Engine start on ground with Engines Windmilling A320
 

Is it recommended by Airbus to do a Manual Start in case the engines are already windmilling on ground ? The airlines I fly for insists on doing a MAN start, but I fail to find Engines windmilling among the listed conditions necessitating a MAN start in Supplementary procedures

I assume the manuals are referring to engines windmilling in the wrong direction?

Some engine types have EECs which monitor rotor* speed and won't allow an autostart if rotor speed is not great enough. This, I believe, is to stop hot starts due to insufficient airflow through the engine.

If the EECs on your aircraft don't monitor rotor speed, then it's up to you to ensure engine airflow is sufficient.

*(EDIT) I mean rotor speed other than the starter-driven rotor. The rotors can be turning in different directions, slowing down the gas flow through the engine.

The engine cares about the rotor that pressurizes the combustor, that pretty much excludes the fan rotor windmilling backwards.

The windmill is insignificant on the ground to any work from the compressors. However any flow through the rear compressors has to pass through into the turbine and the turbine driving the rear compressor pretty much works as designed in turning that rotor in the right direction as long as there is flow through the engine and not just out the fan pipe.

Of course what instruments do and operating manuals say always takes precedence in discussions like this.

QUOTE: "The rotors can be turning in different directions, slowing down the gas flow through the engine".

Not so. All shafts will rotate in the same direction if an engine is windmilling irrespective of whether it has 2 or 3 shafts. The only exception of course are engines that have counter rotating shafts, Trent 1000 etc.

Lomapaseo, you're talking through your "fan pipe"! :yuk:

Bkdoss: My experience of CFM56 engines over the last twenty years shows -

The N1 fan can rotate quite fast in a tailwind. If the FADEC is powered one can see readings of up to 2% N1 rpm - however this is in the wrong direction!
The N2 fan does not usually windmill on the ground because of the high drag/load from the ancillary gearbox.

When the engine starter is engaged, the N2 shaft is driven in the correct sense and one can watch the N1 slowing down and then building in the correct sense.
Another good clue is the EGTs increasing and then decreasing as the correct direction of airflow is established through the engine prior to fuel in and lightoff.
The start then progresses normally.

Some large fan engines like the Trent700 have maximum tailwind limitations and with the A330 we used to push to crosswind for a start in these situations.

Thanks everyone for your insights.I'm a beginner on the Bus, so excuse my naivety

Meikleour, glad to hear out the views of veterans like you on this forum.

Reading your replies, I had a couple of other questions sprout up.

a) As you've pointed out, windmilling in the wrong direction can be quite a stress on the engine while starting. In fact, of the two engine variants my airlines has, CFM-5B6 and IAE V-2500, Airbus recommends manual start on IAE V-2500 when starting with tail winds more than 10 kts, but such a condition is non-existent in CFM engines. So would you suggest throwing caution to the winds and invariably using a manual start whenever Engines are windmilling in the right or wrong direction


b) I don't yet understand the fundamental difference between a manual start and an automatic one from an engine point of view. Agreed, the difference basically lies in either FADEC monitoring the start sequence or the pilot taking over control in case of man start choosing when to start the fuel pumps and so on. But in principle, while starting we follow the same sequence as the FADEC does, on when to time the opening of the fuel valves and so forth. In such a scenario, can you explain how doing a MAN start scores over an automatic one in case of tailwinds, or starting at high altitude airfields ?

Bkdoss: a) the N1 & N2 shafts rotating in different directions does not "stress" the engine - it just means that the driven(by the starter motor) N2 spool has to run for longer to establish the correct airflow direction through the engine. So, the short answer to your question is NO. Follow the manufacturer's recommendations. ie. for the CFM56 no problem. For the IAE engine - again follow the recommendations. Perhaps your airline has more aircraft with the IAE engine and they have decided to "standardise" procedures for simplicity?

b) With a manual start two things are different: Dual Igniters are used and the fuel is added by pilot input. For instance, with an autostart on the CFM fuel goes in at 22% N2 but for a manual start you may want to wait for a higher value of N2 before introducing the fuel. In addition control of over EGT is entirely the responsibility of the crew cf. the auto start.

