Dash-8 Q400 Critical Engine

[adamlouis]

Very simple question, what is the critical engine on the Dash-8 Q400.
Really struggling to find something to verify this online from any official source, I have a feeling it does not have counter-rotating propellers...

[adamlouis]

Thanks for the reply, do you know where I can find any official documents from Bombardier that confirms that its engine no.1 and that the propellers are rotating clockwise? Not sure where to find this... :ugh:

[SloppyJoe]

Something I have never really understood, who cares which engine is critical? I can understand test pilots worrying about this stuff but once certified being flown by a line pilot, why does anyone really care? Not like you say "oh it's number two that's failed, lets trim 5 kts or whatever off the speeds, we can make it over that hill".

[+TSRA]

and that the propellers are rotating clockwise?

The vast majority of North American built propeller aircraft have clockwise rotating engines, and I've never seen a specific reference to this in any other propeller or turbopropeller AFM / AOM for the other types I've flown. Not saying the reference is not there in other manuals, but this is just put down to general knowledge (in fact, this side of the pond we're often told the only people doing counter-clockwise propellers are the Russians).

But, to identify which way the propeller will turn doesn't require one to watch an engine start; just look at how the props are bolted onto the hub, as the position of the blade face (and, consequently, the blade back) will tell you immediately which way the propeller will turn.

who cares which engine is critical?

You're right that the average line pilot does not care. However, this knowledge is another tool in the toolbox that permits a pilot to know why they are "putting in more rudder input than would otherwise be expected." I've never subscribed to the idea that any knowledge is useless - who knows when you might just need to know that a certain engine is the critical engine? Better to know it than not.

Also, it helps to explain why some aircraft are constructed with a rudder that can travel a certain number of degrees further in one direction than the other.

[dixi188]

Sometimes that knowledge can be too useful.
On the Electra #1 was the critical engine (IIRC) and during base checks (no sim) the EFTO was normally #1 pulled back to 200hp at Vr to simulate the failure. The young and keen F/Os would be all geared up for this.
Occasionally though #4 would be pulled back and that got very interesting when the wrong rudder was applied.
My job in the middle was to call "Engine Fail" but not identify which one.
Sometimes an inboard engine would be failed and that also caused issues as very little rudder was required.

[Dan Winterland]

Occasionally though #4 would be pulled back and that got very interesting when the wrong rudder was applied.

And that's what happened to Transasia 235. In the sim, the pilots always trained for the loss of the critical engine. When 235 had an engine failure for real, the pilots did what they had always trained for and shut down the critical engine - and crashed!

Be careful how you train!

[Clandestino]

Very simple question, what is the critical engine on the Dash-8 Q400.

Very simple answer: none.

Due to P-factor (downgoing blade producing higher lift as it has higher AoA and a bit of forward velocity imparted by the aeroplane, when the aeroplane alpha is positive) the thrust centerline on the Q400 (and most western twinprops) is offset to the right, therefore No2 has longer moment arm and No1 failure requires more rudder than No2. Practically, the difference in handling is imperceptible.

There are some airframe/powerplant combination where the phrase "critical engine" really means something. It is kind of useless and potentially dangerous "knowledge" to assume that because your twin trainer behaved differently whether you lost left or right donkey, all the multiprops in your flying life will do the same.

BTW, there is no critical engine on ATR-42 300 either.

[4Screwaircrew]

Sometimes that knowledge can be too useful.
On the Electra #1 was the critical engine (IIRC) and during base checks (no sim) the EFTO was normally #1 pulled back to 200hp at Vr to simulate the failure. The young and keen F/Os would be all geared up for this.
Occasionally though #4 would be pulled back and that got very interesting when the wrong rudder was applied.
My job in the middle was to call "Engine Fail" but not identify which one.
Sometimes an inboard engine would be failed and that also caused issues as very little rudder was required.

I remember a less than young Captain doing this and the TRE taking the other side on the next departure, not a lot of room at Southend to correct that!

[Sleepybhudda]

Clandestino, no manufacturing doc for the ATR aircraft 300 and 500 series mentioned an offset thrust line for the engines or longer moment arm ie AFM,FCOM,IPC,MM etc. It would make sense though. Do you know where I could find any reference to that please?

