View Full Version : Critical engine, jet

mkll zeypher
5th Aug 2001, 13:21
Hi all. I was wondering if somebody could give me a quick run down on why a jet has a critical engine. I think I have it straight but just want to make sure for interview. Thanks

Mister Geezer
5th Aug 2001, 20:04
Hi there.

Mmmm I recall that only prop aircraft that do not have counter-rotating props will have a 'critical engine'

I have never known a jet of any type having an engine designated as a 'critical engine'



5th Aug 2001, 21:38
This topic has already been discussed last month : check here. (http://www.pprune.org/cgibin/ultimatebb.cgi?ubb=get_topic&f=57&t=001904)

Basically it depends whether or not U take the wind into account.


mkll zeypher
6th Aug 2001, 07:43
Thanks Manflex. I checked out the thread and, after sifting through a number of replies from people who had no idea what they were talking about, found an answer. In most jets (not all) it is the up wind outboard that is most critical.


lets go nads
9th Aug 2001, 20:14
mister geezer to make sure you understand what a critical engine is let us clarify a few things.The failure of an engine which causes the most amount of YAW is known as the critical engine.Hence in propeller aircraft (2 engines)and contra-rotating props (in the same direction) the tip of the blade going down closest to the fuselage has a smaller yaw moment than that of the other prop with its downgoing prop (because it is closest to the fuselage .) Hence failure of the engine with the downgoing blade closest to the fuselage will cause more yaw because of the longer yaw moment of the other engine.From memory it is usually the left engine in contra rotating props.

Now then what about Counter-rotating props and jet engines? All being equal and wind straight down the runway there is not (theoretically a critical engine) because failure of an engine either side of the aircraft will cause the same amount of yaw.

Now what if there was a direct crosswind? say from the left. The left hand engine fails, yaw has now been created from the right engine to bring the aircraft nose to the left the left crosswind exaserbates the problem with the wind striking the tail causing shuttlecocking AGAIN to the left. HOWEVER mr Geezer if the right hand engine fails the aicraft yaws right but shuttle cocking brings the nose to the left hence less of a problem. Hence the left hand engine is the most critical and visa versa for a Right crosswind. So as you can see there is such a thing as a critical jet engine.Sorry to all those who think i am teaching people to suck eggs this is not aimed at you.But for the Mr. Geezers of the world.

9th Aug 2001, 23:05
I agree that the wind argument is the major factor. Not sure if this is particularly relevant to jets, but I remember being told one time that it also would depend on what systems would be lost as that particular engine died. I know that most jet systems are not engine critical and most of us would except that this is a handling argument and not systems. I think this was in particular reference to an electrical generator on the Seminole.

Just another point to throw into the discussion.

Air Conditioned
10th Aug 2001, 06:45
The earlier forum discussion also mentioned that a jet can have a Critical Engine due to loss of systems, IE if the landing gear is on one system and the associated engine fails, the extended time taken for the auxillary pump to retract the gear in the first segment is used for certification. The engine is declared as such in the AFM.

10th Aug 2001, 16:23

The only Contra prop system I can think about was the Shackleton (piston x 4 egines)and the Bear(turboprop x 4 turbo), whilst I follow your reasoning for critical engine and yaw, are you referring to yaw caused by lack of power input or yaw caused by dead engine props not feathered, therfore causing huge drag on the relevent wing?

[ 10 August 2001: Message edited by: Vfrpilotpb ]

10th Aug 2001, 19:51

Now that you have that worked out, do you increase your VMCG to account for the wind?

Mutt. :)

Kep Ten Jim
11th Aug 2001, 19:00
lets go nads -

your statement - 'contra-rotating props (in the same direction)'.... well, contra = contrary to = opposite direction! - get it?

12th Aug 2001, 02:20
Good on you, Mutt .... took the words out of my mouth ....

12th Aug 2001, 03:26
mutt and jt-

I have charts for my DC-8 for both "dry" and "slippery and icy" Vmcg. The "dry" Vmcg is approx. 30 KIAS less than the "wet and slippery" values.

These lower numbers are not published for the use of line crews, but I find them valuable for test flights and 3 engine ferries. I've tested them in the simulator, and they do work. The reason, of course, is that in real life, the rudder pedal steering IS connected, and there IS friction working on the nose wheels.

You guys are the engineers, not me. Are there other a/c types that have those charts available? I would imagine there are.

On the subject of which engine is critical in a crosswind takeoff, the trick is to predict WHICH engine is going to fail!


12th Aug 2001, 12:30
Hi Quid, how’s it going?

I delved into the bottom of the broom closet to find a DC8-72 AFM with CFM-56 engines. I found charts which give VMC limited weights and speeds under the headings “COLD WET ICE” and WET DRY”. To the best of my knowledge these charts are unique to that aircraft.

I checked out a B707 AFM which is from the same era, they only have dry VMCG, as does the 737-200/747/757/777.

I then checked to see if it was a MDD way of presenting things, but once again they only have dry VMCG for the MD90 and MD11.

Do you know if this information is also available for the DC8 equipped with the older engines? Or is it specific to the high powered CFM’s?

We are presently working on a program which will predict which engine will fail, presently on the 777 we have a 50% success factor……… :) :).


12th Aug 2001, 15:20
Getting back to the “Critical Engine”, in the older B707 AFMs, they state that VMCG is based on the “failure of an outboard engine”. In the newer aircraft, this has changed to the “failure of a critical engine” which now corresponds to FAR25-149.

But can anyone point me to a Boeing or FAA definition of a “critical engine” which discusses the effect of wind?


Mutt :)

14th Aug 2001, 07:20

In my -62 and -62F manual only, I find:

"Vmc Ground NORMAL - demonstrated with rudder pedal steering operating, with a positive push force on the control column."

"Vmc Ground ADVERSE - the benefit due to rudder pedal nosewheel steering is not incorporated." [and those speeds are the same as everyone elses]

It goes on to list the various environmental conditions for the use of each.

I can see some conditions where the shorter fuselage -62 models could benefit from a lower V1 min that wouldn't be necessary on the longer stretch versions of the DC-8.

I think that "demonstrated" means someone got a STC to use the lower speeds in certain conditions. I know we have shorter Landing Field Lengths than anyone elso flying the DC-8, because we have a STC for our fleet due to our demonstrating to the FAA that we can get the airplane stopped in shorter distances than the certification data in the AFM indicates.

Re: your testing on the 777. ROTFLMAF! But, mutt, you should know better than anyone that Murphy's law would reduce that to 40/60, or maybe even 30/70!