Autostart program details

[Hardy Heinlin]

Good evening,

I'm working on engine model specific, detailed autostart programs.

Here's a question ... maybe someone has an idea:

How does the EEC define a so-called "impending hotstart" to which the EEC reacts automatically; is it the event when the red EGT line is actually exceeded, or is it a combination of EGT trend and distance to the red line?

I think, by the word "impending", it takes the EGT trend into account; i.e. the EEC may react shortly before the EGT shoots above the red line.

Some documentations don't use the word "impending"; e.g. RB211 related manuals just mention EGT >730° as a hot start condition. But I think this is a simplified description, and the real system reacts way before 730° if the trend is very high.

Can anybody remember any hot starts in the big sims or real aircraft, using autostart? Did the EGT indication get a red box after the first attempt?

The RB211 autostart progam even keeps fuel and ignition on for 2 seconds after a hot start detection; it just closes the start valve immediately. If this initial reaction happened at 730° (red start line amber line), the EGT might probably rise above 999°. Or -- perhaps -- the actual critical point is the upper red line, whereas the "red start line" is just there to allow the following overshoot up to 999° 730°.


Edit:

On the RB211, the red start line is at 600°, and the amber line is at 730°. So I conclude, the start line is, in fact, just your reaction point when the EGT is shooting up for a hot start; when you cut off the fuel right at that 600° start line, you will be on the safe side while the EGT is rising further into the really critical range above 730°. The range above the start line is sort of a stopway that allows some overshooting. In other words, the critical EGT limit is always the same: during start and during flight. 730° is always the limit. Whereas the start line (600°) is not a real limit as long as the EGT varies very slowly.

I think the EEC reacts at 600°. And the EGT will rise further for a short moment, reaching 730° or more. Now I wonder whether the red reminder box should be set at 730° or at 600° already. In the existing PSX versions, during start, the red box is set at 600° already. (In the RB211 model.)


Thanks,

|-|ardy

[Britjet]

My very poor pilot understanding - as per the limitations, is that the 600C limit is for a start where the N3 is below 50%, such as a typical ground start.
The 785C limitation is generally considered an in-flight start limitation above 50% N3, and then only for a second or so.
I haven't seen a hot start in donkey's years. The newer RB211's don't seem to have this problem. The big sim models it however, albeit not very well, and I might be able to get a video of it next weekend.
An impending hot start is very easy to spot on an RB211 as the engine makes a rather unusual "whoosh" noise as it winds up.
I don't think I've seen red EGT boxes for real. It I think the big sim does show this for exceed an exceedances above 600C.
I'm sure John will have loads of info on this..
Peter

[Will]

An impending hot start is very easy to spot on an RB211 as the engine makes a rather unusual "whoosh" noise as it winds up.

Upcoming PSX feature? ;-)

[Jeroen Hoppenbrouwers]

Ask these guys to help:

https://www.youtube.com/watch?v=z8bIXeM0RJ8




:-D

[John H Watson]

I get the impression that the system is looking for high temperatures at low rpms.

In the fault isolatiion manual, in the faultfinding flow chart, one of the steps asks:

"Did the engine start begin with the EGT above 100C? Yes/No".... If Yes, the manual says simply dry motor the engine until the temperature is below 100C and try to start the engine again. This is strange, however, as one of the prerequisites for autostart "fuel on" is EGT <100 (I can only assume that spinning the engine to 25% (or max motoring) will cool the engine a little prior to start. We dry spin an engine to cool it down for maintenance, but I don't have any average spin times/temperatures. Engines can be spun for relatively long times before starter duty cycles are exceeded.

For a manual start on a ground test run, you abort the start if the EGT reaches 350C at 35%N3  (nice numbers... I think Jon mentioned these previously). I assume the FAFC does something similar.

If you drew a graph starting at 25% N3 (or max motoring) and 100C and drew a line to 35%N3 and 350C, everything above the line might represent an impending hot start to the FAFC. I think we would need data from Rolls Royce to know for sure.

[Hardy Heinlin]

That "whoosh" effect is pretty quick. I set the reaction point to 350° (correlates to ca. 35% N3). From 350° to 600° it takes just about one second. The start valve takes 0.6 seconds to close, and then there is a 2 second delay until the autostart logic shuts down fuel and ignition. All in all it still reaches 730° and so the box turns red in any case.

[John H Watson]

I think the EEC reacts at 600°. And the EGT will rise further for a short moment, reaching 730° or more. Now I wonder whether the red reminder box should be set at 730° or at 600° already. In the existing PSX versions, during start, the red box is set at 600° already. (In the RB211 model.)

Chapter 31 of the AMM (RB211 effectivity) says that when the engine is not being started, the box changes from white to amber to red as the respective amber and red bars are exceeded. During start, the box changes straight to red if the start red line is exceeded.

AMM 31-61-00

There are charts which tell us when inspections should be done.. and the degree of inspections... based on temperatures and the time spent at those temperatures. According to some old manuals I have, no action is required if 700C is not exceeded during start. The next stage requires just a tail pipe inspection. The one after that requires internal engine inspections. The final stage means compulsory engine removal.

[Hardy Heinlin]

Thanks!

Chapter 31 of the AMM (RB211 effectivity) says that when the engine is not being started, the box changes from white to amber to red as the respective amber and red bars are exceeded. During start, the box changes straight to red if the start red line is exceeded.

Is this text about the latched reminder box or just about the box displayed during the exceedance?