[AerocatS2A]

Surely it is assumed knowledge that comes during basic flight training, not something that needs to be spelt out in a type specific manual?

[+TSRA]

I seem to say this on every second post nowadays, but Sleepybhudda Google is your friend.

https://www.skybrary.aero/index.php/Critical_Engine

As both Aerocat and myself have tried to say, this is not type specific knowledge but is knowledge gained during your initial multi-engine flight training. It is akin to knowing the secondary effect of flight controls.

Clandestino, do you realize in your previous post you gave the exact definition of a critical engine, thus invalidating your whole post?

There is a critical engine on both the ATR and the Dash. It is the #1/left engine.

Just because the handling qualities during a failure of the critical engine are offset by the design of the fin and the rudder and thus typically imperceptible to the pilot, does not mean the critical engine does not exist. If it did not, the manufacturer would not have to make design considerations for it.

[Sleepybhudda]

+TSRA google does not show information on ATR or Dash 8 offset thrust lines.

I am aware of what a critical engine is. That was not the question I was asking.

Let me ask you directly, do you know if the Dash 8 100-400 series or the ATR 42-72 have an offset thrust line for their PW120 series turbo-props to help counteract the affect of the failure of the critical engine. If so what official manufacturing document contains that information?

AerocatS2A and +TSRA you are correct that Critical Engine theory is covered in the Multi Engine Class Rating theory and ATPL theory, not the aircraft type rating.

[AerocatS2A]

The offset thrust line is a side effect of the propeller rotation, it is not there to counteract the effect of the failed engine. It is not a design feature, but an undesirable side effect of prop rotation. ALL props have it.

[Sleepybhudda]

Ahh I see what has caused the confusion.

I think what myself Clandestino are talking about is that the thrust axis / thrust centreline of the the ATR or DHC-8 may have be offset to counter asymmetric effects. i.e the engine is mounted slightly offset by however may degrees to the longitudinal axis of the aircraft.

But having a look through the nacelle chapter of the ATR Maintenance Manual there is no mention of an offset on one of the engines / nacelle.

[AerocatS2A]

I'm pretty sure Clandestino was talking about the thrust being offset due to the P-factor, not as a design feature.

[Sleepybhudda]

Hmmm upon reading it again I think your correct in which case yeah it has a critical engine same as any other multi-engine propeller driven aircraft.

Happy New Year!

[+TSRA]

Sleepybhudda,

The engines of the Dash and ATR are not offset, of that you are correct. Both engine are bolted equidistant from the fuselage. Although you could offset an engine such that a critical engine was effectively removed, that would create a spare parts and maintenance nightmare as things like fuel lines, bleed air lines, electrical cables, etc. would suddenly have to have left and right components to account for the differing lengths required.

Now, as I explained earlier, both Bombardier (Boeing and de Havilland before that) and ATR did add design considerations that assists pilots with critical engine loss control and makes it seem as though there is no critical engine. I speak to the Dash as I can converse intelligently about that plane having taught Ground School on all variants for over 10 years now, but the ATR is similar.

The fore rudder, on the classic Dash (100/200/300 and the applicable Q variants) is permitted to travel 16 degrees left and 18 degrees right; the extra two degrees allow for extra rudder input for a failure of the left engine (Note: fore rudder specifically as the trailing rudder is mechanically joined and, thus, not directly controlled by the pilot).

On the Q400 this difference was removed and the rudder is able to travel 18 degrees left and right and this was probably just to save system complexity and a small amount of weight in the Rudder Feel Trim and Summing Unit (about the only device Bombardier did not add an acronym for).

I should also note that this travel is permitted only with flaps extended (5 degrees or more). With flaps retracted (less than 5 degrees), both the Classic Dash and Q400 rudder is permitted to go 12 degrees left and right.

One has to ask what is easier when it comes to aircraft design; is it easier to change the whole mould of the aircraft to counteract an undersireable consequence of physics, or it is better to slap something onto the side of the airplane or make a control go a little bit further to do the same job?