Do identical aircraft behave the same?
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Flying about 20 planes of the same kind make and model I must say that basically they all behave the same.
Same flap setting, same handling characteristics at similar mass. The noticeable differences come in little details.
E.g. seating position. There are planes you always think that you'll never find the correct seating position. Some make noises from the TRU (transformer rectifier units) during engine start. Some make a whistling noise until you climbe above 5'000 feet and the seals get into place.
These are the real differences I see.
Same flap setting, same handling characteristics at similar mass. The noticeable differences come in little details.
E.g. seating position. There are planes you always think that you'll never find the correct seating position. Some make noises from the TRU (transformer rectifier units) during engine start. Some make a whistling noise until you climbe above 5'000 feet and the seals get into place.
These are the real differences I see.
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Even the same aircraft gains several hundred pounds during the course of it's life. Most have different aileron trim settings. A few seconds of arc difference causes differing lift from each wing. Each wing is built in a different jig which has to be calibrated every 12 months.
Back to the original question about identical aircraft:
A general point: no two airframes or engines are identical, even off the same production lines, as all engineering products are built to defined acceptable tolerances.
From my experience, a couple of examples:
First, on a Hunter squadron, we had one particular FGA9 airframe that always burned more fuel than any of the other dozen or so, even after more than one engine change. For some reason that was never never established its drag must have been higher than the norm. Usually, after we learned that fact, it was flown as a formation leader, but even so was often most limiting on fuel of a 4-ship. XF442, also famous for another unrelated reason.
Second, some years later, in a fleet of 20+ B737-200, there were 2 airframes (built consecutively at Boeing) that had much heavier ailerons than the rest of the fleet. When snagged (more than once) the response was always "ground tested and control forces within limits".
So, yes, they differ.
A general point: no two airframes or engines are identical, even off the same production lines, as all engineering products are built to defined acceptable tolerances.
From my experience, a couple of examples:
First, on a Hunter squadron, we had one particular FGA9 airframe that always burned more fuel than any of the other dozen or so, even after more than one engine change. For some reason that was never never established its drag must have been higher than the norm. Usually, after we learned that fact, it was flown as a formation leader, but even so was often most limiting on fuel of a 4-ship. XF442, also famous for another unrelated reason.
Second, some years later, in a fleet of 20+ B737-200, there were 2 airframes (built consecutively at Boeing) that had much heavier ailerons than the rest of the fleet. When snagged (more than once) the response was always "ground tested and control forces within limits".
So, yes, they differ.
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Yeah there are slow pokes and races horses in the fleet...most guys won't notice unless they are sort to sit up there and pull out the book out of curiosity.
I flew one plane that was 17kts faster then book at FL410...translates to about every 5 hours I flew, the other guy in the same plane was 100 nm behind me... at the end of the year in a busy flight department...he's sitting up there 50 hours a year longer...translate that to duty time...that's about a month.....which has all sorts of effects on maintenance, down time, extra stops, extra brake wear, not being at home, the length of the days, time to climb...
Then you have the dogs of the fleet that are say about 5% on the back side of book...combine that with guys who can't flight plan efficiently....the numbers really get stupid...
I flew one plane that was 17kts faster then book at FL410...translates to about every 5 hours I flew, the other guy in the same plane was 100 nm behind me... at the end of the year in a busy flight department...he's sitting up there 50 hours a year longer...translate that to duty time...that's about a month.....which has all sorts of effects on maintenance, down time, extra stops, extra brake wear, not being at home, the length of the days, time to climb...
Then you have the dogs of the fleet that are say about 5% on the back side of book...combine that with guys who can't flight plan efficiently....the numbers really get stupid...
I used to fly Victors. Each one was habd crafted by artisans and all were different to the extent that often, you couldn't swap componenets from one airframe to another. And of course, in the air each had it's own character.
The Chief of the US's Strategic Air Command visited the production line to see where such a fabulous aircraft was built. He looked at Handley Page's WW1 hangar and asked why it was being built in a barn.
The very best of esoteric British excellence.
The Chief of the US's Strategic Air Command visited the production line to see where such a fabulous aircraft was built. He looked at Handley Page's WW1 hangar and asked why it was being built in a barn.
The very best of esoteric British excellence.
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Dave R - I think he did mean it... all Airbus wings are built separately.
Go to the Airbus factory in Chester and you will find that the port jig runs from Rib 2 outboard. The starboard jig is 2 metres away (one of the main reasons they keep them close because the ambient temp in parts of the factory is up to 10 degrees different and this expansion / contraction difference would seriously distort the wing during the "bolt up" phase).
It is only when they get to the Final Assembly Line (FAL) that the wings are attached to the cetre box section. The jigs are calibrated but still drift slightly over time (which is one of the reasons why no two aircraft are ever truely identical). Airbus couldn't actually sell one aircraft for over a year because it had to use 50% of roll trim to maintain level... serious fuel penalty.
Go to the Airbus factory in Chester and you will find that the port jig runs from Rib 2 outboard. The starboard jig is 2 metres away (one of the main reasons they keep them close because the ambient temp in parts of the factory is up to 10 degrees different and this expansion / contraction difference would seriously distort the wing during the "bolt up" phase).
It is only when they get to the Final Assembly Line (FAL) that the wings are attached to the cetre box section. The jigs are calibrated but still drift slightly over time (which is one of the reasons why no two aircraft are ever truely identical). Airbus couldn't actually sell one aircraft for over a year because it had to use 50% of roll trim to maintain level... serious fuel penalty.
Last edited by RMC; 21st Oct 2012 at 08:12.
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I've been on quite a few 747 (classic) test flights and they all performed to the book when it came to handling and response. Twas a shame they stopped doing the full stalls because as the airframes got older there were concerns on the stab. integrity. 
The only thing that did vary was the relight performance of the early RB211 b2.
The only thing that did vary was the relight performance of the early RB211 b2.
