Autostart program details

[Hardy Heinlin]

Is there, or should there be, protection against an automatic inflight windmilling relight below the min N3?

There are 3 autostart subprograms:

(A) Ground start, starter assisted
(B) Inflight start, starter assisted
(C) Inflight start, windmilling

(A) and (B) initiate starter motor operation only when N3 is below a certain limit. (C) has no such limit because it doesn't use the starter motor.

(A), (B), (C) have in common that they monitor the acceleration up to idle; if an EGT exceedance is detected, ignition and fuel is turned off. (A) and (B) also stop the starter motor. In (C) the starter motor is never used.

So in (C) the only protection lies in the fuel control.

The point is that (A) and (B) don't even begin to add fuel if no reasonable start conditions exist. In (C) there is no such initial check; it does not check N3. It adds fuel immediately and looks what is happening ...

[Will]

Seems like adding fuel directly in C could lead to a busy cockpit if the engine fails when N3 is low. I wonder if that's a current "feature" of the aircraft?

In a way it seems like bad design if it is, because you're taking an automated system, which should relieve workload, and building in the capacity for great damage if it's not monitored completely, with the potential to turn something fixable and not completely urgent (engine failure) into something unfixable (hot start).

I'd expect perhaps an "LOW N3 NO AUTOSTRT" caption or something. But that's just me being creative.

[Hardy Heinlin]

Yes, that's the question.


Another question ... (there's another funny effect) ... I'm also adjusting my N3 model so that it better matches the inflight windmilling start IAS envelope displayed on the upper EICAS. I'm not sure how far I should tune the N3 windmilling down for very low speeds. On the ground, fuel cutoff, at groundspeed zero and headwind 50 knots, my current setting generates 8% N3. Might be a bit too high*. At 8% N3 the EEC is powered by its own dedicated alternator (in PSX and in real life). When the EEC is not powered, the EICAS blanks the EPR, EGT, and oil temp indications. The fuel switch in RUN powers the EEC (by other busses), thus the indications appear. In CUTOFF, the EEC can only be powered by its own alternator. If that achieves 8% N3 RPM, the indications appear. I wonder whether anyone has ever seen EICAS indications flashing on and off at the gate in stormy weather? :-)


One more question ... Is it really true that the EICAS inflight envelope indication is removed when the fuel switch is in RUN, even though the engine is not running yet? I find the information useful as long as the engine is not started yet, especially during climb or descent when the data is changing according to the changing flight levels.


|-|ardy


* One the other hand ... Below FL140 there is no minimum IAS for a windmilling start indicated. So I assume at that low flight levels, that even a low IAS of just 100 kt will generate about 30% N3 (which is the minimum N3 for windmilling starts). If I wash this out exponentially down to zero IAS, there is still about 8% N3 at 60 knots. I think this makes sense. Maybe I should even reverse the exponential function so that the RPM rises very quickly when accelerating from low airspeeds. As we know, ram air drag is a square function of airspeed. Drag drives the windmilling. The rotor fans have nearly zero friction. In that case you would get 8% RPM at pretty low winds already.

[Jeroen Hoppenbrouwers]

I have zero experience with jet engines.

When inflight, there is a natural minimum airspeed that can be considered "flight". Say, this is 100 kts KIAS.

Even if an engine does not windmill fast enough, there will be significant airflow through it due to the airspeed. This effect will not occur on the ground while standing still. So it may be that the 30% N3 limit is more a guideline with a high chance of reliable success, than a hot start limit. The always existing inflight airflow by itself may be enough to prevent a hot start and, sometimes, to light up the engine.

In the end it is always airflow that matters, not rotation. Rotors don't burn (usually).


Hoppie

[Hardy Heinlin]

... more a guideline with a high chance of reliable success, than a hot start limit.

Well, that leads back to my first question on the previous page. If there is no success due too low RPM, I assume the low RPM will cause a hot start. The injected energy must end up somewhere: Either in normal engine thrust -- or in a hot start.

