why is ac wild power used to operate hydraulic pumps

[stubby1]

In a recent check ride on ATR, an examiner asked 'why is ac wild (variable volt & freq) used to operate hydraulic pumps '

Can anyone offer any reason?
thanks in advance:ugh:

[777AV8R]

Probably satisfies a 'know-it-all' examiner's ego to try and stump a good
performing candidate so as to make him look better than the guy he is
testing.

***Current qualified RETRE who has seen it all.

[hetfield]

Don't know the ATR

Anyhow, no need for CSD/IDG means less weight.

[John Farley]

Possibly because it is cheaper and lighter than the alternatives. (A common reason in aviation).

[763 jock]

Can one assume that you are none the wiser? In which case, your examiner has failed to enlighten/educate you with his/her knowledge. :=

[hetfield]

BTW, it's also used on A380....

[FLEXPWR]

Many items do not require a frequency stable power output. That is the case for the HYD pumps, but also the electrical heating to de-ice props and windshields.

The fact is that the ACW generators are mounted on the reduction gearbox of each engine, so the prop rpm will vary the frequency as the operations require during different phases of flight.

Note that during normal cruise/climb (especially since the -500 variant) the frequency IS in fact very close to 400Hz. There will be no need for a frequency rectifier (whatever the name is) therfore same weight, cost of building, and fuel overtime.

777, your post is pointless, unless you wanted to be seen as a smart ass... Believe if or not, as incredible as it may seem, TREs and TRIs, in the great majority, do their training job because they like to share the knowledge and experience, not to make you look bad.

Hope this helps.

[White Knight]

777, your post is pointless, unless you wanted to be seen as a smart ass... Believe if or not, as incredible as it may seem, TREs and TRIs, in the great majority, do their training job because they like to share the knowledge and experience, not to make you look bad.


Believe it or not flexxyP, in some parts of the world the TREs and TRIs have no idea what they are talking about!!!!!!

Ex TRE (and TRI if you must) in the UK:E:E:E

[FLEXPWR]

Ah yes...Thank White Knight, I have trained people on 4 continents (plus India...) and for sure one can meet funny birds, some of them you jsut wonder how they got there...
They would not try to ask anything if they don't know the answer...
:}

[STBYRUD]

My answer would be that on a plane where that actually is the case (on my 737 it isn't to my knowledge) the electrically driven hydraulic pumps draw the most current, the generator output can be used directly, the pumps don't need to run at a precise speed (since the pressure is usually regulated otherwise)...

[mustafagander]

In general, who cares??

[barit1]

I'm with John Farley, if it makes any difference. William Stout (http://en.wikipedia.org/wiki/William_Bushnell_Stout) is reputed to have admonished his designers "Simplicate, add more lightness". :)

[Slasher]

"Dunno. Of what practical purpose would it serve me to know
anyway, short of pranging in the desert and having to build a
new single engined ATR out of the wreckage of the old one
without the benefit of having a German toy plane designer?"
would be my reply to that smart aleck checkie.

[CliveL]

Just a thought, but if the pumps can take a frequency wild supply, then with a windmilling engine (or engines) where RPM drops as speed drops, or the same with a RAT, wouldn't you be able to hold hydraulic pressure down to lower airspeeds?

[hetfield]

Modern electrical power generation types (http://www.scribd.com/doc/33704941/117/Modern-electrical-power-generation-types)

page 137

[FLEXPWR]

Great idea, but try keeping a turboprop engine windmilling more than a couple of minutes, and soon you won't need hydraulics, or any other aircraft systems...you'll need a coffin. :E

[CliveL]

Ah - good point!
I must admit I had turbofans in mind ....

[barit1]

Inasmuch as a turbofan gearset drives/is driven by the core rotor, and the core windmills pretty slowly (maybe <10%) - I don't think it can do much good for you.

[FE Hoppy]

On your type it was the lightest solution.
On many other types the AC is regulated normally to 400hz. In those cases your TRE may find that if electrically driven hydraulic or fuel pumps are fitted they will be designed to best use the constant frequency supply.

[ShyTorque]

'why is ac wild (variable volt & freq) used to operate hydraulic pumps '

They are probably insensitive to variations in either so there is no point in making things more complicated.

(Reading back, this is what others have also written on the subject).

