Useless ATR question

[JammedStab]

Can someone tell me why the #1 throttle knob is almost twice as wide as the #2.

Useless info but I am curious.

[BrasiliaCaptain]

I don't know the answer; my training department colleagues and I once pondered this question. The best answer we came up with was to allow for proper finger clearance space between the right power lever and the left condition lever.

[Yousef Breckenheimer]

Because No.1 is the critical engine.

[powerstall]

I second the motion,

Engine #1 is the critical engine, which is wider to remind the pilot of that fact. :E

[ross_M]

Ergonomics? So that you can identify them by feel?

[Clandestino]

We had this discussion a few times before:

ATR thrust levers (http://www.pprune.org/tech-log/119515-atr-thrust-levers.html)

ATR power levers (http://www.pprune.org/questions/247370-atr-power-levers.html)

I still maintain it was design screw-up, corrected in crude way. Still it works fine, sole unintended consequence being some puzzled pilots.

[airman13]

yousef is right.

[Clandestino]

Why would no1 be critical engine on modern turboprop heavy twin, which is almost, but not quite, entirely unlike Piper Aztec?

Where in the ATR manuals is mentioned that the aeroplane has a critical engine?

What's the difference in control forces and authority between No1 and No2 failure on ATR?

[Roger Greendeck]

If the props turn the same way (which they do on the ATR) then there will be a critical engine. Because the the thrust is further from the CofG on the right engine (with clockwise rotation), all other things being equal, you will need more rudder to keep the aircraft straight if the left engine stops.

Now as to why you would need to know this during an actual engine failure in a modern turboprop is another question. The actions for handling an engine failure are the same regardless of which engine fails.

[Clandestino]

It's all very well to know about P-factor and I have no doubt it exists yet it is so weak on ATR there's no perceptible difference in handling the no1/no2 failure.

Critical aeroplanes have critical engines.

[BrasiliaCaptain]

There is no doubt in my mind that the ATR has a critical engine. However, I don't really see the logic in designing one power lever to be larger than the other as a reminder.

Someone pointed out that there is no perceptible difference in control forces required to handle an engine failure on takeoff. While I've never had one in the airplane, I've had many 'V1 cuts' in the simulator and I've trained dozens of pilots to deal with them, and I agree with that observation. None of my students ever remarked on any differences in control forces required in this scenario, and I imagine that only an experienced engineering test pilot would be able to readily perceive any difference. Besides, in that scenario, a pilot must reflexively and promptly input what is required to keep the airplane from departing controlled flight. It is unlikely that a tactile reminder of which engine is critical would be useful; for him/her, the one that failed is now the critical engine in any case!

I would also go as far as to say that knowledge of the critical engine (and its application) is a double-edged sword. Pilots should know that even in modern airplanes, differences in required control pressures MIGHT exist depending on which engine fails. However, in the US the FAA also dictates that applicants must demonstrate handling a failure of the critical engine during a check ride. So an applicant taking a check ride might not know when an engine will fail, but he will know which one will fail. Since identifying the failed engine is of some importance, the evaluation of this skill is compromised by the FAA's requirement.

[FLEXPWR]

Clandestino,

I beg to differ. The ATR has a critical engine and it is the No1. It is mathematical, not about what you feel, and there is a notable difference which one fails, but you will feel this difference close to VMCA mostly. If you have the opportunity to practice in a simulator that can record and later playback your flight (like ATC in Toulouse), you will see that the displacement of the rudder is higher when the No1 engine fails than with the No2.

Anything away from VMCA will leave you with litlle or no noticeable difference. Flying at VMCA with the critical engine failed leaves you with very little to play with. This is one of the reasons that made the rudder bigger on the ATR 42-500 (they took the -72 design) to cope with extra torque and allow you a lower VMCA.

As to getting back to basics, the P factor is much more significant on the ATR than you would have on the Aztec a you mentioned.

The power levers difference in size was mentioned rightly by Smiert Sponiom (I think it's the poster's ID) on another thread. It has to do with a design cock up involving Go around buttons on both sides of each lever. Nothing to do with critical engine identification, but it makes for a good reason nowadays when people ask.

You could ask another one about the rudder cam, very few people know WHY it is here, but most people know roughly WHAT it does, and that you have to center it before take off. (bear in mind rudder cam and rudder trim are two different pieces of equipment)

[Mach E Avelli]

The critical engine of any multi-engine aeroplane is of academic interest only. Something useless to file away for the anal check pilot given to asking petty questions.
When an engine quits, you don't have the luxury of selecting which one to shut down, or for that matter the luxury of treating the less critical one (if it has been considerate enough to be the one which has failed) as less serious than its cohort with regards to correct handling technique.
Back to the original question, I go with the idea that originally they needed a larger housing on one throttle lever to accommodate the Go-Around switchology, probably ordered 10,000 of them and now need to use them up.

[JammedStab]

Someone pointed out that there is no perceptible difference in control forces required to handle an engine failure on takeoff. While I've never had one in the airplane, I've had many 'V1 cuts' in the simulator and I've trained dozens of pilots to deal with them, and I agree with that observation. None of my students ever remarked on any differences in control forces required in this scenario, and I imagine that only an experienced engineering test pilot would be able to readily perceive any difference.

I wouldn't trust a simulator to give you an accurate feel for an engine failure in terms of critical engine. I have flown 7 sims and none of them were accurate representations of the actual aircraft. Or at least there were significant differences.

