B737-800 Engine Start N2 question

[cpt_shawky]

From FCOM

ENGINE START switch ................................................GRD F/O
Verify that the N2 RPM increases. C, F/O

When N1 rotation is seen and N2 is at 25%, or (if 25% N2 is not possible), at maximum motoring and a minimum of 20% N2:
Note: Maximum motoring occurs when N2 acceleration is less than 1% in approximately 5 seconds.

My question is moving the engine start lever to IDLE detent preferred once the N2 25% or you wait maximum motoring as I see some captains wait till it reach 29 to 32% and they said this to avoid hot start problems.

also what is the exact definition to the term "Maximum motoring" as I want you to correct my understanding to the above paragraph as I believe it means not more than 25% [[ (if 25% N2 is not possible), at maximum motoring]] this why I wonder why captains wait till the reach higher N2 i.e: 29-32% ?

[clonemole]

Well there must be some wisdom in boeing saying that you need to put the start lever to idle at 25% or they would have mentioned to wait till the N2 reached max motoring(which is only if it doesnt reach 25%).
as far as max motoring goes, as it says, it is the stage whn ur N2 has reached an almost constant figure(<1% change in 5 secs)
Even I have encountered some capts who wait for increased motoring(some saying they want to decrease the EGT) i guess its their own personal procedure so better to check with them :}
Cheers!

[Denti]

Sounds like an old version of the FCOM you have there. We used to have the same description a couple years back, but it got changed by Boeing to say max motoring or 20% whichever is higher only. No 25% at all anymore.

[cpt_shawky]

my FCOM is the current & its for B737-86N date 27 March, 2009

[Denti]

From the customer code it looks like its been tailored for GECAS (which is a leasing company) or the specific configuration of that plane.

As i said, we had that until two or three years ago but then it got changed by Boeing. So some of your captains might be used to that if they joined after that time. It does indeed decrease thermal stress on the engine and leads to longer engine life (or so Mr. Boeing told us back then).

[muduckace]

Not on a 737-800 but as a basic rule, we tried to get our drivers to throw fuel in past minimum start % just as there was a noticable decrease in N2 rotation acceleration. Unless you are high and hot where thin air may make a hot start likely don't do it, you will save the company starters as they take the abuse. We determined that wear and tear in the hot section that some worried about was not a factor.

Not to mention the increased chance of a starter coming apart, if the shaft does not shear and a starter comes apart the damage it can do is extreme, those things spin at insaine RPM's.

It is a bad habit to throw fuel in past max motor.

[RAT 5]

Reducing EGT with motoring engines was something a BIG fan engine would do. On these smaller versions I doubt it is significant.
Regarding N1%. There is no tailwind limitation on starting engines, but would anyone put fuel BEFORE N1% was showing correct rotation? I note he engines on NG's don't show -ve rotation with a tailwind, only 0%. Then they often beccome +ve aroubnd 27% N2. Thus I wait to confirm all the roatating bits are going in the correct direction before giving them a large shock. Thoughts?

Further, but connected. Inflight Start. I teach to wait until 25%N2 if possible, when at lower altitudes. The 11% minimum means for me it is available at the top of the flight envelope and might well be max motoring up there. Between 5000' & 25000' there might be some line graph which means you'll get only e.g. 17% at a higher FL. But surely, injecting fuel at 11%N2 at low levels can not be a sound thing to do. Could well produce a hotter start, even though the forward speed will help. Thoughts?

[RMC]

RAT 5 is correct the temp reduction in degrees C is in single digits on this engine.

I personally don't move that lever until I see N1 rotation (used to say it in the old checklist). I try and delay start until there is no significant tailwind which means N1 rotation always occurs before 25% N2.

[captjns]

Under normal conditions, one can expect N1 rotation at about 18% to 20% or N2 rotation.

CFM places no tailwind limitation regarding engine starting.

The EEC monitors the engine for impending hot starts while on the ground.

