2 Engines are Safer than 4
Ken Hamer
>
>
> After reading the recent thread, I thought I'd offer my opinion as to why 2
> engines are safer than 4 engines on any aircraft.
>
> The main issue is performance. Each aircraft is required to meet certain
> climb performance requirements on takeoff in the event of an engine
> failure. The climb performance requirements are virtually the same,
> regardless of whether the aircraft has 2, 3, or 4 engines. A 2 engine
> aircraft will therefore have about the same performance on 1 engine as a 4
> engine aircraft has on 3 engines (each has one engine failure).
>
>
Does this imply that 2 engine planes are 100% over powered, while 4
engines are only 33% overpowered?
Ken
Mark Rogers
After reading the recent thread, I thought I'd offer my opinion as to why 2
engines are safer than 4 engines on any aircraft.
The main issue is performance. Each aircraft is required to meet certain
climb performance requirements on takeoff in the event of an engine
failure. The climb performance requirements are virtually the same,
regardless of whether the aircraft has 2, 3, or 4 engines. A 2 engine
aircraft will therefore have about the same performance on 1 engine as a 4
engine aircraft has on 3 engines (each has one engine failure).
Most accidents do not involve engine failures. In situations where all the
engines are operating, 2 engine aircraft will have much better climb
performance than 4 engine aircraft. This enables 2 engine aircraft to have
a much better chance at surviving encounters with windshear, or perform
GPWS escape maneuvers (to avoid hitting the ground). They will also have
better performance during a go-around.
There are also problems with 4 engine aircraft. Many people point to
instances of dual engine failures on 4 engine aircraft as an argument
against 2 engine aircraft. In reality, I would argue the opposite point.
The dual engine failures on 4 engine aircraft that I can think of are: El
Al in AMS, the United 747 in HNL, and Evergreen in Alaska. In each of
these cases the 2 engine failures were related. In both the El Al and
Evergreen cases, the explosive failure of the #3 engine (inboard right
side) caused the failure of the #4 engine. In the United case, debris from
the cargo door, etc. caused both the #3 and #4 engines to fail.
If these same events occurred on a 2 engine aircraft, only 1 of the engines
would fail. While it might appear that both aircraft have lost half of
their engines, the 4 engine aircraft is in a much more serious situation.
The 2 engine aircraft still has 100% of it's required engine out
performance; the 4 engine aircraft only has 66%. The 4 engine aircraft
doesn't even meet takeoff climb performance requirements. If the
initiating event occurs very soon after takeoff, it is quite possible the
aircraft will not be able to clear surrounding terrain.
The argument is also made that flying with 2 engines over the water is not
safe. In the modern era, there has never been an aircraft accident due to
a loss of thrust from an engine during cruise (I'm specifically excluding
UA 232, which resulted from a hydraulic problem, not a loss of thrust).
There has also never been an instance of dual, unrelated engine failures.
Under ETOPS rules, there are strict limits on the operation of the aircraft
which seriously lessen the risk of related engine failures. In fact, a 3
engine Eastern L1011 almost ditched in the water off of Miami due to
related engine problems with all 3 engines (which would not have happened
if the aircraft was operated under ETOPS rules).
In my opinion, overwater flight under ETOPS is safer. I'm not worried
about flying on 1 engine after a failure; the airplane does that just fine.
I am worried about not having an alternate to which to divert. Under
ETOPS, there are strict rules on diversion airports, including the weather
at the alternate, and the time required to get there. There are no such
limitations for non-ETOPS flights. If an airplane is on fire out over the
water, it won't matter how many engines there are. The availability of an
alternate airport might make a difference, however.
2 engine aircraft are much better equipped to survive windshear and CFIT
encounters, two of the leading accident causal factors in recent years.
Unrelated independent engine failures is not a leading accident causal
factor. In addition, 2 engine aircraft under ETOPS are better protected
against related engine failures than 4 engine aircraft. That is why I feel
safer boarding a 2 engine aircraft.
--Mark Rogers
Steve Kropla
fear or better judgement, wrote:
>
>After reading the recent thread, I thought I'd offer my opinion as to why 2
>engines are safer than 4 engines on any aircraft.
Excellent post, Mark. It probably won't end the 2-3-4-6-8 debate, but
I was glad to read your sensible and reasonable take on the subject.
