Flying with 2 eng out

[Hardy Heinlin]

(Tu-ench-out is also a pharaoh, I think.)

...

Can the jet pilots in this forum help to refresh my memory? It's about the difference between track and heading during a stable eng-out-flight.

I've just rediscovered old video clips taken in a Lufthansa sim in 1993, in this section I was filming, not flying, so I can't tell nor recall how aileron and rudder were set.

I'm surprised that the track line is nearly equal to the heading while the slip "ball" is way off center. It looks like the rudders were centered and only aileron were applied against the asymmetric thrust. OK, still: heading and track are almost identical. It's probably the same effect as with all engines running and side-slipping against a cross wind during the landing in order to avoid a crab angle, keeping track and heading equal.



(By the way, note how nicely she climbs with 2 eng out.)

Cheers,

|-|ardy

[Qavion]

Prior to and after the last shot, were the heading/track/slip values fairly constant? (say, within a 30 second timeframe). Or is it simply a momentary indication?

I thought it was necessary to keep the slip indicator centred during engine out (where possible) to keep the wings as level as possible. (Rhetorical question) Have they run out of both rudder trim and rudder pedal movement?

Rgds.
Q>

[Hardy Heinlin]

It was a calm, relaxed ride, everything was constant.

This phase shows an intentional left turn to heading 090.

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I thought it was necessary to keep the slip indicator centred during engine out

No, you need to give more rudder so that the slip ball goes off-center to the opposite side. Ironically, I learned that only later on some 737 sims. PS1 is not quite correct in this area.

In this video the ball is on the good engine side, so there's obviously no rudder kick into the good engines.

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[74pilot]

2 engine out (on the same side) demands a lot of rudder. But it depends on the weight and how much thrust you apply. I can only speak for the 747 Classic (for now), but it should be pretty similar to the -400. As the Vmca2 is around 160 knots, at that speed, you will need maximum rudder and a lot of aileron just to fly level, if you apply full thrust. If you are on approach, you will need less thrust, and less rudder obviously, but during 2 engine out simulator sessions I rarely find I use LESS than half rudder until I get to the flare. For survival aspects, it is all about not getting low on speed AND need to be level or even climb. Even if you can fly down to Vmca2 it is quite challenging, and you don't get a second chance. The best option is - when possible - to establish at 10000 feet on profile, maybe around 30-40 miles out, and descend on profile (to stay with the ILS until you can reach the glide path), and keeping descending, no intermediate level offs. While you are descending, you can extend all the flaps and eventually gear, and you will be committed to land. It can be quite manageable this way, but level-offs or climbs when you start to slow down requires a lot of focusing on the flying. Every time the thrust moves you have to move the rudder along with it to stay stable.

So, to answer your question you need to know

speed
weight
thrust on live engines
configuration

Level flight around 200 knots I would say you need more than half rudder and some bank, just for level flight. Just about any climbing requires more or less full rudder, unless you are flying really fast. And it is normal with the ball out a third or half way, during stable flight, and some bank, just to be level.

[Walter Kranl]

I remember well a sim session in Frankfurt when the instructor cut out 2 engines on a side (on approach). Before I realized what's going on, the bank angle was very steep. The forces on the yoke were so big, I barely could move it in direction of the good engines. I also forgot to use the rudder pedals. Of course, a crash was the result.

Cheers
Walter

[Hardy Heinlin]

Thanks.

What I would like to find out is why track and heading are nearly equal in the above scenario.

(By the way, there's no crosswind.)


Cheers,

|-|ardy

[Hardy Heinlin]

I thought it was necessary to keep the slip indicator centred during engine out

No, you need to give more rudder so that the slip ball goes off-center to the opposite side. Ironically, I learned that only later on some 737 sims. PS1 is not quite correct in this area.

Correction: Off-center on the good engine side, not opposite side.

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[Hardy Heinlin]

In that video the slip indicator (the "ball") is offset by one ball width. Can I assume that without rudder the ball would be approximately twice as far away from the center?

Unfortunately, the video doesn't reveal that.

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P.S.: Considering the fact that on traditional mechanical instruments the slip indicator is as wide as the attitude indicator, I conclude the width of the EFIS slip indicator in PS1 is waaaaaaaay to small. Ha! In the above video, one could say the ball is "slightly" offset; if the ball were at max, the ball would sit almost at the edge of the attitude indicator! Correct?

[74pilot]

I think I understand your question now.

Without rudder the ball will be way out, yet, and you will soon find yourself in a spiral dive you cannot recover from. The rudder offsets the asymmetry produced by the engines, almost, just like one engine out, but it needs more obviously. If the track and heading were far off each other (in a no wind condition), you would induce so much drag by uncoordinated flight that the aircraft would not only be uncontrollable, but also not able to maintain altitude.

The two engines on one side makes a lot of yaw, just as if you floored the rudder in one side. By giving opposite yaw you can keep the aircraft flying AND controllable, at the expense of some drag created by the rudder itself (You don't get anything for free).

[Hardy Heinlin]

Actually, there are two questions now :-)

1. Where are the graphic left/right limits on the EFIS slip indicator? – I assume the ball width is the same as that of a traditional mechanical ball. If that's correct, the freedom left/right is on EFIS as wide as on mechanical instruments, i.e. almost as wide as the attitude indicator.