I do remember in a former life, during intermediate stops, we motored the RB211 until EGT was below a certain value.

[BOAC]

The variations are endless. I used to fly on 737 with Captains who would wait till max motoring or motor until the EGT was less than x or y or z, despite company published procedures. I think with all of this just do what you are told by your company (and HOPE that your company has cleared the procedure with the manufacturer.)

Regarding tailwind starts, I think I picked up from BM a few years back a simple and effective technique of starting with reversers deployed until N1 rotation - not approved but *** useful in a 60kt tailwind if you cannot be moved.

[captjns]

Regarding tailwind starts, I think I picked up from BM a few years back a simple and effective technique of starting with reversers deployed until N1 rotation - not approved but *** useful in a 60kt tailwind if you cannot be moved.

We would deploy the T/Rs before engine shut down on the JT8, for that exact reason where dust storms and or strong winds were forecast.

While some actions are not approved, sometimes you need to adapt, improvise, and overcome.

Thorough briefing with ground and flight crew is a must. But hey... you do what you gotta do in order to get out of Dodge.

I may have seen that technique with the RB211:E wink wink nudge nudge say no more.

[muduckace]

I know a guy who knows a guy who lost a starter on #1 of a dc8. There was a sistership on the ground, got her started by using the thrust of the sisterships #4.

[RAT 5]

muduckace: firstly I'm trying to decypher your code name and failed. Secondly, this was often a curiosity in the bar as to whether it would work as a get you out of jail technique in some unsavoury dump. But surely, it would give a very hot start blowing large quantaties of hot air down the throat of a healthy colleague. A bit like a few C.P's I've known. Sent we into a red flush rage (inside) a few times.

[vwreggie]

Was at some time on 747 classics (rolls royce engines) a reccomendataion or requirements to motor the engine until egt had decreased to below 150 and we also had this on the 737 300/400 models for a while. Ex engineers turned pilots would also say that the worst thing to do to an engine is to let it peak and then make adjustments so they might move the start lever to idle at 25 % because the momentum of the start was happening and the acceleration would continue rather than wait for a peak and then add fuel which might begin another acceleration ie engine load. That kind of thinking is similar to the stable thrust requirement of 40% before setting take off thrust. The stability of the engines is more important than the amount of thrust. That is you wouldnt set say 50% then pull them back to 40 and then hit the toga buttons for takeoff as that would be an up down in thrust and engine wear/load which is pointless. What you are seeing in your captains is a reflection and variety of past experiences, past SOPS which are broadly within the requirement to not put fuel in too early, and dont let them burn at the top end of the start, dont run starters for longer than 2 mins. Ask them the question about their actions and they should have a story or 2 for you which will make the sops and individual actions stick in your mind for the understanding you have gained:8

[BOAC]

whether it would work as a get you out of jail technique in some unsavoury dump. - yes - seen it done a few times. The 'C P's':) output is not so much of a problem if you use the exhaust gas to get the necessary RPM and then cut the 'feeder'. The biggest problem is getting the 'broken one' close enough. Beats runway jump starting:ok:

To add to 'odd' starting techniques, I flew with one Captain who used to move the 737 start lever up so slowly one thought he might have died. Apparently came from the 1-11 and "you get a cooler start if you bring the fuel in slowly". (Don't tell CFM...)

[FCS Explorer]

nice try. to bad those new micro-switches (supposedly 6 at the "idle" postion and 6 at "cut-off") only know ON or OFF.:ooh:

[alexban]

Denti, we have the FCOM version from 27 march 2009 for the NG , and for start is says:
" When N1 roatation is seen and N2 is at 25% or (if 25%N2 is not possible), at maximum motoring and a minimum of 20%N2..'
also
'Note: Maximum motoring occurs when N2 acceleration is less tahn 1% in approximately 5 seconds".