_________________________________
Steve Kropla
Kingwood, Texas USA
To e-mail, remove "dot" (You know which one)
Help for World Travelers: http://kropla.com
Mark Rogers
Ken Hamer <KenH...@ibm.net> wrote in article <36DCD7...@ibm.net>...
> Mark Rogers wrote:
> >
> >
> > After reading the recent thread, I thought I'd offer my opinion as to
why 2
> > engines are safer than 4 engines on any aircraft.
> >
> > The main issue is performance. Each aircraft is required to meet
certain
> > climb performance requirements on takeoff in the event of an engine
> > failure. The climb performance requirements are virtually the same,
> > regardless of whether the aircraft has 2, 3, or 4 engines. A 2 engine
> > aircraft will therefore have about the same performance on 1 engine as
a 4
> > engine aircraft has on 3 engines (each has one engine failure).
> >
> >
> Does this imply that 2 engine planes are 100% over powered, while 4
> engines are only 33% overpowered?
>
Essentially, yes. The numbers won't work out quite that well, as 4 engine
aircraft are required to have a little better engine out takeoff
performance than 2 engine aircraft (3.0 percent climb rather than a 2.4
percent climb - 14 CFR 25.121), but the point is still valid.
Anyone who's watched a heavy 747 or A340 depart knows what I'm talking
about. You never see twins lumbering into the air. Also, we routinely
easily outclimb a 727, although they cruise faster than our 737.
--Mark Rogers
(my argument is not susceptible to the "if two engines are better than 4,
then 1 should be better than 2" counter. With a loss of the only engine,
adequate takeoff performance will obviously not be obtained!)
BAP
that you are talking about the MINIMUMS and that a four engine aircraft with
one out may very well exceed those minimums, perhaps by a substantial
margin? On the other hand, the twin engine probably is engineered to make
the minimum, but without a lot of extra room to spare?
As to the availability of alternates, with a 180 minute ETOPS, you may well
be over two hours away from an alternate during a significant portion of the
flight. If the problem lasts that long, it probably isn't going to matter
if there was an alternate or not.
What about fuel usage on a twin when operating with one engine? Probably
have to have a usable alternate.
And what about Boeing lobbying hard for a non-ETOPS exemption for the 777?
All that said, I have no problem with a twin operated under ETOPS.
Mark Rogers wrote in message <01be651d$6b609020$263993d0@default>...
Robert Carpenter
>
> After reading the recent thread, I thought I'd offer my opinion as to why 2
> engines are safer than 4 engines on any aircraft.
**** Excellent post snipped ****
While you pointed out that the EA L1011 3-engine problem most likely
couldn't have happened under ETOPS rules, I think you missed a stronger
argument.
Forgetting two-emgine ETOPS rules, even if the failure cause is totally
uncorrelated, there is twice as high a likelyhood of engine failure with
four engines than with two. And as you pointed out, you are not in a
happy situation with one-out on a 3- or 4-engined plane.
ETOPS rules bias things even more strongly in favor of the twin, if not
voluntarily followed for the more-engined planes. Which brings up the
point: do any airlines voluntarily follow two-engined ETOPS maintenance
rules for their over-water planes with more engines? I'm a bit
apprehensive on ancient 727s on long overwater legs (WAS-SJU).
Bob C.
Mark Rogers
BAP <bapat...@ibm.net> wrote in article <36dd...@news1.us.ibm.net>...
> Well written. However, when comparing engine out performance, isn't it
true
> that you are talking about the MINIMUMS and that a four engine aircraft
with
> one out may very well exceed those minimums, perhaps by a substantial
> margin? On the other hand, the twin engine probably is engineered to
make
> the minimum, but without a lot of extra room to spare?
>
No, not really. The climb performance requirement is one of the factors
that limits the allowable takeoff weight for an airplane. If the aircraft
is lightly loaded, the climb performance minimums will be greatly exceeded.
When a full flight is planned, however, often the minimum climb
performance (often termed Performance Limit Weight) will restrict the
amount of passengers and cargo. When you hear the term that a flight is
"weight restricted", this is often one of the reasons. Other reasons
include Runway Limit Weight, Structural Limit Weight, Landing Weight Limit,
and Enroute Limit Weight. This is all part of the performance problem
performed by a crew before every takeoff.
