View Full Version : Do identical aircraft behave the same?

14th Oct 2012, 02:53
Always wondered if two indentical triples or any other jets,behave the same?Are there subtle differences between the same planes?Do pilots prefer one 777 to another?In my profession,two identical maschines never behave the same,so how is it with jets?

Dan Winterland
14th Oct 2012, 04:41
There are differences, even in ones straight out of the factory. But less so in fly by wire aircraft where the artificial stability of the control systems usually blend out the differences.

14th Oct 2012, 15:05
Do you mean different models? A 767-200 vs. a 767-300? A 777-200 vs. a 777-300ER or 777-200LR?

14th Oct 2012, 15:46
Do you mean different models? A 767-200 vs. a 767-300? A 777-200 vs. a 777-300ER or 777-200LR?

I think he means identically built aircraft; one that rolled out of the factory this month versus the last.....

14th Oct 2012, 17:05
From an engineers point of view you always get the one that was built on a Friday afternoon & gives you s-- t even on a good day.

14th Oct 2012, 18:16
From an engineers point of view you always get the one that was built on a Friday afternoon & gives you s-- t even on a good day.

I recall at BA we had at least a couple of Tridents that either couldn't cruise as fast as their stablemates or flew one wing low.

Loose rivets
14th Oct 2012, 18:31
Then there was the DC4 or 6 or some-such, that after hours of circuit work asked if they could fly around the circuit in the opposite direction, cos the flight engineer said he could see a curve in the fuselage.

Apocryphal? It was a story that hung around for years.

blind pew
14th Oct 2012, 18:31
Probably bent like X-ray Mike Dave

14th Oct 2012, 18:32
Can't comment on FBW aircraft but whilst all a/c have very tight "interchangeability" limits 9which allows one part from an aircraft to be replaced by another from stock - unlike hand made cars) they are all different. Look at the weights and no two are the same (and this changes through an aircrafts life). Some will need different amounts of trim ...have better climb performance etc.

14th Oct 2012, 23:36
I think he means identically built aircraft; one that rolled out of the factory this month versus the last.....
You nailed it,now how about FBW aircraft like the E-Jets,777 series,Airbus family etc?I fly on a 195 pretty often,same airline,same plane,similar weather but some of the jets just feel rougher or it maybe just a beginner pilot.lol!

tubby linton
15th Oct 2012, 00:14
We have four A300s that were built within two years. They are almost the identical spec but each has its own character. I suppose it comes down to how he artificial feel has been set up and how each airframe has been used or abused over the years.

15th Oct 2012, 11:12
I flew two 727-200's which were externally pretty much the same although they did have different hushkits. One was a bit of a dog compared to the other.

15th Oct 2012, 11:18
I fly a fleet of brand new 738s. I can almost tell which possible registrations I am sitting in by the seat position. For the same index, some aircraft are perceptibly a little forward, back, higher or lower. Also I know in which aircraft the toilet seat stays up :\ Other than that, they feel and fly pretty much identically.

15th Oct 2012, 11:38
Regarding to my feelins the A320-21X and A320-23X behavior quietly differs during landing. I believe one of the reasons may be different flap extension degrees in config FULL

15th Oct 2012, 16:44
Computer monitors produced in a same company, same model, same time with the exact spec have color differences :) So i heard Color sensitive assignments do have minimum of 3 monitors manufactured in same brand, same company, same plant, same time and with same machinery and located in same location!

i guess it's a relative thing to compare as how far you expect any sort of two identical machines to behave, it may differ it's behavioral characteristics with age: the way they fly, way they being looked after, climate, etc

I assume two similar models with same specs, one flying in a tropical weather and other flying in far arctic will eventually have two different destinies with different characteristics of flying! :)


15th Oct 2012, 18:05
any other jets,behave the same?Are there subtle differences between the same planes?

I do not have the privilege of flying 777s so I can not comment on that model.

I fly 737-NGs from my vantage point; provided the same model are compared at the same weight and density altitude then yes they do behave and fly the same. I am unable to tell the difference between one jet and another. I do have a “favorite” aircraft in the fleet, and think it “feels” great to fly but I suspect my evaluation is purely subjective and emotional.

If you try and compare different versions of the same jet, then no they are not and do not "fly" the same. The smaller lighter 737-700 will feel and fly quite differently than the 737-800 or -900.

I spent a dozen years flying the MD-80. That jet was unusual and I would say that two MD-80s built next to each other on the assembly line in a few years could feel and fly very differently, it was the strangest thing.

The MD-80 series jet manipulates its flight controls with a byzantine series of cables and trim tabs rather than hydraulic or fly by wire systems (affectionately knows as the Long Beach Cable Car {manufactured at Long Beach California}). Because the cables wear and stretch differently each aircraft develops it own unique feel. It was also by far the most difficult aircraft I have ever flown to get a smooth landing out of. Every time I thought I had it figured out, I would embarrass myself.

16th Oct 2012, 00:51
Thanks for all your replies,very interesting read :)

16th Oct 2012, 03:18
Doing the flight control checks you'll sometimes notice the controls are slightly freer, or stiffer, than typical. Minor difference.

The non FBW a/c typically have to be trimmed, primarily rudder. Sometimes it's slightly right, sometimes slightly left. Some use the same trim setting at different speeds and some need retrimming as the airplane slows or accelerates.

Engines can have slightly different spool characteristics, especially off of idle.

Small differences, sometimes noticeable. Probably no different than if you drove 20 different cars of the same make and model, especially if they're 5, 10, or 15 yrs old and well maintained. They'd be the same, but not 100% the same.

A Squared
16th Oct 2012, 04:11
All identical aircraft fly exactly the same.

The thing is, there are no aircraft which are truly identical.

Lightning Mate
16th Oct 2012, 11:40
I know that EE Lightnings had differing characteristics

I never noticed any, unless you are talking about different marks.

19th Oct 2012, 14:46
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.

20th Oct 2012, 05:42
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.

20th Oct 2012, 07:49
Each wing is built in a different jig

I don't think you really mean that.

20th Oct 2012, 21:07
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.

20th Oct 2012, 22:05
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...

Dan Winterland
21st Oct 2012, 02:39
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.

21st Oct 2012, 09:11
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.

gas path
21st Oct 2012, 19:55
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. :uhoh:
The only thing that did vary was the relight performance of the early RB211 b2.