Hope this is of help?

Meikleour,

Your response to Bkdoss is absolutely correct. As most people know, the fan is connected to the LPT. One can start the fan and LPT rotating using a finger, that is how good and balanced the bearing system is. With a tailwind entering the exit end of the LPT, it turns the LPT in the opposite direction of intended design, thereby turning the fan in the opposite direction as well. Besides the points you mention regarding the HPC and HPT, I don't think the energy of a tailwind ever gets that deep into the engine, the energy is dissipated in the LPT. As you point out there is no mechanical stress on the engine.

In CFM engine manual start when EGT exceeds starting limit before reaching 50% N2, FADEC will abort start. This is the only exception.

If one rolls back the calendar 45 years or so, PanAm's very first 747-100 revenue flight (JFK-LHR) was parked with wind up the tailpipe; it suffered hot start(s) on JT9D-3's, forcing the callup of a backup ship. Ever since then some engines have been sensitive to tailwinds, and others not so much.

I think the feedback in posts above is representative of most of the industry.

There's always the old trick - crack the reversers.

Meikleour.

Thanks for the elaborate reply . The dual igniter part slipped my mind.

I invariably put the Master switch on at 22% or at the max motoring speed i.e when I feel that the acceleration on N2 rpm has flattened out above 20%. Are there any other cues to help me decide when to switch the master ON ?

Never seen reference to that in either my airline's procedures or in Airbuses. Or on the 737. Last time I came across this was on the Quadropuff.

Someone's making up unnecessary procedures to justify his paygrade, I think.

"Well, we did it on the DC8, so we'll do it here!" :ugh:

If you want to throw away the FADEC's safeguards and unnecessarily revert to an unfamiliar procedure (I've never done a manual start out of the sim) that could be more hazardous I'd say it's a bloody silly policy, but hey ho!

I would bet the fan is turning the lpt...

Agaricus bisporus

Precisely what I thought.

On the Bus, really ? Have you ever ended up with start faults when you have have disregarded the man start conditions ?

FC,
Good bet if the wind is blowing into the fan, bad bet if the wind is blowing up the LPT from the rear…

In normal engine operation, do you think the fan drives the LPT, or does the LPT drive the fan?

So, you really think the lpt is turning the fan vs the air going through the bypass turning the fan? Considering none of the core is rotating, how much air do you think is going through the lpt?

Which one has more surface area and access to free flowing wind?

Tailwind starts
 

Turbine D:I respectfully disagree. The fan, even in reverse flow, can generate a lot more torque at low P/P than can the smaller diameter LPT, which is designed for much higher P/P.

And BOAC's remark about deploying the fan reverser makes sense in this context.

This has nothing to do with the very low P/P case.

QUOTE: "..........about deploying the fan reverser makes sense....."

Had to chuckle, it sounds like the reverser rotates the fan opposite to the normal direction of rotation. It simply diverts the direction of LP airflow forwards out through the cascades (or buckets), but then you knew that, right?. :hmm:

Anyway, arguments on fan rotation vs. p/p (wtf?) etc isn't what the OP asked.

Bkdoss, if Airbus doesn't specify it as a procedure it just isn't necessary. It may or may not be potentially harmful, but anything that reduces the automatics' protection levels must increase potential risk of damage. It does, imo, indicate that the person/persons/department that wrote and approved that requirement is less than ideally qualified for the job. It also suggests a lack of understanding of the Airbus philosophy and probably an excessive belief in their own technical superiority over Airbus themselves, the first of which is a highly undesirable trait in tech management, and the second is pretty damned unlikely.

Beware people like that, they can lead you into grief in several ways.