means the ability to set power by matching equivalent settings, such as fuel flow, and enables a rapid scan for a problem during an emergency. This is not the case if multiple types of makes or models of engine are used.
 
strfyr51
Posts: 3812
Joined: Tue Apr 10, 2012 5:04 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Mon Apr 01, 2019 10:09 am

kalvado wrote:
Starlionblue wrote:
Well, not all the numbers, but you do have to know a smattering of reference settings, in particular for unreliable airspeed situations.

Let me rephrase the question: how many engine specific numbers do you actually keep in mind?
Or, same question in a different form: how it works with type rating? E.g. if you fly A330 with RR, and your company adds PW birds for some reason - what kind of training is required for you to get on one? Can you fly both, or you're locked to one fleet? For me such 2 subfleets at least somewhat resemble different engines on one bird scenario. Are there a lot of numbers to memorize from that class; and how it works in general?

yes you could but you'd have to be trained on Both.
 
meecrob
Posts: 101
Joined: Sun Jun 19, 2016 6:15 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Tue Apr 02, 2019 8:43 am

747Whale wrote:

It's one thing for the captain to have the fish while the first officer eats the chicken; taking a shotgun approach to aircraft safety by trying to spread the odds of failures between a smattering of different accessories, equipment, etc, is more like guesswork, than science. A great deal of effort goes into tracking component performance on board an aircraft, and in knowing what we can expect from a given item in terms of reliability, life, maintenance needs and intervals, and so forth. Operators and manufacturers are very good at predicting and writing maintenance intervals that allow servicing, replacement, overhaul, inspection and so on to optimize cost and safety. Consistency and standardization is very, very important in aviation; attempting to distort that by using chaos theory to spin the odds not only will upset the apple cart, but compromise safety on many levels.


I'd have to say this answer and the one downthread mentioning putting two types of tires on your car are the answer. It costs money to run a maintenance department and if you can reduce your variables you can be much more efficient with the tracking of maintenance and thus increase safety. For instance, you only need one database as opposed to two to track engine performance. You would obviously check if there were any amendments, but you could say "this maintenance item needs to be carried out every x hours" not "check which manufacturer it is, then I'll give you an answer, just to doublecheck we don't carry out an incorrect procedure." You basically removed a chance of possible error by having twin engines.
 
User avatar
flyingturtle
Posts: 5613
Joined: Mon Oct 31, 2011 1:39 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Tue Apr 02, 2019 10:27 am

One aircraft mechanic on a.nut said that 40% of his job is finding out if part numbers / serial numbers are matching. (Slightly exaggerated).

With two different suppliers, you double that problem...
Reading accident reports is what calms me down
 
strfyr51
Posts: 3812
Joined: Tue Apr 10, 2012 5:04 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Wed Apr 03, 2019 6:24 am

747Whale wrote:
Faro wrote:
Theoretically --and only theoretically-- if different engines with substantially the same weight, thrust, spool-up/down times, etc were used on the same airframe, would it lead to a significantly safer aircraft?


Absolutely not.

While a nuclear powerplant does not need to lift its own weight off the ground and fly, an aircraft does. Minimum weight is crucial, and attempting to mix and match numerous parts, integrating systems from one manufacturer and another will add complexity, weight, and create more procedures to deal with in flight, ultimately decreasing safety.

Aircraft operators know that if their fleet uses CF6 engines, they have one engine to maintain, which makes it easier to manage parts, engine replacement, service, and to ensure a uniform standard of repair and maintenance. Having two different engines on the aircraft would add levels of complexity far beyond simple plumbing and fitting of fuel lines and electrical, hydraulic, pneumatic, etc. They aircraft would require unique systems capability on each side of the aircraft, separate support, separate maintenance tracking, separate interface, procedures, parts, etc. Every one of those comes with a plethora of complexities, each of which is another layer of "swiss cheese." Another opportunity for a problem.

On a given engine, there may be two engine driven hydraulic pumps, each identical, each capable of doing the same job. An accurate understanding of the longevity of these pumps, necessary preventative maintenance intervals or replacement intervals, etc, come with fleet use and observations. Component failures are rare. Engine failures rare. Where multiple engine failures occur at the same time, invariably the issue is lack of fuel, ingestion of foreign material (birds, volcanic ash, etc) ice, or a catastrophic uncontained failure of one engine that damages another. In any of those events, there would be nothing gained by having multiple manufacturers.

Where multiple manufacturers do exist on an airframe for a given component or accessory, the parts are compatible and made to a particular standard. It's not uncommon to have a particular part with several manufacturers, but each must be built to the same standard, whether it's a bleed valve or electrical relay, pin connector, or so on.

Can you imagine in an emergency calling for the quick reference handbook or checklist, and having to decide which manufacturers procedures are needed because the equipment is different? 80 knots...V1...Rotate, V2...engine failure...positive rate, gear up...autoflight, no, wait, it's number one, or number two? Yeah, that's the pratt, uh, lets go with...

The engines are identical; they're trimmed the same, operate the same; the parameters should match up fairly closely in operation; same N1, same N2, same fuel flow, same temperatures, etc...because they're all the same engine, all operated the same, and only minor differences will be apparent. Part of a scan of engine instrumentation or indications is looking for something different. If you're running two different manufacturers and models of engines, you'll always have different indications between them; now each must be individually monitored and simultaneously compared against memorized numbers, independently. That can only reduce safety.