A 2 engine aircraft is just as likely to be not heavily loaded, and
therefore also able to perform above the minimum climb performance
requirements.
> As to the availability of alternates, with a 180 minute ETOPS, you may
well
> be over two hours away from an alternate during a significant portion of
the
> flight. If the problem lasts that long, it probably isn't going to
matter
> if there was an alternate or not.
>
There are some problems where it won't matter, but some where it will. My
point is that ETOPS rules require that certain alternate requirements
exist, while non-ETOPS rules do not. Therefore, ETOPS flights will have a
greater margin of safety, while I concede that the margin may not always be
sufficient.
> What about fuel usage on a twin when operating with one engine? Probably
> have to have a usable alternate.
>
I guess I don't understand this question. Both types would operate the
remaining engine(s) at maximum continuos thrust (for best driftdown
performance), and would probably have similar increases in the percent of
fuel burned.
> And what about Boeing lobbying hard for a non-ETOPS exemption for the
777?
>
This is politics. While I have said I think twins are safer, I am opposed
to operating the 777 non-ETOPS. It's not because of the single engine
issue; it's because I don't think _any_ flight (2 or 40 engines) should be
operated without suitable diversion airports (with appropriate weather and
crash/fire/rescue capability, as is currently required with ETOPS).
--Mark Rogers
Mark Rogers
Robert Carpenter <rca...@erols.com> wrote in article
<36DD3F...@erols.com>...
>
> While you pointed out that the EA L1011 3-engine problem most likely
> couldn't have happened under ETOPS rules, I think you missed a stronger
> argument.
>
> Forgetting two-emgine ETOPS rules, even if the failure cause is totally
> uncorrelated, there is twice as high a likelyhood of engine failure with
> four engines than with two. And as you pointed out, you are not in a
> happy situation with one-out on a 3- or 4-engined plane.
>
I agree with this, also. I could have expanded greatly on some of my
points, but wanted to keep the post to a reasonable length. Thanks for
including this issue.
> ETOPS rules bias things even more strongly in favor of the twin, if not
> voluntarily followed for the more-engined planes. Which brings up the
> point: do any airlines voluntarily follow two-engined ETOPS maintenance
> rules for their over-water planes with more engines? I'm a bit
> apprehensive on ancient 727s on long overwater legs (WAS-SJU).
>
I know of at least one airline that designates certain portions of the
world as "ETOPS areas", and operates all of their flights to these areas
under ETOPS rules, regardless of the number of engines, or if the route
actually requires ETOPS operation.
--Mark Rogers
Mark Rogers
Steve Kropla <kro...@ibmdot.net> wrote in article
<36dcd41c...@news.supernews.com>...
>
> Excellent post, Mark. It probably won't end the 2-3-4-6-8 debate, but
> I was glad to read your sensible and reasonable take on the subject.
>
That reminds me of an old aviation joke about the number of engines a pilot
would like to have on the aircraft. The ideal scenario would be:
FLIGHT ENGINEER: "Captain! We just lost # 57"
CAPTAIN: "Which side?"
:)
--Mark Rogers
Neason
Boeing has a program in which they encourage operators to adopt
practices very similar to ETOPS practices in their entire maintenance
operation. The program is outlined either in an Aero mag or one of the
other Boeing website features. (I'm running late today or I'd go find
the article for you.)
Steve
Simon Hargrave
<mmro...@tcsn.net> writes
>
>Anyone who's watched a heavy 747 or A340 depart knows what I'm talking
>about. You never see twins lumbering into the air. Also, we routinely
>easily outclimb a 727, although they cruise faster than our 737.
Yes, I know what you mean about lumbering take-offs. My first flight on
an A340, en route for IAD from LHR, was somewhat scary for the first few
moments after take-off - it's a LONG time since I've seen the sewage
works and the M25 from so close, whereas I'm usually in a twin-engine
plane which climbs much quicker before the noise-abatement cut-off. As
for a 757, well, that has to be my favourite for high-powered leaps into
the air!
..which reminds me of an Aer Lingus 737 driver who nearly tail-scraped
right in front of me when he tried to get airborne a wee bit too quick!
>
>
>--Mark Rogers
>
>
>
--
Simon Hargrave