2. Why can heading and track be equal on a level flight with 2 eng out and with stabilizing opposite rudder and with no wind? – I think this is absolutely impossible and that video looks like a miracle to me. I mean an aircraft is not a train on rails. If the thrust vector is no longer in the center of the fuselage, the fuselage will be drifting sideways. The rudder will just compensate the yaw, so that the aircraft stops turning. But this will not cause the fuselage (heading) to be in line with the track. I may be wrong. I'm confused :-)


Cheers,

|-|ardy

[MEade]

Hardy,

The EFIS slip indicator would be at it's max deflection in the video (as far as I can tell). 74Pilot should be able to dig out the exact numbers for the 744 (or Qavion for that matter), but in the aircraft I fly, full deflection represents 0.08g, any further then that, and the slip indicator turns amber, but doesn't move any further.

http://files.me.com/markeade/x1z83v shows the graphic to explain this (couldn't figure out how to post an image)

[Hardy Heinlin]

While we're at it ...

... at the expense of some drag created by the rudder itself (You don't get anything for free).

I know. And that leads me to question #3 that I had in mind for a very long time already:

Performance tables and FMC VNAV features provide data for 2 eng out situations. These data are used for symmetric as well as asymmetric eng out situations. Is the difference in drag so small that it can be neglected?

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[Hardy Heinlin]

Hardy,

The EFIS slip indicator would be at it's max deflection in the video (as far as I can tell). 74Pilot should be able to dig out the exact numbers for the 744 (or Qavion for that matter), but in the aircraft I fly, full deflection represents 0.08g, any further then that, and the slip indicator turns amber, but doesn't move any further.

http://files.me.com/markeade/x1z83v shows the graphic to explain this (couldn't figure out how to post an image)

Thanks. Yes, these g numbers are pretty the same on the 744 PFD. Your picture just reminded me of something: if one ball width means 0.08 g, what width would you get at 1 g (e.g. at bank angle 90, uncoordinated)? (1/0.08) * ball width = 12.5 balls ... that's a bit too much :-) OK, we won't do aerobatics, but the ball should be able to indicate more than just 0.08 g. No?

Cheers,

|-|ardy

[MEade]

Theoretically yes, the ball SHOULD indicate more than just 0.08g, but in reality, unfortunately it doesn't. So it displays 0 to 0.08g, and turns amber (but remains at its limit) if the value gets any greater than 0.08g. I suppose when it comes to instrument scanning during an engine out situation, you don't want to have to search the PFD for the slip indicator. As for the scale of movement, it was probably a compromise to keep the movement of the indicator "perceivable."

[Hardy Heinlin]

OK, that answers question #1.

And PS1 is correct then :-)


Thanks,

|-|ardy


P.S.:

74Pilot should be able to dig out the exact numbers for the 744 (or Qavion for that matter), ...

We "touched" this topic some months ago, but handbooks and real life sometimes disagree. Never trust the documents :-)

[MEade]

but handbooks and real life sometimes disagree. Never trust the documents :-)

Ever so slowly, I'm learning this very fact! So far in my short career, I have found the books either completely incorrect, or completely lacking in any useful detail. I'm sure that in the "early" days they probably gave the pilots the engineering manuals...these days they might as well give you picture books that you can color in with crayons! Oh well, at least it teaches resourcefulness.

[Qavion]

Theoretically yes, the ball SHOULD indicate more than just 0.08g, but in reality, unfortunately it doesn't.

I've seen the "ball" go beyond the edge of the pointer during an IRU test (It doesn't do it in the videos I sent you, Hardy, but it has happened on random occasions). My 744ER notes show examples of skid pointers at a full skid bar width away from the roll pointer. I can't recall whether this was on an LCD or CRT.

Not very clear, but....



During random IRU tests, momentarily the display looks a bit like the last picture on the right.

I've only seen the ball turn amber on LCD displays.

Cheers.

Q>

[Qavion]

I thought it was necessary to keep the slip indicator centred during engine out

No, you need to give more rudder so that the slip ball goes off-center to the opposite side. Ironically, I learned that only later on some 737 sims. PS1 is not quite correct in this area.

Correction: Off-center on the good engine side, not opposite side.

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Isn't this non-centered slip indication simply a consequence of not having enough rudder authority, hence the additional right aileron input (in your picture)?

Cheers
Q>

[Garry Richards]

2. Why can heading and track be equal on a level flight with 2 eng out and with stabilizing opposite rudder and with no wind? – I think this is absolutely impossible and that video looks like a miracle to me. I mean an aircraft is not a train on rails. If the thrust vector is no longer in the center of the fuselage, the fuselage will be drifting sideways. The rudder will just compensate the yaw, so that the aircraft stops turning. But this will not cause the fuselage (heading) to be in line with the track. I may be wrong. I'm confused :-)

I'm not a pilot but this question set me to thinking about the physics of this situation. The asymmetric thrust is relatively close to the centre of gravity, while the rudder force is relatively far away. The torque generated by the engines involves a large force multiplied by a short distance, while a smaller force is required by the rudder to generate the same counterbalancing torque. These two forces are perpendicular to each other, but unequal so there should be a small net thrust towards the engine-out side. If a little more rudder were applied to bring the aircraft track onto its heading, it would start to bank. Some counteracting aileron could then be used to level it. (And the skid indicator would be off centre.)

My non-pilot thoughts.

Cheers

[Hardy Heinlin]

Isn't this non-centered slip indication simply a consequence of not having enough rudder authority, hence the additional right aileron input (in your picture)?

Maybe partially. But even when enough rudder is applied and the yawing is stopped, the ball will be off center. – When I tried this on the 737-8 and -4 and gave so much rudder that the ball was centered, the yaw was overcompensated and the aircraft rolled to the opposite side. If I recall correctly, I had to keep half a ball width off center to stop all yawing and rolling. But the 737 thrust vectors obviously sit more inboard than the middle vector of the 2 engines on a 747 wing, so the ball might be more offset there – and that would agree with that video.

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