... no action is required if 700C is not exceeded during start.

This could mean that the 600° exceedance will not latch the red box as long as 700° hasn't been reached.


|-|ardy

[John H Watson]

I'll have to get back to you on this one... My training notes seems to be conflicting with the AMM...  There is no mention of delays in the AMM for the red boxes or of red box latching times.

The notes mention 20 second and 0.5 second delays for various things (red box appearance, red box latching). Red box latching is a sign that exceedance data will be available to engineers on the CMC maintenance pages. They also mention a higher transient limit of 805C which has no delays.

[Hardy Heinlin]

The 20 second delay is modeled in PSX and mentioned in the PSX manual on page 512. As we know, this is not for engine start. In addition to that, there are 5 and 10 minute delays for all-engines takeoff and E/O takeoff.

If the red box latching requires the exceedance to last 0.5 seconds, then that's rather irrelevant; it will always last longer than 0.5 sec anyway.

[Hardy Heinlin]

The starter assisted RB211 inflight autostart program is now implemented exactly by the book; now I see an interesting consequence: If the fuel-switch-run timer has reached 300 seconds (this is one of the engine-is-running status conditions), the ENG FAIL caution message will beep up after each failed start attempt. With an unresolvable hot start problem and repeated start attempts, you will hear the beeper about every minute :-)

[Britjet]

Which doesn't happen in real life...the engine hasn't failed - it just hasn't started...

[Hardy Heinlin]

Yes, but the (engineering) trick is that in certain parts of the system you can simulate a started, running engine by leaving the fuel switch in the RUN position for 5 minutes (on the real aircraft and in PSX), while the engine is in fact shut down.

A lot of systems work with engine-is-running information signals, e.g. certain EICAS messages, EECs etc. This signal is set when certain conditions are met; one of these conditions is the fuel switch being in RUN position. In many cases this not really just a check of the fuel switch position, but a combination of relays and a 5 minute timer.

I think the purpose of this trick is to allow engineers to simulate an "engine is running" condition for system tests. The chance of a pilot noticing this effect at the gate is probably very low; who would leave the fuel switch in RUN for 5 minutes with the engine shut down?

The timer is indicated on Instructor > Analysis > Miscellaneous in the upper right-hand corner.


|-|ardy

[Britjet]

Interesting Hardy -  I thought you were referring to the in-flight start situation. I don't remember that happening with protracted in-flight starts in the sim - where for instance if the engine didn't start you would leave the Fuel Control Switch ON in the hope that it would eventually burst into life - but I can't remember exactly. ;-)
Peter.

[Hardy Heinlin]

Maybe the message is inhibited during autostart operation ...

[Britjet]

Possibly....

[John H Watson]

All my books say is...

(PW) N2 less than 52%*, oil pressure less than 70 psi*, fuel control switch ON, GCB is open and airplane in air mode.... but I think they used the default  PW values)

(RR) <50% N3, 25psi
(GE) <50% N2, 10 psi

What oil pressure does a windmilling engine produce in cruise (before the aircraft starts descending)

I don't think I've ever seen this message in the maintenance world (even after 5 minutes of fuel on and no engine start). However, you do get Lower EICAS EICAS engine paramater pop-ups after 5 minutes (and a bunch of messages) if you leave the fuel cutoff levers on too long without starting an engine.

(EDIT: I don't recall the position of the Autostart switch during my ground tests)

[Hardy Heinlin]

In PSX, the ENG FAIL message is inhibited on the ground.

I could also inhibit it when the autostart program is in a certain phase. I can't inhibit it during the entire autostart process because it's always in process when windmilling with the fuel switch in RUN; and that's always the configuration when an engine fails -- it will start windmilling while the fuel switch is still in RUN.


Edit: I've rechecked the conditions. I can't inhibit the ENG FAIL message in that relevant autostart phase. The engine failure detection always happens in the same phase no matter whether the failure occurs the first time in normal flight (when you really need the alert) or during restart attempts. When an attempt fails, the autostart program loop returns to its initial task, and this initial task will also run when the engine fails the first time. My ENG FAIL message conditions are exactly the same as described in engineering documents. I'll keep it as.

(Actually, the ELEC GEN OFF message is the original ENG FAIL trigger, among others; that ELEC message should be inhibited, if at all. But the fuel switch condition is the same. It's the same problem.)

[Hardy Heinlin]

As we know, fuel-on conditions require at least 25% N3 on the RB211. If fuel is injected too early, a hot start may occur.

For an inflight windmilling start, at least 30% N3 is required (which is about max motoring speed).

What will most likely happen when attempting a windmilling start below, say, 20% N3?

Will this cause a hot start?

In PSX, when this is done during a ground start, you will get a hot start. But not so during an inflight start. I was considering adding this hot start effect for inflight starts as well. 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

So when you get a flame-out at low IAS (below inflight start IAS envelope) with N3 dropping to 20%, with the three switches in the above setting, autostart will be trying windmilling restarts as long as the start switch is IN. And as soon as the first windmilling restart begins at 20% N3, a hot start would be the logical consequence. I think that shouldn't happen.

But then, what's the point of having a minimum N3 for windmilling? If a hot start won't occur below minimum N3, it should spool up normally from below minimum N3. And if it does stall below that minimum, there must be a hot start effect. But if there is a hot start effect, windmilling assisted autostart shouldn't even try to open the fuel metering valve as it is sort of "forbidden" to do windmilling starts below the minimum.


|-|ardy

[Will]

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