Asymmetric flight (twin jet with 1 engine out) without banking.

[manucordier]

Hi,

I am at Oxford OAA doing an MCC/JOC (great experience here by the way) and we flew yesterday on the sim (B737-400) with one engine out and rudder trim to control the aircraft (we put the ball in the center again only with rudder trim).

We were flying on a radial 180° outbound when we got a right engine failure. Then the instructor told us to level the wings, center the ball with the rudder trim, and to maintain a heading of 180°, maintaining the wings level. I then thought the aircraft would be drifting in the direction of the dead engine. But it did not and I don't understand why. The aircraft maintained a heading of 180° and stayed on the radial without drifting.

However if you have a look at the following picture I just drew,

http://cours-de-math.eu/jet-asymmetric-flight.jpg

It is clear that if you control the aircraft only with the rudder and if you don't bank towards the dead engine to balance the F2 force due to the rudder, you drift !

Don't you agree ?

Am I missing something ?

I drew another sketch with banking to show the effect of bank in asymmetric flight, have a look:

http://cours-de-math.eu/jet-asymmetric-flight-bank.jpg

Do you agree with my sketch ?

Thank you for your help.

[wiggy]

As far I see it ( in very simple terms as always) the error is you've drawn the force generated by the live engine and the force generated by the rudder as if they act both act directly/linearly through the C of G of the aircraft, whereas instead the forces are generating couples around the C of G. One is the couple generated by the asymmetry (clockwise in the case of your drawing), hopefully opposed by the (anticlockwise) couple generated by the rudder.

Standing by to be shot down or someone to draw a nice diagram!!!

[manucordier]

Wiggy,

For an object to be in equilibrium, it has to verify two conditions:

1). Sum of forces = 0
Here we work in two dimensions so we consider only the forces along an X axis (longitudinal axis of the aircraft) and an Y axis (which is the lateral axis of the aircraft).

Sum of Fx = 0
Sum of Fy = 0

2). Sum of moments = 0
We consider here the moment around CG.

We can always apply the forces at the CG of an object if we consider the moment of each force.

We can perfectly well have an object with a total moment = 0 so it doesn't rotate. But the sum of the forces is not zero so it undergoes a translation motion.

By the way, if you cut the shape of a basic aircraft in a sheet of paper, put it on a table, then you push forwards with your finger its left wing (this simulates the thrust of the engine alive), and at the same time you prevent the rotation of the aircraft by pushing the tail to the right (this simulates the force of the rudder), the aircraft is drifting ! But it doesn't rotate. Try it and tell me your conclusions !

[wiggy]

Ah, Well I was trying to keep it simple, but until one of the experts arrive I'll offer you this (prop biased) paper:

http://www.cast-safety.org/pdf/5_asymmetric_flight.pdf

Out of interest how much "drift"/wing down did you expect to see in the sim?

[manucordier]

I think that I was expecting let's say between a 3 and 5 degrees drift on my required track. But I just made that number up. I really have no idea because I never did that exercise with a real aircraft.

[Edgington]

You have pretty much answered your own question, if you use enough rudder you can maintain heading even though you will sideslip. When flying a track just adjust your heading to maintain the required track or let the A/P do it for you.
Using bank increases performance by using part of the lift vector to reduce yaw, hence requiring less rudder and decreasing sideslip angle. Less sideslip is less drag, thus increasing excess thrust and performance. Which is not really an issue during cruise.

[punk666]

Regarding the technique used...dont follow the ball!! especially in the sim.
Best technique is to use enough rudder input to keep the wings level using the control wheel as a reference.

[john_tullamarine]

Unless I missed it along the way, I don't think anyone commented on the main reasons you were advised to maintain wings level - which is a common technique on large jets.

(a) drag with wings level is not critically greater than with a few degrees into the operating engines. Note that bank into the operating engine(s) is relevant for Vmca determination (as it reduces Vmca for the OEM - the certification limit of 5° keeps everyone honest) but is not usually necessary for continued OEI flight. In general, optimum bank angle is around 2-3° for OEI climb performance (for the reasons cited).

(b) some jets have AH/ADI which are not suitable for maintaining small bank angle indications.

(c) the 737 lateral control system includes both aileron and spoilers. If control inputs get to the stage of commanding spoiler activation then drag increases significantly. Trying to maintain a constant bank angle requires a control wheel input. While I can't recall the angle at which spoilers will rise, it is easy to get into that situation.

Overall, the compromise is to accept a very slightly degraded climb performance for operational simplicity and reduction in pilot manipulative workload.