[EEngr]

... on the type of pump (motor) and supply. Some 400 Hz equipment needs 400 Hz, regulated voltage. But it is becoming more common to see AC motors driven by solid state variable speed (frequency) drives (VFD). This may be to implement variable speed control or to 'soft start' the motor from a weaker source. Once the pump system has been designed with a VFD, the input frequency is less critical. Most VFDs are made up of rectifier, DC link and inverter stages. The inverter stage is much less frequency dependent than line powered AC motors, so connecting them to a filtered, constant frequency source is pointless when all you are going to do is to make DC out of it. Less load is placed on the constant frequency supply equipment, allowing them to be reduced in size and increased in reliability.

[Blue Coyote]

is the simple answer, if you go on the Embraer 135/145 they are DC pumps, go on an ATP (unlucky) and it too has a DC pump. Go on a Saab 2000 and it has 1 DC motor driven pump and 1 AC motor driving 2 pumps. It's whatever the manufacturer feels is right for his aircraft and they all have different logics based on which systems they are interfacing with.
Your TRE is a numpty. Tell him to get a life.:suspect:

[cavortingcheetah]

As I remember things they sort of go like this...
AC or alternating current power on an aircraft is usually provided from two sources.
Inverters are used to convert DC or direct current to alternate current, usually 115v. This is stable and frequency regulated and is usually used for intruments and electrical consumers on the aircraft that do not require a lot of power but do need a stable source of alternating, regulated power.
For the heavy consumers such as hydraulic pumps, AC generators typically 26v are provided where the current drain is large but considerations of current stability are not important.
A DC motor could no doubt be used to power hydraulic pumps as well but an AC generator can provide much more power than a DC generator for any given size and weight.
An inverter with regulated frequency and voltage won't do the job?
Humpty Dumpty Numpty.

[glad rag]

Constant Speed Drive Units [CSDU's] were the bees knees 30-40 years ago allowing frequency stabilisation, along with multiple generator parallel operation and real power load sharing [reactive being matched by the electronics].

But they are heavy, costly, require line maintenance and when they go "bad" they lead to load shedding, single channel operation and associated system failures, not the best of things on a dark stormy night [and it will be].

With the reduction in both size and weight of solid state frequency stabilisation and/or freq wild rectification now down to consumer unit modules the need for stabilised frequency generation is past and modern aircraft now mostly convert the prime current down into their own needs at unit.

[Clandestino]

In a recent check ride on ATR, an examiner asked 'why is ac wild (variable volt & freq) used to operate hydraulic pumps
...and didn't provide an answer later on?

6 years on ATR, never had any hydro problem. 4 years on aeroplanes with engine driven main pumps, no probs there either. Seems to me that as long as it works, it's designer's choice.

[cavortingcheetah]

I presume that the hydraulic pumps on an ATR 42 are not supplied with power from separate AC generators but run from one or two respective CSDs or IDGs. But that really doesn't matter. All that the examiner did was to ask a reasonable question based upon general technical knowledge. Presumably candidates for professional licences are still required to know the basic differences as between AC/DC electrics and power sources? It'll certainly be important if you jump into an HS748 or an F27 which of course push button pilots won't do. Rather than being an ego question it's a one stop shop to find out how much interest the pupil really has in exploring the technicalities of his chosen profession. No one except the Russians uses Doppler today but you could be interested enough in your career to know why the train comes early through the tunnel. The specific question, the candidate's reaction to it and the manner in which he fails to answer give the examiner an instant insight into the trainee's command potential. It's an excellent idea.

[FLEXPWR]

I agree with cavortingcheetah.

Although we're going off topic, being a pilot means working in a technical environment, and you gotta have some interest in what makes an aircraft and how things work.
If one doesn't like tech stuff, they shouldn't be in the pointy end of an aircraft, more in a field growing potatoes or working in a flower shop.

memo: the ATR has indeed seperate ACW generators that drive the hydraulic pumps and anti-ice. It is a tri-phase with 115v variable frequency for most items, except using the 200v phase for the front windshield heating.

Flexy Numpty

[FE Hoppy]

@cavortingcheetah
26V AC? this is a stepped down supply and not directly generated and very rarely used to power high demand services such as pumps. Your memory must be fading.

@FLEXPWR
Your mentality is very old fashioned. Most manufacturers have for the last 30 years attempted to shift pilots away from deep technical knowledge. I deal directly with one of the major aircraft manufacturers and can tell you with certainty that the engineers themselves do not have an intimate knowledge of things outside their own speciality so when seeking an answer as to "Why" something is designed a certain way there are very few who can give a definitive answer. You yourself have not answered the question as to "Why" freq wild is used you simply stated that it is. If you know WHY then enlighten us.

