View Full Version : Settle a disagreement!

20th May 2002, 21:08
Engine number 1 is considered the critical engine on a propeller powered aircraft. Do jet aircraft also have a critical engine?

What are active controls?


Captain Stable
20th May 2002, 22:20
No, jet aitcraft do not have a "critical engine" in the same way as prop aircraft, in which the thrust is not symmetric about the centreline of the fuselage.

This is caused by the aerodynamic effect of the prop, in which the downgoing prop blade produces more thrust than the upgoing one. Hence, on an engine with the prop rotating clockwise (seen from behind looking forward) there is more thrust from the right side than the left.

So an aircraft with two identical engines (designed as above) will, in the event of the left (#1) engine failing, experience more assymetrical thrust than in the event of the right engine failing, since the left engine has more of its thrust closer to the fuselage. Therefore the left engine is referred to as the "critical" engine.

If the props rotate anti-clockwise, the right (#2) engine will be "critical". There are many aircraft out there on which the props rotate in opposite directions. These, therefore, have no "critical engine".

Depending upon the design of the systems (mainly on older aircraft) some jet aircraft will experience different problems with loss of aircraft systems, with ease of restart, etc. if this engine fails instead of that one. But this is not referred to as a "critical engine" problem.

21st May 2002, 04:28
Captain Stable.

The FARS refer to "failure of the critical engine", so which engine are they referring to on a jet aircraft?

Do you consider that the wind has any effect on your choice of critical engine?



(Is there a PPrune Safety Forum?)

Captain Stable
21st May 2002, 09:05
Since I'm not familiar with FARs I don't know what they are referring to.

But you don't choose a critical engine, unless you are an aircraft designer. It is imposed on you.

And, as I have pointed out, jet aircraft do not have a critical engine. Nor do some propeller-driven aircraft.

For testing and certification purposes, where you do have a critical engine, however, you almost always assume the worst-case scenario, and have that one fail.

PS, No, there isn't yet a PPRuNe Safety Forum. It's on the way.

21st May 2002, 09:21
I'm sure that, somewhere in the deep and distant past, I've come across reference to jet ac having a critical engine in the context of a X-wind and the ac's weathercocking tendencies. Most ac (I think) have a positive tendency ie they tend to turn into wind if left unchecked and, thus, failure of the upwind engine would pose the more handling difficulties. Conversely, the HS/BAe 125 Series 4 to 700, for example, have a negative tendency due to the ac geometry and therefore the downwind engine's failure is critical. Unless, of course, you know different!!

21st May 2002, 10:26
It is not a tech-legal answer, but for me, in a twin engine aircraft, I you "lose" an engine the one that remains operative is the critical one, because if you lose it, there is nothing but the ground!

Regrading the FAR´s remember that they were originally wirtten by the CAA, if I am not mistaken they were called CAR´s and then there were not Jet arcraft around.

Genghis the Engineer
21st May 2002, 13:29
All multi-engined aircraft have a critical engine, although the design and airworthiness team may not trouble to mention it to service aircrew. They don't really need to - the procedures and speeds are written around worst case, stick to them and things should work out okay, it's just that failure of the critical engine is likely to then give the lowest remaining margin of safety.

In a 4-engined jet, it may well be that the outboard engines are equally critical, but the selected critical engine during certification trials will be that into any crosswind on the day. Similarly on a twin.

But there are other issues, which get a little more complex. Not every system on an aircraft is powered by the same engines - for example on the C130 the critical engine generator happens to also power the galley (says a lot about Herc aircrew I've always thought). This is an undesirable feature of the need to keep system weight and complexity to a minimum, but can mean that control over the aircraft is, at-least initially, (until buses can be switched returning full power to whatever control was degraded by loss of the powering engine) more critical for one engine than another.

FARs are Federal Aviation Requirements. Some of them were originally based upon BCARs (British Civil Airworthiness Requirements), although in practice the two have run (almost) hand-in-hand for more years than anybody can remember. BCARs aren't used much now, having largely been superceded by European JARs (Joint Airworthiness Requirements).

Aren't all of the Pprune forums safety forums?


23rd May 2002, 02:11
A few observations, if I may ...

(a) I like Mago's idea the best ....... unless you are a sailplane pilot, of course ...

The CARs to which he refers predated the FARs and the CAA (actually the CAB - originally the Civil Aeronautics Authority (CAA) dating back to the '30s - which subsequently split into two organisations, one of which, the Civil Aeronautics Board ..) was the predecessor to the FAA. Some of the old yarns about how the original rules were dreamed up make for wonderful bar tales ...

(b) the downgoing blade idea, while appealing and of considerable use in pilot education and understanding, is only a part of the problem/answer ... the airflow patterns downstream of the prop discs are influenced considerably by the prop rotation and their interaction with the aft keel and empennage surfaces is not as simple as might be thought .. djpil, if he sees this thread, might be enticed to talk on the matter at some length ..

(c) further to mcdhu's observations, and totally off topic of course, some, if not all, of the 125s have a Va limited by directional control considerations for the same reason ...

