Re: 2 Vs 4 engines - CFM56 Engineering Dept Costs

Date:         03 Dec 99 02:10:11 
From:         James Matthew Weber <>
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At 11:56 PM 11/21/99 +0000, you wrote:
>If you were an airline running a CFM56 fleet B737 / A320 etc. and you
>wanted to buy a wide bodied Airbus would the A340 cominality (sp?) with
>the CFM56 be much of an advantage when compared to the greater fuel
>burn etc. of a 4 engined aircraft?

The CFM56 is part of the A340 problem in the first place. The A340 was
supposed to use the v2500 super fan, however IAE got cold feet, and left
the A340 without an engine. The CFM56 on the A340 involved a BIG
stretch, and BIG stretches to a design tend to do  ugly things. The
CFM56 on an A340 and the one on a 737 have about as much in common as
JT9D-3 has with a JT9D-7Q, which is not a whole lot.

In the past such commonality has been attractive. Most D10 operators who
later bought 747's, bought the 747's with GE engines, and there isn't a
lot of difference between a CF6-50 on a D10-30 and CF6-50 on a 747

Most airlines who bought the A340 did so for one of two reasons:
1). They wanted the extreme range, but didn't have the load to justify
a 747-400. Interestingly enough, many A340's are being removed from the
ultra long haul missions they were purchased for. According to Singapore
Airlines at least, SQ doesn't like the airplane all that much and
neither do the passengers.
2). They had long over water segments, and wanted 4 engines either
because they couldn't live with 120 or 180 minutes ETOPS, or political

Most of the people making decision about regulations started in the
business in the days of the Constellation and DC6/DC7, when engine
lifetime was maybe 2000 hours. Some CFM56's and RB211-535's have now
been on the wing for more than 30,000 hours!!! These engines are
incredibly reliable relative to the engines of 40 years ago.

Engine commonality just wasn't part of the decision process, its way
down the list of important considerations, especially since the only
engine you can have on an A340-200/300 is a CFM56.

Perversely, a 4 engine aircraft is less likely to get you where you are
going than a 2 engine most of the time. Engine failure is more likely on
a 4 engine aircraft because there are more engines (D10's have to turn
back because of engine failures less often than 747's, even though 747
engines are more reliable, the problem is the 747 engine needs to be at
least 33% more reliable)...

James Matthew Weber   (623) 587 7514 .  Fax  (623) 434 7598