What's this sound on the 146?

[Malybenek]

A question to 146 / Avro experts:

Could anyone explain the strange sound coming from the engines when the aircraft stabilizes in the cruise? The sound can be heard on every flight and is very similar to the sound of an engine spooling down during shut down. Could the reduction from climb to cruise power cause such a dramatic effect?


MB

[alf5071h]

Malybenek as you state it could be the change in engine speed at top of climb, or a mis set/mismanaged synchronisation system (TMS); on the RJ series it could be a ‘hunting’ autothrottle.

For the techies, consider a cycling PTU. There were problems in the past; newer systems / setups were introduced as was, if I recall correctly, the option to turn the PTU off during the cruise to save wear.

[tallsandwich]

As a non-expert but regular commuter with Brussels SN Airlines, are you referring to the unusually loud noise created when the flaps are deployed/withdrawn on 146/Avros? Either way, I would love to know the engineering reason why this type has this very unfortunate (and I think unique) characteristic. From the ground it completely dwafs the engine noise and so must therefore have got endless attention of anyone considering the noise pollution of this type. Judging by the quantity/quality of input to the 146 refit thread concerning Fire Fighting I expect it will be answered very quickly.

[No_Speed_Restriction]

i was under the assumption that the noise heard during flap deployment was purely an aerodynamic induced side effect and nothing more.

[tallsandwich]

Sure its just a noise from fluid rushing around a bit of metal - but why does the 146 have such an unusually pronounced noise when other aircraft do not ? Is the wing design or flap linkage so very different from other aircraft?

[Malybenek]

Tallsandwich, this is definitely not created by the flaps. Not in this phase of flight.

Alf5071h, would the PTU be enough to create such an effect? Believe me, the sound causes some pax to think that at least one engine is being shut down :eek:

MB

[dhc83driver]

As a current rj100 driver, the big noise is the flaps traveling from 18 to 0 or 0 to 18. Because of the high wing design and the big flaps for good short field performance they come all the way along the wing to the cabin. The gap is filled by a flexable seal. this allows air to rush thru against the side and you can hear it very loud in the cabin, (not so loud outside) a bit like making a blade of grass whistle, its only really noisey for about the first 9 degrees then it gets better. At Flap 24 you can still hear it if you listen carfully but by then the gear is normally in transit.

The RJ has a slightly different mod than the older 146 so is a bit better. They have tried several times to reduce the noise but i find it very reasuring up front as you can hear what the aircraft is doing.

Other noise are the aircon being changed for recirc to fresh air, (with my outfit this occurs at fl100) but thats only really noticable by the increase in airflow in the cabin.

If you are climbing up to anything above fl240 then the engines are still running flatout to accelerate to cruise speed for a long time after the aircraft levels and on a long flight (heavy weights) they only come back a couple of percent and so its not very noticable.

The PTU on the 146 is on all the time but can only work if the pressure on the driven system falls below its output. so in effect it dosent turn until the green system fails.

The noise that i notice in flight is the ovens in the front galley. you don`t notice how noisy they are untill they are shut off. (big fans) You don`t notice it as they are on warming up onthe ground and then cooking for 20 -30 mins and then suddenly the cc turns them off and its very quite.

Any noises from the hydraulics bay are noticable below the floor in the middle of the cabin. The AC and DC powered pumps and the PTU make a loud whineing noise but these are only used on the ground or if you loose a engine driven pump. A cycling ptu sounds like a banging / whineing noise that repeats itself so i don`t think its that.

System wise i can`t think of anything else that changes at top of climb. big power reductions are only possible at light weights and even then its obvious its engine noise.

Any other clues?

[tallsandwich]

Many thanks to dhc83driver for the flap noise explanation, and my apologies to Malybenek for the distraction to his original question.

[Capt Claret]

My guess.

Climbing at TGT840 or Climb mode (depending on whether ALF502 v LF507). After cruise sped is reached and the TMS selected to N1 Sync or TGT sync, there can often be a largish change in N1, hence noise, before pilot bloggs corrects with the thrust levers if N1 Sync, or the engines settle if TGT Sync.

[JW411]

The BAC1-11 flaps made a very similar noise.

[safetypee]

Previous threads have discussed the flap noise and give a more accurate answer, see here. (http://pprune.org/forums/showthread.php?s=&threadid=138702&perpage=15&highlight=flap%20noise&pagenumber=2)

The PTU is armed all the time, but only runs if the hydraulic pressure falls to a predetermined value; this often happens when lowering the gear and selecting lift dump on the ground. On some aircraft with worn parts, low tolerance settings, and often with alternative vendor PTUs, the system will cycle at random.

The PTU noise can be heard in the forward cabin, and a single burst can sound like an engine running down.

[WHBM]

There was some recent discussion here on PPRuNe about the 146 flaps in transit noise which you could search for. One informed commentator said that when the prototype was first found to do this a mod was developed but it weighed 50 kg = 1 revenue pax = no modification.

For those outbound from 28 at London City (which I think includes your good self dhc83driver !), be assured that the flap noise from 3,000 feet is quite audible from the ground when standing on Stratford train station platform as you finish the 180 degree outbound turn and pull the flaps in.

I've never seen the adverse reaction by passengers to it that others write about, it's not nearly as bad as the fan noise from an IAE-powered A320 on departure/climbout, a buzz-saw-like whine, again audible from the ground and quite irritating when seated right alongside the fan inlet. If you want concern from us pax on a 146 start with the condensation "steaming" out of the air-con vents in the cabin on the ground on high humidity days.

[tallsandwich]

Can't resist another question on 146 noises...so many to choose from :D

On windy days when the engines are off, the airflow sometimes causes the fan to rotate slowly. Once I heard quite a loud clicking (proportional to the speed of fan rotation) coming from only one of the 4 engines as it rotated (can't recall if it was 146/rj85/rj100). I enquired and one of the FA's assured me that was normal, after flight deck had checked it.

I never got a decent explanation (they didn't know what is was or just knew that it was normal). Anyone here happen to know what casues this noise? why would I only hear it from one engine (port inboard) when the outboard engine was turning in the wind but did not make this noise?

[Capt Claret]

tallsandwich

Think back to how youngsters draw a christmas tree, with the zig-zag shape.

N1 fan blades are held in place by a fir-tree (christmas-tree drawing) type root, sliding into the fan hub.

Centrifugal force hold the blades steady when the engine is operating but when the N1 fan wind-mills, there is enough slack in the root/hub to allow the blades to fall a little as they rotate up and over, or down and past, the two verticals.

This is the source of the clacking noise heard when a fan wind-mills.

As an aside, I have heard of a full scale bomb inspection being caused when a new F/O told his new Captain that he could hear a ticking sound coming from the aircraft, during a pre flight inspection. Red faces all round by all accounts! :}

[tallsandwich]

Thanks Cpt C, that description certainly matches the noise I heard.