(d) following on Genghis' thoughts, in the normal certification sense, we are principally concerned about the failure of whichever engine results in the aircraft's exhibiting the most critical handling characteristics. If there be no immediate systems consequences (hydraulics etc) which might affect the controls, then a jet is not usually going to have much of a problem when compared to a prop aircraft.

One handling factor to which mcdhu, Mutt, and Genghis are alluding is wind. Crosswind will provide a handling concern, regardless of certification considerations. I gather that some airlines, and I seem to recall CX's being mentioned in this regard in other threads, take the view that the upwind engine is, for directional control reasons, the "critical" engine during the takeoff. This concern is valid and is addressed in the various test pilot assessments of the aircraft throughout the flight test program.

More importantly, but largely ignored, there is one area of operations where wind is a critically important factor which is ignored by the design standards.

For a FAR 25 twin, the minimum V2 (often referred to as minV2 or V2min) is limited by either 1.2 Vs or 1.1 Vmca and the minV1, similarly, is limited by Vmcg. The FAA (and I presume the JAA, likewise) makes the assessment of Vmcg for nil wind conditions (AC 25-7 refers), while the BCARs used 7 knots crosswind (I had a reference long ago .. but can't recall it these days).

The problem arises when one considers that the AFM-scheduled speeds, which in the case of V1 can be based on this Vmcg-limited case, raises no concern with the crosswind. The "actual" Vmcg, with a crosswind, increases substantially above the certification value for the same reasons as discussed above. On one narrow-bodied twin jet, for which I have manufacturer's test data, this increase is half of the crosswind. On another thread it was reported that, for a particular 4-engined jet, the increase is more than the value of the crosswind. Of far greater importance is the observation that Vmcg handling problems are characterised by a very rapid onset, regardless of the wind. On one aircraft test program with which I was involved and where we looked at this in detail, you are looking at 5 knots or so making the difference between a comparatively "no-sweat" operation and quite significant centreline excursions ... another knot or two and it is a case of onto the grass ....

The problem now, is quite clear. If the takeoff be predicated on a low weight minimum speed schedule and the crosswind is significant, then the pilot WILL lose control of the aircraft and likely exit the runway edge - unless the runway is very wide and the V1/Vr split is small. Even if the pilot manages to drag the aircraft off the ground, in the handling confusion, there is a very real risk of the bank angle's being permitted to be driven by yaw-roll coupling .. which, itself, increases the Vmca and sets the pilot up for a Vmca departure. This is, as I have observed on other threads, an interesting sim exercise and, for the crews, very informative ...

Operational circumstances permitting, ths problem can be avoided on takeoffs where the RTOW is considerably higher than the actual TOW by using an appropriately higher weight speed schedule. The TOW used, naturally, still has to be not greater than the RTOW permitted value.

Upnorth ... "Active control", as a term is used in a variety of applications so it really depends on the context in which you saw the term used. Ignoring non-aviation areas, such as the common computer lexicon use, in the aerospace arena you are possibly looking at the use of computer controlled feedback systems which provide primary control inputs for gust load alleviation or flutter control. Alternatively, I notice that BAe is using the term to describe some of their work in the pilot control interface on the JSF program.

As a polite request it would be better if the topic header were more appropriate to the subject matter .. I only discovered this one after Mutt steered me to it by email .. you don't think that it might be useful if you changed the topic header to a more appropriate one ... ?

24th May 2002, 03:12
The definition of a critical engine is the engine, that it failed, will have the most adverse affect on aircraft performance.

In a single engined aircraft, it's obviously that engine.

In a prop, it is the engine with a downgoing blade closest to the fuselage (unless the props are contra rotating).

In a jet (more than 2 engines), it is the outboard engine.

Crosswind will also effect the resultant performance following a failure, and thus in a twin jet, or a prop with contra rotating props, the critical engine is the upwind engine.

E.g. B747, xwind from the right, critical engine is #4.

E.g. DC10, xwind from the left, critical engine is #1.


24th May 2002, 04:31

Do you have an official reference for this?? or is it a case of personal experience?


24th May 2002, 04:33
On a lot of twin jets, the critical engine (= the one that's going to effect your systems the most) will be No. 2. The reason is that the Captain's systems (Comms, Nav etc) are on Essential Bus but the Co-pilot's are on plain No. 2 Main - any engine problems & you're likely to lose ILS & Autopilot just when you don't want it.

More modern twin with solid state switching don't have a problem but on the older/less sophisticated models it can ruin your approach.

24th May 2002, 05:39

Both! :) If you want a reference there are a lot of textbooks around. The training manuals for any aircraft should cover it as well...


The 'critical engine' is one that affects performance rather than systems capability. ETOPs aircraft have a number of unique terms used to desribe events and contingencies that differ slightly from normal OPs aircraft (e.g. 'adequate' airports), whereby the definiations may get crossed over a little.


Genghis the Engineer
24th May 2002, 07:54
Lancer, my point about systems is that sometimes a systems degredation can also degrade handling - hence the significance in determination of the critical engine.