Below reasonable windmilling RPM: Failed start, i.e. hot start (or hung start, also with increasing EGT)

Above reasonable windmilling RPM: Good start, i.e. no hot start

There is a hot start protection for inflight starts. But the other question was why the windmilling autostart tries to start the engine even when the N3 is extremely low. The starter assisted autostart will not try any starts if the conditions are not reasonable. Maybe the windmilling mode is sort of a desperation mode :-)


|-|ardy


P.S.: Another conclusion: On the ground you can't autostart when the wind is such that you get 10% N3 by windmilling (autostart won't begin if N3 is 10% or higher). In that case you need to start manually solely with the help of the winds ...

[John H Watson]

But then you would get hot starts whenever you get a momentary flame-out while the deck is normally configured. Normal configuration is:

Starter switch IN
Fuel switch RUN
Autostart ON

Which aircraft? If you get a momentary flameout on a RB211 the ignitors are fired up before the engines have had the chance to spool down. Note that autorelight is a separate entity from Autostart. The Autostart switch doesn't have to be on. I thought the CF6 had something similar, but I believe it wasn't as good as the RB211 system, hence the need to make sure (CF6) CONT IGN is on in adverse conditions.

CF6
During a windmill start, the CF6 monitors for stalls (N2 rate of change <15rpm/sec) and EGT >750*C, and will shut off ignition and fuel for six seconds if this happens, then tries again with reduced fuel. If a stall occurs and EGT<750C, the fuel is increased for 15 seconds or until the N2 rate of change is greater than 70rpm. Hot start limits in flight are greater than on the ground. In flight, the limit is 960*C.
Having said that, if no stall and the EGT reaches 960*, the fuel schedule is also reduced. I guess we can assume that the engine is attempted to be windmill started contrary to the EICAS advisory (X-BLD) hotstarts are a possibility, but the engine will attempt to reduce the longevity of those hotstarts (othewise shut down the engine).

[Jeroen Hoppenbrouwers]

Well, that leads back to my first question on the previous page. If there is no success due too low RPM, I assume the low RPM will cause a hot start. The injected energy must end up somewhere: Either in normal engine thrust -- or in a hot start.

What I meant to address is that IN FLIGHT, the actual RPM may be much less relevant. Yes there will be a relationship between RPM and KIAS but in the end it's not RPM that matters but air mass flow. Unless the engine is blocked up by ashes or so, there will always be significant air mass flow due to KIAS.

Need jet engineer here.


Hoppie

[Hardy Heinlin]

Which aircraft? If you get a momentary flameout on a RB211 the ignitors are fired up before the engines have had the chance to spool down.

Yes, but for my thought experiment I assume the momentary flame-out drops the N3 below windmilling speed so that the autostart immediately takes over. Just assume there's also a momentary igniter problem. I guess the windmilling program is just not as strict as the starter assisted program; when windmilling is the last chance at low airspeeds at FL350, the system just doesn't care anymore and risks some EGT exceedances as long as it is able to cut the fuel anytime thereafter. -- Desperation mode.

Yes there will be a relationship between RPM and KIAS but in the end it's not RPM that matters but air mass flow.

Maybe. But the RPM-KIAS correlation is very dominant nevertheless. The starter assisted inflight autostart program won't begin if 30% N3 is exceeded. N3 is the only parameter for this decision. This is valid for all flight levels. The minimum IAS ranges from 280 kt at high altitudes down to 160 kt and less at low altitudes. These airspeeds must more or less correlate with the 30% N3 figure, because that is the decision maker re windmilling mode versus starter mode.


|-|ardy

[Hardy Heinlin]

On the RB211 EGT indication, should the red 600° start line be removed during an inflight start? If an inflight start allows the EGT to reach the maximum red or amber line at the top, that lower 600 line is rather irrelevant, isn't it?

[John H Watson]

Yes, but for my thought experiment I assume the momentary flame-out drops the N3 below windmilling speed so that the autostart immediately takes over.

This is getting confusing.. . I think we need to go back to the basics and also define Autostart. As I understand it, "Autostart" involves the cycling of the fuel cutoff levers (but only after you have followed the procedures for an engine start in flight). Cyling the fuel levers sends a reset signal to the EEC's, including the resetting of the start timers. Software is also reset. Also, Autostart in the air may or may not require the start switch to be pulled (but the start valve won't open automatically in flight without manual intervention)

On the other hand, auto-ignition/ auto-relight is fully automatic. Subsequent fuel control is a combination of thrust lever position and EEC computer control (The EEC is monitoring a lot more parameters than the pilot is). 