[JammedStab]

You could ask another one about the rudder cam, very few people know WHY it is here, but most people know roughly WHAT it does, and that you have to center it before take off. (bear in mind rudder cam and rudder trim are two different pieces of equipment)

Flex

Would love to hear your detailed description of the rudder cam.

[FLEXPWR]

Off topic, but to answer your question JammedStab, there is flow separation on the upper side of the rudder, due in part to interference with the horizontal plane of the elevator. It is mildly corrected by wortex generators fitted on either side of the fin, close to the elevator.

Yet there was still some rudder aerodynamic fluctuations even after this addition, which may or may not be felt at the pedals, since the whole rudder system is on a spring tab design.

One solution to dampen these unwanted fluctuations was to encase a set of springs in a box, with a linkage from the rudder pressed between these two springs. The fluctuations were stopped, but one problem remains: if the system was locked in one position, the rudder travel would be greatly limited (the rudder cam box is about the size of a pack of cigarettes). If left as is, regular trimming of the spring tab would be acting against the springs in the dampening box. Therefore it needs to be moved (or displaced) when the rudder is being trimmed, and re-locked when the new trim position has been attained.

Hard to visualize the whole works without a picture, but this is the very reason why the rudder cam was designed. Very small cost, rather than redesigning the whole rudder-elevator assembly.

Although I do not have documentation to sustain this, I have worked for ATR as an instructor in Toulouse, and got this information first hand from the assembly line. There could be of course other reasons I am not aware of.

Hope this helps.

[fantom]

I respectfully suggest some of you re-assess what 'critical engine' means.

If the props turn the same way (which they do on the ATR) then there will be a critical engine

That's the important thing; it's all to do with the way prop-wash hits the fin (or, rather, rudder).

It makes no difference with counter-rotating props.

[no-hoper]

From the maintenance manual:
http://imageshack.us/a/img40/9835/rcutext.jpg (http://imageshack.us/photo/my-images/40/rcutext.jpg/)
http://imageshack.us/a/img832/6284/rcuys.jpg (http://imageshack.us/photo/my-images/832/rcuys.jpg/)
http://imageshack.us/a/img703/4530/springtab.jpg (http://imageshack.us/photo/my-images/703/springtab.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)
Uploaded with ImageShack.us (http://imageshack.us)
Uploaded with ImageShack.us (http://imageshack.us)
http://imageshack.us/a/img40/9835/rcutext.jpg <a href=http://imageshack.us/a/img832/6284/rcuys.jpg target=_blank>http://imageshack.us/a/img832/6284/rcuys.jpg</a> http://imageshack.us/a/img703/4530/springtab.jpg
http://imageshack.us/a/img40/9835/rcutext.jpg <a href=http://imageshack.us/a/img832/6284/rcuys.jpg target=_blank>http://imageshack.us/a/img832/6284/rcuys.jpg</a> http://imageshack.us/a/img703/4530/springtab.jpg

[Roger Greendeck]

Fantom, more information on what you think a critical engine is and why counter rotating props don't make a difference.

[DaveReidUK]

Fantom, more information on what you think a critical engine is and why counter rotating props don't make a difference.

Could have been phrased better, but he is referring to the fact that that C/R props result in there no longer being a critical engine (i.e. no difference whichever one fails), not that they don't make a difference to whether there is or isn't one !

[FLEXPWR]

Fantom,

No, it is not ALL to do with prop wash hitting the fin. In fact it has almost nothing to do with it. It is a matter of thrust location/distance from the center of gravity of the aircraft, the farther away, the more trouble with engine failure on a twin/ multi-engine.

[twochai]

Sorry folks, there seem to be many mis-conceptions here.

The reason multi-engined propeller driven aircraft have a critical engine is due to the impact the rotating vortex trailing from each propeller makes on the wing and vertical surfaces behind, it while producing thrust: i.e. the angle of attack of the vortex on the wing and the vertical surfaces is different, depending upon the direction of rotation of the prop, the thrust being produced and the indicated airspeed of the aircraft.

The critical engine will be determined during flight testing for Vmca, which is defined as the minimum speed at which directional control can be maintained under defined circumstances (i.e. sudden power cut, maximum bank angle of 5 degrees, etc.).

On aircraft with propellers rotating in a clockwise direction, as viewed from behind, the critical engine will usually (but, not always) be the left engine. In other words, the failure of the left engine will cause loss of directional control at a higher speed than a failure of the right and therefore is more critical. On some aircraft, an engine failure at Vmca will result in a significant difference in performance between a left engine failure and a right engine failure.

[john_tullamarine]

Several observations ..

is due to the impact the rotating vortex trailing from each propeller makes on the wing and vertical surfaces behind

Probably a tad simplistic.

sudden power cut, maximum bank angle of 5 degrees, etc.

Generally the FT workup involves establishing what generally is referred to as the static Vmc .. bit like the pilot endorsement demonstration activity. Once this is established, checks will be made with sudden failures to make sure that the (you guessed it) dynamic Vmc situation doesn't provide any unpleasant surprises .. if it does, then the numbers will be revised to suit.

Important to keep in mind that real Vmc stuff potentially is VERY dangerous and one is best advised to stay some speed above the real world Vmc on the day.

an engine failure at Vmca will result in a significant difference in performance

True, depending on Type, but not overly relevant to anything. Vmc is concerned with handling rather than performance. Unless and until the pilot gets the aeroplane back up to a speed appropriate for OEI climb things generally are going to be untidy so far as climb is concerned ..

[twochai]

John:

All true and your cautions are wise.

Twochai