For the Islander interestingly, the performance and handling critical engines are different. Even BNG's flight test department have never quite worked out why.


24th May 2002, 11:00

While the FAR definition of critical engine addresses both handling and performance (Part 1 refers), I suspect that the topic originator was more concerned with the traditional engine failure handling considerations .....

.. perhaps we ought just to give up, Genghis and Mutt .... ?

27th May 2002, 06:25
Lancer - how about posting some specific references, especially for the twin jets (that contain the "critical engine" phrase)?

27th May 2002, 22:45
Hye guys.
it depends whether you are left or right handed or lets say footed, think about it. will explain latter got to go.

28th May 2002, 05:57
Okay, so some references...

The ATSB defines "the critical engine is the engine that would most adversely affect the performance or handling qualities of the aircraft". This definition is with reference to a B747-300 as part of a take-off incident investigation at http://www.atsb.gov.au/aviation/sdi/sdi20000022.cfm

Where there is no 'standard' critical engine (twin engine jet, or counter-rotating prop), the engine that will most adversely affect performance and handling, is the upwind engine in a crosswind. This is because the upwind engine will cause further drift into wind, and thus an increased degradation in performance.

To visualise exactly how that can affect the handling of the aircraft, consider a 747 taking off with the maximum steady crosswind of 30kts. At V1 the upwind, outboard engine fails... It is impossible to rotate safely without departing the runway! This occurs because there is not enough rudder authority to counteract the yaw from both sources... The same is true for many, if not all jet aircraft because the certification requirements don't take into account engine out performance in maximum crosswind.

When the maximum crosswind for takeoff is calculated it is based on the maximum demonstrated performance of the test pilot. The best performance is limited by the amount of force the rudder will provide to counteract the weathercocking effect during the takeoff roll. Losing a 'critical' engine at the most critical time supplies more yaw than the rudder can counteract, and thus a loss of controlability. That 'critical' engine is the upwind engine.

I'm trying to find somewhere that actually details all this - but for now, give it a go yourself! :D


28th May 2002, 08:48

.. you are talking rather selectively ... why not expand your commentary to make it more useful ?

Capt Claret
28th May 2002, 10:49
This question was covered in significant detail some time ago. Perhaps it was lost in one of the GSCs (Great Server Crashes).

The AFM of the BAe146 states that the critical engine is the outboard engine on the upwind side.

29th May 2002, 06:53
Just as a reminder: a prop does not always produce assymmetric thrust as may be implied in previous posts! Indeed the downgoing blade has a greater (local) AOA than the upgoing one and therefore produces more 'lift', but only if the aircraft is at a positive AOA. If the aircraft is flying straight and level there's no such thing as assymmetric thrust from a prop disc.

Obviously if you're going to lose an engine on any twin prop, the critical phase for this would be just after take-off, and the climb angle will then indeed produce the assymmetric thrust case, and yes you will have a 'critical engine' which indeed is the one with the downgoing blade closest to the fuselage.

29th May 2002, 08:23
Mutt, which part of the FAR's are you reading about the critical engine, because certain FAR parts do not apply to jet aircraft.
There is no definition of critical engine on twin jet aircraft as either engine will have the same effect to controllability, in zero wind conditions. Now what is mentioned earlier by others such as Lancer with reference to the x-wind effects are correct but that is just an external effect.
On 4 engine jets we have a speed called Vmcl-2 which is the lowest controllable speed with loss of both engines on the same side which is the worst case senario.

29th May 2002, 09:44

Sec. 25.107 Takeoff speeds.

(a) V1 must be established in relation to VEF as follows:
(1) VEF is the calibrated airspeed at which the critical engine is assumed to fail. VEF must be selected by the applicant, but may not be less than VMCG, determined under Sec. 25.149(e).
(2) V1, in terms of calibrated airspeed, is selected by the applicant;
however, V1 may not be less than VEF plus the speed gained with critical engine inoperative during the time interval between the instant at which the critical engine is failed, and the instant at which the pilot recognizes and reacts to the engine failure, as indicated by the pilot's initiation of the first action (e.g., applying brakes, reducing thrust, deploying speed brakes) to stop the airplane during accelerate-stop tests.

You are correct in saying that the “defined” critical engine doesn’t apply to twin engined jet aircraft, primarily because the aircraft was tested in perfect conditions with NO WIND.

Are we therefore right in allowing VMCG limited V1 speeds when there is a crosswind ?

Would love to continue, but I have an appointment with a pint of Guinness in the Irish Village.


29th May 2002, 11:00
.. which is why those in the know, when circumstances permit, don't needlessly expose themselves to very low speed schedules in crosswind conditions .... that way, the crosswind related increase in Vmcg can still be kept below the nominated V1 ...

30th May 2002, 10:53
Jhieminga, an aircraft flying straight and Level is at a positive angle of attack, and this only gives a low assymetric blade effect (AKA "P" factor) - you are forgetting about torque reaction, slipstream effect and the gyroscopic effect.