Ignition will come on when a flameout is detected. A flameout can be caused by lack of fuel or bad airflow (or torrential rain flow) through the engine. Assuming the ignitors haven't been turned on beforehand and the engine flames out and the ignitors fail at the time the engine passes through 50% (or perhaps sooner on the RB211, because it's sensing deceleration rates above 50%), then what happens next depends on what caused the problem, what the EEC does (when not in Autostart mode) to fuel flow and how long the ignition failure lasts.

I'm not really sure what would happen if you Autostarted or even manually started an engine on the ground with the fuel levers at 100%, but I think the EEC may be smart enough to NOT schedule tonnes of fuel during initial start (something to try in the big sim). It may be the same in flight in your scenario. The EEC's are looking at rpms and air pressures and adjusting fuel flow accordingly, so EGT exceedance may not be as easy as you think it is.

Cheers
JHW

[John H Watson]

The rotor fans have nearly zero friction. In that case you would get 8% RPM at pretty low winds already.

Just to clarify, this may apply to fan rotors (they are pretty big), but N3 is attached to a monstrous mechanical gearbox which is driving fuel pumps, hydraulic pumps and electrical generators (even when they are not generating). Also, the air going through the core is relatively small compared to the bypass (and the air will want to take the path of least resistance).

I don't know how much the other end of the N1 rotor is assisting the airflow through the core. Engine experts?

[Hardy Heinlin]

In PSX too, the EEC is so smart and will not go above idle fuel flow during start, even when the levers are out of idle.

RB211 relight function: I think this only works as long as N3 is not too low for too long. When a relight is not promptly successful, the conditions for windmilling autostart are met, and the program begins with its timers and cooling phases; I doubt that you need to cycle the fuel switch in this case. I think this is only necessary when a pre-start-condition timer is running or an attempt counter is to be reset. The windmilling program has no such features.

"Monstrous mechanical gearbox" -- Ah, right! I overlooked that :-) OK, let me guess: At the gate, a stormy 70 knot headwind can drive N3 up to 7%. This way we will never see 8% at the gate in PSX.


Cheers,

|-|ardy

[Phil Bunch]

I've been trying to find basic tutorial information re hot starts, hoping to bring my understanding to a higher level.

Is this excerpt from Wikipedia reasonably accurate and relevant to the 747-400's engines?  Does anyone have a link or two that might be more helpful in a tutorial sense?

https://en.wikipedia.org/wiki/Hot_start

In a jet engine - be it a pure jet, a turbofan or a turboprop - a great amount of the air ingested by the engine runs around the combustion chamber or around its flame, instead of being mixed with fuel and burned. The purpose of this air is to keep the temperature of the chamber within its limits. If it weren't for this cooling effect, the chamber would get too hot because of the combustion, and it would then be burned or even melted.

In a turbofan, the engine is started when compressed air is blown through its high-pressure stages. In a turboprop, the engine is started by the starter-generator, which is the engine's own electric generator - during the start, an electric current runs through it, making it spin, and then making the whole engine turn. In both cases, the engine must be already spinning before igniting the fuel, so as to have the compressors attain enough speed to draw air and make it flow though the engine. When there is a sufficient amount of air flowing, the fuel is injected, and then the engine will be able to run by itself.

The critical part is injecting the fuel. If the fuel is ignited before there is enough air flowing around the chamber, its temperature will increase dramatically and exceed the design limits of the combustion chamber and turbine blades, thus causing a failure. This condition is known as a hot start.

In some modern jet engines FADEC system prevents such condition from occurring. When FADEC is not present the flight crew has to monitor the engine parameters and manually shut off the fuel valve if the exhaust temperature exceeds its maximum allowed value.

<snip>

A hot start is aviation pilot slang referring to the condition during a turbine engine (turboprop, jet, turbofan, etc.) start in which the Turbine Inlet Temperature or Interstage Turbine Temperature exceeds the turbine manufacturer's prescribed starting limits. Hot starts damage the turbine engine usually by causing damage to the metal structure of the engine. A hot start may occur when fuel is introduced into the engine combustion chamber while the compressor RPM of the turbine engine is too low. When fuel is introduced without adequate air pressure from the compressor section of the turbine engine, fuel will burn in combustion chamber without adequate outflow to the turbine section of the engine, thus creating an extremely hot temperature in the combustion chamber.