It's one thing for the captain to have the fish while the first officer eats the chicken; taking a shotgun approach to aircraft safety by trying to spread the odds of failures between a smattering of different accessories, equipment, etc, is more like guesswork, than science. A great deal of effort goes into tracking component performance on board an aircraft, and in knowing what we can expect from a given item in terms of reliability, life, maintenance needs and intervals, and so forth. Operators and manufacturers are very good at predicting and writing maintenance intervals that allow servicing, replacement, overhaul, inspection and so on to optimize cost and safety. Consistency and standardization is very, very important in aviation; attempting to distort that by using chaos theory to spin the odds not only will upset the apple cart, but compromise safety on many levels.



on the practical side? you would have to use the airplanes in a closed loop so that if an engine failed or deteriorated? There might be a spare engine of the type needed to replace it. for Example? Look at the A400, you have left turning prop in one position and right turning prop in another position. So? What engine would you position Where in the event an engine failed? This is what you might face in the same event on your scenario. Having worked with and on 2,3,& 4 engine airplanes for most of my career in the military and Civilian life? It's always simpler to have a single engine type and configuration on any single model. The A-400 could have done that but I suspect with that small Vertical Stabilizer, They elected to go with opposing direction Engines. OR? It could have just been an Idea to be different. Who Knows?? In any case? I submit that the Idea was BAD Logistically.
 
User avatar
Starlionblue
Posts: 19314
Joined: Fri Feb 27, 2004 9:54 pm

Re: Out of Curiosity: Two Different Engines on Same Airframe Safer?

Wed Apr 03, 2019 7:08 am

strfyr51 wrote:
747Whale wrote:
Faro wrote:
Theoretically --and only theoretically-- if different engines with substantially the same weight, thrust, spool-up/down times, etc were used on the same airframe, would it lead to a significantly safer aircraft?


Absolutely not.

While a nuclear powerplant does not need to lift its own weight off the ground and fly, an aircraft does. Minimum weight is crucial, and attempting to mix and match numerous parts, integrating systems from one manufacturer and another will add complexity, weight, and create more procedures to deal with in flight, ultimately decreasing safety.

Aircraft operators know that if their fleet uses CF6 engines, they have one engine to maintain, which makes it easier to manage parts, engine replacement, service, and to ensure a uniform standard of repair and maintenance. Having two different engines on the aircraft would add levels of complexity far beyond simple plumbing and fitting of fuel lines and electrical, hydraulic, pneumatic, etc. They aircraft would require unique systems capability on each side of the aircraft, separate support, separate maintenance tracking, separate interface, procedures, parts, etc. Every one of those comes with a plethora of complexities, each of which is another layer of "swiss cheese." Another opportunity for a problem.

On a given engine, there may be two engine driven hydraulic pumps, each identical, each capable of doing the same job. An accurate understanding of the longevity of these pumps, necessary preventative maintenance intervals or replacement intervals, etc, come with fleet use and observations. Component failures are rare. Engine failures rare. Where multiple engine failures occur at the same time, invariably the issue is lack of fuel, ingestion of foreign material (birds, volcanic ash, etc) ice, or a catastrophic uncontained failure of one engine that damages another. In any of those events, there would be nothing gained by having multiple manufacturers.

Where multiple manufacturers do exist on an airframe for a given component or accessory, the parts are compatible and made to a particular standard. It's not uncommon to have a particular part with several manufacturers, but each must be built to the same standard, whether it's a bleed valve or electrical relay, pin connector, or so on.

Can you imagine in an emergency calling for the quick reference handbook or checklist, and having to decide which manufacturers procedures are needed because the equipment is different? 80 knots...V1...Rotate, V2...engine failure...positive rate, gear up...autoflight, no, wait, it's number one, or number two? Yeah, that's the pratt, uh, lets go with...

The engines are identical; they're trimmed the same, operate the same; the parameters should match up fairly closely in operation; same N1, same N2, same fuel flow, same temperatures, etc...because they're all the same engine, all operated the same, and only minor differences will be apparent. Part of a scan of engine instrumentation or indications is looking for something different. If you're running two different manufacturers and models of engines, you'll always have different indications between them; now each must be individually monitored and simultaneously compared against memorized numbers, independently. That can only reduce safety.

It's one thing for the captain to have the fish while the first officer eats the chicken; taking a shotgun approach to aircraft safety by trying to spread the odds of failures between a smattering of different accessories, equipment, etc, is more like guesswork, than science. A great deal of effort goes into tracking component performance on board an aircraft, and in knowing what we can expect from a given item in terms of reliability, life, maintenance needs and intervals, and so forth. Operators and manufacturers are very good at predicting and writing maintenance intervals that allow servicing, replacement, overhaul, inspection and so on to optimize cost and safety. Consistency and standardization is very, very important in aviation; attempting to distort that by using chaos theory to spin the odds not only will upset the apple cart, but compromise safety on many levels.



on the practical side? you would have to use the airplanes in a closed loop so that if an engine failed or deteriorated? There might be a spare engine of the type needed to replace it. for Example? Look at the A400, you have left turning prop in one position and right turning prop in another position. So? What engine would you position Where in the event an engine failed? This is what you might face in the same event on your scenario. Having worked with and on 2,3,& 4 engine airplanes for most of my career in the military and Civilian life? It's always simpler to have a single engine type and configuration on any single model. The A-400 could have done that but I suspect with that small Vertical Stabilizer, They elected to go with opposing direction Engines. OR? It could have just been an Idea to be different. Who Knows?? In any case? I submit that the Idea was BAD Logistically.



Not as bad as all that. The A400M does have "handed" propellers but the engines all turn the same way. Two of the engines are fitted with a gearbox to make the props go the opposite direction.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

Who is online

Users browsing this forum: No registered users and 13 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos