Define Critical Engine...

[hoggsnortrupert]

I say old boy!

QUOTE: Ctitical engine can be defined in both aerodynamic or system , the pprune experts as usual show great knowledge by bagging the bloke when he ask's the question.


I actually thought this a very good thread up until the above, I was not aware of anyone bagging anyone.

it was actually one of the better questions, one that comes up often, and one that deserves explanation.

Just my opinion, but hey play nice and tidy up your toys before you head off to bed "old boy",Aye wot.

PS: I say stationair 8, your spelling is about as bad as mine.

Chr's
H/Snort

[Aerodynamisist]

The critical engine is the engine that, if lost, has the most adverse effect on the flight.

The critical engine for a contra rotating or turbo jet/fan on take off is the upwind engine. yaw plus weather cocking tendency.

The critical engine on a 337 from a thrust perspective is the front one as the rear is less efficient due to disturbed airflow.

[jxk]

Next question to stimulate debate - assuming 'normal' twin prop (Aztec) which way do you turn, say in the circuit, with the critical engine burning and turning and the other feathered?

[RadioSaigon]

Should be relatively self-evident I would have thought. You turn in the published circuit direction, just very carefully if that happens to have you turning into your dead engine. It's not the time to be getting creative with your airframe or your procedures.

[300Series]

"You turn in the published circuit direction"

I was always taught to turn towards the live engine unless there is some reason where you absolutely must turn towards the dead one.

300

[RadioSaigon]

Circumstantially, quite correct, but...

If you are (for example) established in a right-hand circuit on lets say late downwind or even base for landing and suffered a power-loss in your right engine, you are saying you would re-circuit asymmetric so you could make all your turns into your live engine???

Forgive me, but I would rather not be on your aircraft.

[waren9]

Isnt there a difference between contra and counter rotating propellers? Thought it deserved a mention after reading some of these posts.

[go_soaring]

When I was doing my training in 2002, after a simulated engine shutdown I was somehow (can't remember the exact circumstances) asked which way I would/could turn while on one donk. I replied into the live engine.

He replied as instructors do, really!? I then start second guessing myself, but finally settle on the live engine again. He replied ok, but dont let it worry you if you have to turn the other way.

I was then asked to take the hood off for an enroute exercise. go_soaring, put this PA34 into a 45* turn for 180* into the live engine, then again away from the live engine.

What did I notice, hardly any difference at all! So I'm with RadioSaigon..


I dont want to..
go_soaring! instead in this situation...

[300Series]

"If you are (for example) established in a right-hand circuit on lets say late downwind or even base for landing and suffered a power-loss in your right engine, you are saying you would re-circuit asymmetric so you could make all your turns into your live engine???"

RadioSaigon I didnt explain myself very well.

I was taught to turn towards the live engine with an engine failure after takeoff considering the terrain and weather and LSALT/MSA etc. If an engine failure occurred on approach then you just continue with the turn.

I definitely would not re-circuit assymetric so i could make all turns toward the live engine.

I meant to say if an engine failure occured after takeoff i would turn toward the live one depending on the terrain and weather and LSALT/MSA and that would determine circuit direction but if the engine failure occurred anywhere else in the circuit you would just continue with it.

[john_tullamarine]

if an engine failure occured after takeoff i would turn toward the live one

a very low priority, I suggest ... considering the terrain and weather first might be somewhat higher on your list .. I would hope.

[Healey 3000]

Hi all,

Not sure if its any help, but I've always defined critical engine as “The engine whose failure would most adversely affect the performance or handling qualities of the aircraft”

Healey

[TinKicker]

Here is the definition from "Janes Aerospace Dictionary - 3rd (and last) ed" and the same is in the new replacement for it "The Cambridge Aerospace Dictionary - 2004" both edited by Bill Gunston.

Critical engine - engine, the failure of which is most disadvantageous, due to asymmetric effects, loss of system power or other adverse factors; failure of critical engine at V1 is basis of takeoff certification in most multi-engine aircraft.

So it would appear that this could apply to jet aircraft as well as pistons, it all depends what the manufacturer decides to hang off each engine.

Tinkicker

[Brian Abraham]

The FAR's define critical engine as "Critical engine means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft."

FAR 25.121 (1) is an example of where "critical engine" is further mentioned. "The critical engine inoperative and the remaining engines at the power or thrust available when retraction of the landing gear is begun in accordance with §25.111 unless there is a more critical power operating condition existing later along the flight path but before the point at which the landing gear is fully retracted"

My reading of the FAR's (both 23 and 25) is that they seem to only address performance or handling qualities and not the systems aspect (only one hydraulic pump on Aztec for example). Perhaps you could shed some light John T on interpretation.

[boofhead]

You don't need to know which is the critical engine if you have a failure during takeoff. Fly Blue Line (or Vxse) and remain clear of the Red one and you will be OK. Critical engine has no practical application to flying a light twin. The only time it comes up during multi engine training is when flying the Vmca demo and even then it is not important since few light twins can actually show you a true Vmca loss of control because it is done at an altitude where full power cannot be obtained.

[hoggsnortrupert]

Boof say's!

QUOTE:Critical engine has no practical application to flying a light twin.

That is totally incorrect old boy!

If no rudder trim available, the amount of foot pressure between the L & R sides can be significant.

Even with rudder trim available, in some types the amount of rudder pedal travel available is significant between sides.

A re-think is in order here Boof!

Chr's
H/Snort.

[boofhead]

In order to fly at Vmca you do not need trim. OK, I used to use trim in the DC3, but that is not a light twin. I would never allow a student to use trim in a Vmca demo, which is the only time you will run out of rudder. If you fly at the correct speed (Vyse or Vxse) you do not need trim and you do not need to worry if you are operating with the critical engine failed. Just fly the airplane, keep it straight and remain on speed. And, in case you don't know, using trim will reduce the rudder effectiveness (if it is a trim tabbed rudder, that is...got be pedantic on this forum).
If you cannot hold the airplane without using trim maybe you need some gym work to strengthen your leg muscles??
Two other points:
1. It is tough to hold the DC3 after an engine failure; should there be a strength test before flying this type?
2. Why would anyone in his or her right mind think it necessary to put contra-rotating engines on a light twin? What possible benefit could that provide?

[Brian Abraham]

Further to my previous I found the FAA Training Handbook has the following to say re critical engine.

The critical engine is the engine whose failure has the most adverse effect on directional control. On twins with each engine rotating in conventional, clockwise rotation as viewed from the pilot’s seat, the critical engine will be the left engine.

Multiengine airplanes are subject to P-factor just as single-engine airplanes are. The descending propeller blade of each engine will produce greater thrust than the ascending blade when the airplane is operated under power and at positive angles of attack. The descending propeller blade of the right engine is also a greater distance from the center of gravity, and therefore has a longer moment arm than the descending propeller blade of the left engine. As a result, failure of the left engine will result in the most asymmetrical thrust (adverse yaw) as the right engine will be providing the remaining thrust.

Many twins are designed with a counter-rotating right engine. With this design, the degree of asymmetrical thrust is the same with either engine inoperative. No engine is more critical than the other, and a VMC demonstration may be performed with either engine windmilling.

With regard to climb performance, the multiengine airplane, particularly in the takeoff or landing configuration, may be considered to be a single-engine airplane with its powerplant divided into two units.

There is nothing in 14 CFR part 23 that requires a multiengine airplane to maintain altitude while in the takeoff or landing configuration with one engine inoperative. In fact, many twins are not required to do this in any configuration, even at sea level. The current 14 CFR part 23 single-engine climb performance requirements for reciprocating enginepowered multiengine airplanes are as follows.

More than 6,000 pounds maximum weight and/or Vso more than 61 knots: the singleengine rate of climb in feet per minute (f.p.m.) at 5,000 feet MSL must be equal to at least .027 Vso. For airplanes type certificated February 4, 1991, or thereafter, the climb requirement is expressed in terms of a climb gradient, 1.5 percent. The climb gradient is not a direct equivalent of the .027 Vso formula. Do not confuse the date of type certification with the airplane’s model year. The type certification basis of many multiengine airplanes dates back to CAR 3 (the Civil Aviation Regulations, forerunner of today’s Code of Federal Regulations).

6,000 pounds or less maximum weight and Vso 61 knots or less: the single-engine rate of climb at 5,000 feet MSL must simply be determined. The rate of climb could be a negative number. There is no requirement for a single-engine positive rate of climb at 5,000 feet or any other altitude. For light-twins type certificated February 4, 1991, or thereafter, the single engine climb gradient (positive or negative) is simply determined.

boofhead - with a normally aspirated engine Vmca at sea level is an extrapolated figure, since no test pilot is going to put himself in that position. In aircraft certification, dynamic Vmc is determined under the following conditions.

Maximum available takeoff power. Vmc increases as power is increased on the operating engine. With normally aspirated engines, VMC is highest at takeoff power and sea level, and decreases with altitude. With turbocharged engines, takeoff power, and therefore Vmc, remains constant with increases in altitude up to the engine’s critical altitude (the altitude where the engine can no longer maintain 100 percent power). Above the critical altitude, Vmc decreases just as it would with a normally aspirated engine, whose critical altitude is sea level. Vmc tests are conducted at a variety of altitudes. The results of those tests are then extrapolated to a single, sea level value.

The above indicates that what systems are hung off which engine are not a consideration when talking of critical engine in the take off phase. Off course that does not infer that a engine may not have a critical function when other considerations are made. ie only one hydraulic pump, pressurisation off only one engine and every engine becomes a critical engine should the LSALT be above your single engine absolute ceiling.

[john_tullamarine]

The critical engine is the engine whose failure has

that's the principal concern but not complete .. I'll take you up on the earlier invite .. but I need to review the FARs and ACs to make sure I don't leave too much out along the way.

[boofhead]

A light twin's critical engine is of academic interest only. Once established and published, the only time it is used is during the Vmca demonstration, which has no impact on actually flying the airplane.
From the explanation above, having contra-rotating engines allows for the demo to be done with either engine. Woop de doo.
If the demo was to be tried with the non-critical engine, surely the airplane would stall before losing directional control, as it often will even when done correctly, since most light twins cannot obtain full power at the required demo altitude. A degree or two of bank either way will have a much greater effect on handling than will the particular engine that has failed.
But I cannot think of any other occasion when critical engine would be "critical". And knowing which is critical has no effect on normal or abnormal operations. If you find yourself flying on one engine after takeoff with your speed at Vmca or less, your flight will be short enough that you can surely hold the rudder until impact. Or not; it will matter little at that point, and I promise that it will not matter if the critical engine is failed or the other one.
The rudder force and deflection required to maintain straight flight reduces as speed increases, thus if flown at blue line will be about the same regardless of which engine has failed. I consider the "rudder force" argument similar to the specious opinion of a "safe" turning direction.
When I fly a new (or regular) twin, I use the ASI to determine my single engine handling procedures, and I look at the red line, for my decision on aborting the takeoff, and at the blue line for continuing. When (if) I lose an engine I fly as close to the blue line as I can, and I know that if I go below the red line I risk a loss of control. As a CFI/MEI I see this often.
I never look at the POH to determine which is the critical engine (I can work it out if I care to) nor do I ask my students to do so.
I currently teach and operate the E90, C310, PA 23 and PA31 so I do not come from just the academic arena.