[John H Watson]

RB211 relight function: I think this only works as long as N3 is not too low for too long.

Autoignition takes over when Autorelight is not available.

The Autostart Switch has no effect on the Autoignition features of the FAFC.

Down to 35%N3, the RB211 autorelight function overrides the FAFC Autoignition function

The Autoignition function of the FAFC... the logic is active when all of the following occur...

PRSODV (HP fuel valve) is open
N3 <58%
No starting faults are active
Autorelight logic is not satisfied

Autoignition doesn't give in ... which is good when the Autostart system is under MELs.

I'm still not sure about the other stuff... In some sections of my manuals the logic reads "fuel control switch in RUN", in other sections it uses the words "Moving the FCS to run"

You would only know by looking at fuel flow after a flamout with the Autostart switch ON.

Cheers
JHW

[Hardy Heinlin]

OK, I just added the condition that the fuel switch is to be cycled to CUTOFF then back RUN.

But I added this only to the RB211 inflight autostart. I hope this makes sense. For the ground start you need to pull the starter switch; that's a clear signal that an autostart is desired, isn't it? What do you think? (Who would want any auto-relight/-ignition at the gate before engine start? And the fuel switches are already in CUTOFF anyway.)

[John H Watson]

I'll ask around.....  The additional engineering training notes I have use the expression "move to RUN" a lot (more than the RB211 notes). However, pilot procedure may be guiding the wording of  the notes rather than airplane logic.

[Hardy Heinlin]

The previous PSX versions already had the autoignition/autorelight features with the conditions N3 <58% etc. There has been no conflict with autostart so far because those versions had no extended autostart programs. Your hint regarding the fuel switch cycling was very important; it solved all my problems yesterday :-)

The only thing I had to add to the autorelight logic is that it's now disabled when N3 < 35% (there was no minimum previously). During a massive compressor surge or FOD the N3 will quickly drop far below 35%, and thus autorelight will now have no chance anymore. It only works during a short flame-out.

[Britjet]

Going from cutoff to RUN is also part of the relight procedure for the multiple engine failure and volcanic ash QRH items.
Peter.

[John H Watson]

Going from cutoff to RUN is also part of the relight procedure for the multiple engine failure and volcanic ash QRH items.

Understood, Peter. I just wasn't sure whether the QRH was a necessary step to initiate autostart.... or just a backup method of initiating autostart (perhaps with slight variations). On the ground, the EEC software is reset. I'm not sure if this happens in the air. A PPRuNer suggested that the inlet guide vanes were reset on the older RR's... but I'm not sure if the EECs on new RR's are aware of engine conditions (flameout) and vary the guide vanes accordingly.

If your sim models volcanic ash with no chance of restart, the autostart operation might be detectable. The EEC will cut off fuel flow periodically. This won't be the case with AutoRelight and AutoIgnition .

The only thing I had to add to the autorelight logic is that it's now disabled when N3 < 35% (there was no minimum previously). During a massive compressor surge or FOD the N3 will quickly drop far below 35%, and thus autorelight will now have no chance anymore. It only works during a short flame-out.

Good to hear.. but I don't understand why 35% makes a difference. Below this, the AutoIgnition takes over. Just the method of detection changes and the result is the same... The ignitors activate.  Note that the RB211 has high/low ignitor energy states, not just single/dual (as on the CF6). I'll have to do some reading on this to see if it might make a difference to the chances of a relight/restart.

Rgds
JHW

[Hardy Heinlin]

Yes, autoignition takes over. I just wanted to say that autorelight resets below 35% N3. This can be noticed by looking at the ignition power status on the Analysis page; both ignitors ("B") are only powered when autorelight is operating (or when autoignition is in a certain configuration). When autoignition powers a single, good ignitor, and the engine is not damaged, the engine will restart by autoignition. If that single ignitor is contaminated (non-severe ignition malfunction), it won't restart.

The Analysis page just indicates which ignitors are powered, not whether the powered ignitor is contaminated.


|-|ardy