The answer to "why" questions will invariably be that it was the best compromise of cost/weight available at the design stage.

As has been mentioned, there is a general shift towards freq wild AC generation as the electronics required to stabilise the supply for various consumers becomes more mature. The overriding reason as to why this is becoming popular is weight.

As an ex Flight Engineer and now tech instructor I'm more than happy with the new philosophy as in the past there was a tendency for nonsense to become common understanding usually because of someone trying to point score on their "technical knowledge". Pilots need to know what to do. Aircraft systems are now designed in such complexity with huge levels of software automation and digital systems that the idea that a pilot should have a deep technical knowledge is daft.

And just to make the point, how many of the pilots on this site know how many bits are used in the title word on their ARINC or ASCB buses? It's a question just as relevant as why is AC wild used.

Useless knowledge is of no value regardless of how much of it you have.

[FLEXPWR]

@FE Hoppy

I beg to disagree, there is no such thing as useless knowledge. I am not saying everybody should know the color of the lead cables for a starter generator, or that the 26V AC is, as an example used to power the avionics for low voltage in the cockpit (as an engineer, you are surely aware no 115V is directly used in cockpit panels, at least not to push buttons or switches touched by crew, except some CB's). But I hold on to my "old fashioned" belief: a pilot is a technical operator, and having an interest in what you operate seems a logical thing to me.

As to the answer why ACW is used, please refer to my first post, it is far from being exhaustive in description, but gives a start of an answer.

Flex

[barit1]

Is the wild frequency question appropriate for a check ride? I'll let the ATP community decide that one. After all, an engine-driven hyd pump is essentially equivalent to a wild-freq one.

But the guy (gal) who has a better-than-spec comprehension of the aeroplane's systems gets my vote: Better able to handle multiple failures/ops conditions, etc. The ability to draw logical connections is a valuable commodity in my book.

[cavortingcheetah]

Memory fade is a reality brought on by needing to cram too much too often into a small space in a big hurry for a one off emergency evacuation leading to a new type rating. Did I mean 26v AC for ground cooling fan or aircraft lighting and a 22kVA 200v 3 phase system to power de icing and windscreen heat? Who knows? Flick a switch and a system works. Flick one and it doesn't then call for the MEL and an engineer? If you're airborne then just fire up No 5.

[STOLskunkworks]

My 2 cents for what its worth....

On the one hand I kind of agree (based on personal interest of how all aircraft systems work) that an interest in what makes all aircraft systems tick can lead to some helpful insight as a pilot. In many cases a QRH checklist etc can make some very basic assumptions which may not be correct. Once in a while this can lead you down the wrong path. If you do not have adequate systems knowledge and the experience to know when to question the QRH then this could be a problem (although the chances are somewhat rare).

The truth is most failures are covered by the aircraft checklist ECAM etc. and in the real world outside the box stuff is actually very rare. Usually the complex failure requiring great technical knowledge is stuff practiced in the simulator by an instructor who wants to show how sharp they are.

One problem in a human factors is training to fixate on the possibility of some great complex "trick" that needs to be solved. Some reflection on previous accidents can show the human factors trap here.

A great example would be how many techies does it take to fly a perfectly good airliner (minus one gear light) into the Florida everglades?......

Another one I like is the examiner who needs to asks for the ITT limit in deg C of your engine. Well does it matter? Not if there is a big red line that corresponds to that limit. The gauge could be read in any unit of measure you choose and the limit will still be on a specific point on the gauge. You could have :mad: to represent the limit, pink polka dots or the ITT and it would preform the same function.

One of the big problems in a modern cockpit when the s%@it hits the fan is sifting pertinent information from the extraneous and erroneous. Sensory overload is the BIG human factors problem here and being trained to think about: "AC vs DC wild generator hyd pump system wait what was the trick....hey why is that mountain goat in the clouds (loud noise)"...... end of CVR. Could be your undoing.

In my humble opinion this extra knowledge may help maintenance troubleshoot after a flight however at the end of the day you need to concentrate on “Aviate—navigate—communicate.” and a few extras like follow the checklist Manage your resources and do not be afraid to ask for help and get the aircraft on the ground. etc.

So remember while being amateur Aeronautical Engineers can be helpful as a Pilot be the Pilot first and the Engineer second.

submitted from a techie pilot (or is that pilot techie?) with all due respect :)

[Checkboard]

I don't know the ATR systems, however to get back to the original post I would say that the hydraulic pump is a constant pressure variable volume type, and thus the pressure output is dependant on the pressure governor, not the motor speed.