How do you calculate DOI Dry Operating Index?
Thread Starter
Join Date: Jan 2006
Location: EU
Age: 43
Posts: 364
Likes: 0
Received 0 Likes
on
0 Posts
I must say that this discussion has taken an interesting turn, and it's what I intended it to do... There is a lot of discussion at my school about what's relevant to know and not to know and I think that how to calculate the DOI is one such thing that pilots should know? Am I right?
Best Regards
Tim
Best Regards
Tim
Moderator
My view is a bit biased as I am a sort of pilot-engineer mix with a foot firmly in both camps. However, one of Tech Log's strengths is its ability to act as a training facilitation vehicle and the mods revel in this sort of thread.
Generally the pilot is not called on to calculate DOI but there is no reason why you should not be able to do so .. in any case, knowledge is generally a useful thing in its own right ...
Generally the pilot is not called on to calculate DOI but there is no reason why you should not be able to do so .. in any case, knowledge is generally a useful thing in its own right ...
Moderator
Now that I have scratched my head for the requisite period of time, the errors in FLEXJET's URL link reference have become clear.
The manual linked to by FLEXJET contains expressions for index and %MAC both of which contain errors which appear to have been generated by a combination of (perhaps) carelessness and typographical mistakes.
Following the reference nomenclature, the correct equation for index is
I = (W x (Sta - Ref Sta))/C + K
This has been printed in the manual incorrectly by substituting a minus sign for the multiplication sign after W, probably a simple typo. In addition the divisor line has been omitted.
The manual's logic is then a bit unusual in that the IU equation is solved for Sta which results in
Sta = Ref Sta + ((I - K) x C)/W
which is then plugged into the standard %MAC equation of
%MAC = ((Sta - LEMAC)/MAC) x 100
which gives
%MAC = ((Ref Sta + ((I - K) x C)/W - LEMAC)/MAC) x 100
which can be adjusted to read
%MAC = (Ref Sta + ((I - K) x C)/W - LEMAC)/(MAC/100)
Unfortunately, somewheres along the way
(a) I was replaced by 1
(b) the W got lost in the complexity. Interestingly, this error is in the manual but in FLEXJET's transcription the W is resurrected .. I am not too sure how that came about .. ?
Two things come out of this ..
(a) be wary of indiscriminately pinching something from the net as it might be wrong ...
(b) I wouldn't choose to go down the algebraic path chosen by this particular operator ...
The manual linked to by FLEXJET contains expressions for index and %MAC both of which contain errors which appear to have been generated by a combination of (perhaps) carelessness and typographical mistakes.
Following the reference nomenclature, the correct equation for index is
I = (W x (Sta - Ref Sta))/C + K
This has been printed in the manual incorrectly by substituting a minus sign for the multiplication sign after W, probably a simple typo. In addition the divisor line has been omitted.
The manual's logic is then a bit unusual in that the IU equation is solved for Sta which results in
Sta = Ref Sta + ((I - K) x C)/W
which is then plugged into the standard %MAC equation of
%MAC = ((Sta - LEMAC)/MAC) x 100
which gives
%MAC = ((Ref Sta + ((I - K) x C)/W - LEMAC)/MAC) x 100
which can be adjusted to read
%MAC = (Ref Sta + ((I - K) x C)/W - LEMAC)/(MAC/100)
Unfortunately, somewheres along the way
(a) I was replaced by 1
(b) the W got lost in the complexity. Interestingly, this error is in the manual but in FLEXJET's transcription the W is resurrected .. I am not too sure how that came about .. ?
Two things come out of this ..
(a) be wary of indiscriminately pinching something from the net as it might be wrong ...
(b) I wouldn't choose to go down the algebraic path chosen by this particular operator ...
Last edited by john_tullamarine; 15th Mar 2006 at 10:38.
Join Date: Nov 2002
Location: Only upon request
Posts: 874
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by john_tullamarine
(b) the W got lost in the complexity. Interestingly, this error is in the manual but in FLEXJET's transcription the W is resurrected .. I am not too sure how that came about .. ?
In fact I have a copy of this formula at home (taken from an A330 GOM/AHM), as I did some research about this subject earlier this month...
I ended up mixing the web linked and paper formulas as I was certain they were the same !
Originally Posted by john_tullamarine
Two things come out of this ..
(a) be wary of indiscriminately pinching something from the net as it might be wrong ...
(a) be wary of indiscriminately pinching something from the net as it might be wrong ...
Moderator
ATPL Mass & Balance recollection
.. you mean ... ground schools teach this nonsense ?
I have a copy of this formula at home (taken from an A330 GOM/AHM),
.. you mean ... other people do it this way as well ?
Absolutely mind blowing.
Now, I can see no reason for taking a simple equation (surely we agree that the standard %MAC equation is simple ?) and turning it into an unwieldy complicated thing when there is no apparent advantage gained for anyone.
The evidence of the errors which have come to light in this thread's discussions should be adequate indication of the undesirability of this needlessly complex way to do something which, inherently, is very simple ....
Perhaps someone can offer an explanation as to the philosophy behind this apparently strange phenomenon ? Or is it just a case that Aussies like to take the easy way ... ?
.. you mean ... ground schools teach this nonsense ?
I have a copy of this formula at home (taken from an A330 GOM/AHM),
.. you mean ... other people do it this way as well ?
Absolutely mind blowing.
Now, I can see no reason for taking a simple equation (surely we agree that the standard %MAC equation is simple ?) and turning it into an unwieldy complicated thing when there is no apparent advantage gained for anyone.
The evidence of the errors which have come to light in this thread's discussions should be adequate indication of the undesirability of this needlessly complex way to do something which, inherently, is very simple ....
Perhaps someone can offer an explanation as to the philosophy behind this apparently strange phenomenon ? Or is it just a case that Aussies like to take the easy way ... ?
