Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Critical engine, jet

Tech Log The very best in practical technical discussion on the web

Critical engine, jet

Old 5th Aug 2001, 12:21
  #1 (permalink)  
Thread Starter
 
Join Date: Aug 2001
Location: png/nz
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Post Critical engine, jet

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
mkll zeypher is offline  
Old 5th Aug 2001, 19:04
  #2 (permalink)  
 
Join Date: Dec 2000
Location: Samsonite Avenue
Posts: 1,538
Likes: 0
Received 0 Likes on 0 Posts
Lightbulb

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'

Regards

MG
Mister Geezer is offline  
Old 5th Aug 2001, 20:38
  #3 (permalink)  
 
Join Date: Jun 2001
Location: France & UK
Posts: 927
Likes: 0
Received 0 Likes on 0 Posts
Post

This topic has already been discussed last month : check here.

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

MF
Manflex55 is offline  
Old 6th Aug 2001, 06:43
  #4 (permalink)  
Thread Starter
 
Join Date: Aug 2001
Location: png/nz
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Post

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.

Thanks
mkll zeypher is offline  
Old 9th Aug 2001, 19:14
  #5 (permalink)  
 
Join Date: Aug 2000
Location: Everywhere
Posts: 35
Likes: 0
Received 0 Likes on 0 Posts
Talking

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.
lets go nads is offline  
Old 9th Aug 2001, 22:05
  #6 (permalink)  
 
Join Date: Aug 2001
Location: Uk
Posts: 34
Likes: 0
Received 0 Likes on 0 Posts
Post

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.
RegionalFlyer is offline  
Old 10th Aug 2001, 05:45
  #7 (permalink)  
 
Join Date: Oct 2000
Location: Australia
Posts: 23
Likes: 0
Received 0 Likes on 0 Posts
Post

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.
Air Conditioned is offline  
Old 10th Aug 2001, 15:23
  #8 (permalink)  

Senis Semper Fidelis
 
Join Date: Feb 2001
Location: Lancashire U K
Posts: 1,288
Likes: 0
Received 0 Likes on 0 Posts
Talking

LGN Hi,

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 ]
Vfrpilotpb is offline  
Old 10th Aug 2001, 18:51
  #9 (permalink)  
 
Join Date: Sep 1999
Location: ME
Posts: 5,502
Likes: 0
Received 0 Likes on 0 Posts
Post

LGN,

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

Mutt.
mutt is offline  
Old 11th Aug 2001, 18:00
  #10 (permalink)  
 
Join Date: Mar 2001
Location: UK
Posts: 78
Likes: 0
Received 0 Likes on 0 Posts
Post

lets go nads -

your statement - 'contra-rotating props (in the same direction)'.... well, contra = contrary to = opposite direction! - get it?
Kep Ten Jim is offline  
Old 12th Aug 2001, 01:20
  #11 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 7,181
Received 93 Likes on 62 Posts
Post

Good on you, Mutt .... took the words out of my mouth ....
john_tullamarine is offline  
Old 12th Aug 2001, 02:26
  #12 (permalink)  
still learning....
 
Join Date: Jul 1999
Location: USA
Posts: 169
Likes: 0
Received 0 Likes on 0 Posts
Post

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!

quid is offline  
Old 12th Aug 2001, 11:30
  #13 (permalink)  
 
Join Date: Sep 1999
Location: ME
Posts: 5,502
Likes: 0
Received 0 Likes on 0 Posts
Post

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……… .

Mutt.
mutt is offline  
Old 12th Aug 2001, 14:20
  #14 (permalink)  
 
Join Date: Sep 1999
Location: ME
Posts: 5,502
Likes: 0
Received 0 Likes on 0 Posts
Post

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?

Thanks.

Mutt
mutt is offline  
Old 14th Aug 2001, 06:20
  #15 (permalink)  
still learning....
 
Join Date: Jul 1999
Location: USA
Posts: 169
Likes: 0
Received 0 Likes on 0 Posts
Post

mutt-

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!
quid is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.