[RAT 5]

Craig F. This ideaq of not 'shocking' the engines has been there with the debate about use of reverse thrust. You fly a long descent at idle, then apply mid-range power for a short while on approach, then pump the engines upto 75% in a burst, then back to idle. What are your thoughts on that, and do you have any factual knowledge about the effect on the core/turbine with the standard technique. I know many airlines are using brakes and idle thrust, but the quick turnround enviroment we now live in needs careful monitoring. I'm amazed that the speedy LoCo's don't have brake temp displays. There is already so much to do on a trun round that checking brake cooling tables amidst all the other parafanalia will most likely be missed. On other a/c this was only necessary with temps above 4, perhaps 5. They would be very helpful if this was so on B737's. However, many Flt Ops chappies on these LoCo's may not have big a/c experience and not choose a customer option apparently to save cost.

[framer]

I haven't got access to brake cooling scheds right now but I'm pretty sure that on a 30 min turn around in a 737 it's not an issue. Even heavy with 37 degrees celcius ambient . Can someone confirm? Maybe give weights and temps that would cause a problem?

[Tee Emm]

The early 737's with JT8D series engines initially had a start lever actuation at 15 percent N2 during engine start. Complaints were filed to the engine manufacturer that 15% gave high starting EGT that were worrying people. Following further engine testing, the manufacturer recommended a revised figure of 20 percent N2 before the start lever was raised to idle from cut off. The high EGT problem disappeared and everyone was happy.

Then the inevitable happened and some pilots decided they could drop the EGT even further and let the starter wind up to max motoring before introducing the start lever. Of course they hadn't a clue what other technical factors may be affected by the higher N2 rotation.

In other words they disregarded the manufacturer's recommendations in favour of their more expert (in their mind) knowledge. Of course they had not done measured tests to prove their point nor had they written to the engine manufacturer with their research results (apart from personal opinion) to prove the manufacturer recommendations were wrong.

In every airline there are the smarties that insist their personal opinions should be SOP - certainly while they are in command of the aircraft. As a wise man once said "Without data, you've only got opinion".

[BOAC]

I flew with one Captain who used to move the 737 start lever up so slowly one thought he might have died. In every airline there are the smarties that insist their personal opinions should be SOP - certainly while they are in command of the aircraft.- absolutely, Tee Emm - and guess what - did he do it on a sim or line check?:ugh:

[sjm]

Framer!


Performance Inflight – 300
Advisory Information
737-300/CFM56-3_20K
PI.12.8 Revision 0 Jun 2008
ADVISORY INFORMATION
Advisory Information Recommended Brake Cooling Schedule
Reference Brake Energy Per Brake (Millions of Foot Pounds)
-

50tones with a wind corrected brakes on speed of 130kts gives you a min turnaround time of 32 mins @40C.
1 million pounds pressure added for every taxi mile (say 2 miles.)
reverse idle autobrake 1
max wt 51709kgs

doesn't normally cause a problem till f/0 stamps on the brakes like a loon to make a rapid exit.:}

on our 737-3/500 we are taught fuel in at 25% if it cant reach 25% ( dodgy apu)then max motor <1% per 5 sec.

Have however with a engine that is prone to hot starts very near the limit 715/720C mark motored the engine past 25% to get the egt down. generally only when very hot say 35c im AGP.

[vwreggie]

Rat 5. I have no data on "engine shock" only anecdotes which was the jist of my post that new f/o s might ask the question and the anecdote based response will assist them in broader understanding of how procedures are derived. Anecdotes might also lead people to think " thats crap" but that thought will arrive as a result of thoughts/questions which is the nature of this site. As for idle thrust landings we did away with that as a procedure some time ago on 744 however I have been 737 for 4 years now and I am not au fait with 744 procedures. The highest and hottest altitude we fly to is around 2000 and 35 to 40 celsius on a very hot day. Our turnarounds are 35 to 40 mins 400/800. Landing max weight with auto brakes 3 ( we have 1 2 3 max) gives an adjusted brake energy of 25.9 which fits within the caution zone and no take off for one hour however use of reverse thrust and auto brake 3 gives a 14.7. use of autobrake 2 and reverse gives you 30 min turnaround. Using autobrake 3 adds 10 mins to the turnaround which is near to the standard turnaround. In practice we are never scheduled to less than 40 min turnaround and we land with autobrakes 3 and ease them out early on the rollout. The only time idle reverse landings are a consideration might be if we landed on a long runway with a roll through like 34L in sydney and someone may elect to brief and use idle reverse and take the reversers through the interlock for muscle memory. This also allows the 3 minute cool down timing before shutdown to start and remove the need to hold at the gate with engines running for a cool down. We dont as a rule use idle reverse very often. I do agree that the brake temp guages on the 747/744 were handy and would be handy if your company was promoting idle reverse landings as standard.:ok:

[Tee Emm]

Sounds like you are talking about Canberra or Alice. Use of any autobrakes will give you increased brake temperatures since the brakes bite immediately on touch down. This despite reverse will cause the autobrakes to back off. It is braking at the high speed high energy portion of the landing that causes the brake temp increase.

The original history behind setting an autobrake figure was if you needed the brakes immediately on touch down on a limiting runway (and that does not mean you have 1500 ft to spare but a real short runway for the landing weight) - or a strong crosswind on a limiting runway or slippery runways or using a higher than normal approach speed on a limiting runway. Of course if your operation requires an early turn-off which would in fact now convert your landing length into being limiting, then autobrakes would be an option. But to blindly have the autobrakes on for every single landing regardless of runway length well in excess of performance limit, of course is asking for hotter than normal brakes. As was pointed out this can affect turn-around time.

On the other hand where local noise abatement rules dictate the use of reverse idle only then be prepared for hotter brakes. As Boeing recommend, prompt activation of full reverse in conjunction with accurate threshold speeds and touch down point will normally obviate the need to use auto brakes and in most cases the brakes need not be applied until 80 knots of lower where energy is less. In real life autobrake use neatly disguises sloppy airspeed control and long touch downs.

So if your turn-around times are marginal because of brake heat, simply improve your flying skills to tighter tolerances, don't use autobrakes unless operationally necessary and the problem of hot brakes diminishes.

[Ocampo]

Landing max weight with auto brakes 3 ( we have 1 2 3 max) gives an adjusted brake energy of 25.9 which fits within the caution zone and no take off for one hour however use of reverse thrust and auto brake 3 gives a 14.7. use of autobrake 2 and reverse gives you 30 min turnaround

Let me see if I got this one; auto-brakes do their thing at a constant rate, since the figures you just gave are different, I'd say that the "constant rate" the auto-brakes work around is speed, as in, they slow the aircraft down X knots/sec? And, BTW, are those numbers you mentioned PSI? :8

Thanks in advance.

[vwreggie]

There are very few times when use of the auto brake system is not required in our ops. regardless of " covering" up poor speed control aimpoint. flare commencement/technique etcetera, autobrakes are a tool. One could plan to turn up with min.fuel on a regular basis as well and that 1.2 tonne less might factor in a 1-2 knot decrease in Vref and groundspeed but who would do so to save the brakes? Its like a mechanic said to a young rev head years ago about effctive braking using down shifting or foot braking. " brakes are a lot easier and cheaper to replace than a gear box". For some time we did no reverse landings on 744 to save a cost around the reverse actuators I think as well as more efficient wearing of those particular brakes. We dont do that any more.
The figures I referred to are brake energy per brake expressed in millions of foot pounds for a classic.

[sudden Winds]

Not a long time ago...in an airport far away.

737-500, hot day, quick turnaround, F/O starts first engine, motors it till EGT drops below 130°C (residual was much higher). Normal start.
Captain: Why did you do that?? Don´t do it again. It´s not written anywhere!!!
F/O: Sir yes sir.
F/O starts 2nd engine, raises start lever at 25.00000000% N2, aborted engine start due to "egt rapidly approaching the start limit". Peak was 720°C or so. (limit 725°C)
Captain's face::uhoh:
Captain's reaction: ahh welll aahh mmm ok listen...you´re probably right...ahh ok...let´s see...ahh ground...we just interrupted the start because...ehhh...aahh...
A few minutes later, engine is started after some motoring (plus the one called for by the NNC) and engine started normally.
(I was not any of the above, just know the F/O very well).

i flew both jurassics and classics as a captain and I always took precautions to avoid hot starts by motoring (for just a few seconds) engines that had high residual EGTs and having the airplane towed out of a direct tailwind if this exceeded 7 to 10 knots right at the tail, especially in combination with some residual EGT. I enjoy NG now and normal peak EGTs are wayy lower than the redline (which happens to be 725°C).

At times I watched the APU EGT in our Garrets go as high as 710° (max continuous) during start. As soon as I had one engine started I put that generator on bus and then I started the second engine with APU EGT readings 20 to 30°s lower (which is a lot). Some used to de energize the EMDPs but that´s something I chose not to do, because there´s always the risk of not being able to energize them again or forgetting them off.
Regards,
SW.

[grounded27]

With any motor I wait for minimum motor want max motor but do not sit there for 10 seconds waiting to squeeze 1% more n2/n3 out of the starter, you are not achieving anything significant by doing so. You reach a curve where the potential stress on the engine during start dropps off exponentially and the wear and tear on the starter increases exponentially.

[Swedish Steve]

It will load monitor it's own electrical load to maintain air output for the engine start as the priority.

Sorry not licensed on NG, but how does it do this. Has the NG load shedding? Do the galleys and the cabin lights go out during start?


as well as no IDG spar fuel valve indication.

and thats new to me as well. What does an IDG spar valve do?

Quite a revelation that Boeings narrow body has manual start. I thought all new airliners had auto start. A320 with both engines has always had it.

On RR engines, we always motor them down to 100degC before fuel on. The FADEC does the same.

[EW73]

The NGs APU will not reduce the pneumatic output as a result of an increasing EGT, when you're starting a main engine (Pneumatic priority). It will load monitor it's own electrical load to maintain air output for the engine start as the priority.
However, when you are operating both packs, say before engine start, an increasing APU EGT will result in reducing the air supply to retain full generator output capability (Electrical priority).

For me, if the engine N2 is accelerating normally as it progresses thru 25%, the start lever is selected up around 27%.
And since we have fuel-cooled IDG oil systems, I always look up at the fuel panel during the start lever selection (only for a moment) to notice the transient bright blue of both fuel valve indications, as well as no IDG spar fuel valve indication. When my eyes get back to the engine indication screen, it's usually just in time to see 'light-off'!

Works well for me...

[captjns]

I believe that during engine start, when the START SWITCH is placed in ground, a signal is sent to the APU FCU to increase up to a maximum of 110%. APU rpm will adjust to maintain a schedule PSI for duct pressure until its maximum operating temperature has been reached. Then the APU FCU will reduce fuel to maintain the APU within its operating limits.

Doesn’t the EEC monitor the engine and send signals to the HMFCU to provide a uniform rise in EGT which is designed to avoid hot starts?

[Virtual738]

I deleted this post myself.
I am not mocking anyone but am just tired of the " pretending to be pilots " on here.
I am not a RW pilot but have been working with simulators for over 10 years. I have built fixed based trainers for the aviation education sector and have also worked as a sim-tech on 2 level-d 737-800 simulators.
( both were from CAE )
I am professionally involved in flight simulation and sometimes look for answers to questions that I cannot find answers for in the CBT or Fcoms.

My appologies if I offended anyone.

Jason Grimes.

[framer]

Virtual 738..... what do you gain from coming onto this thread and mocking pilots, when you yourself don't add anything to he subject except for your mockery?
I'm sure everyone on here is quite capable of starting an engine and even if some do need to spend more time in the books, it is not your place to give advice, it's debateable if it's your place to contribute at all actually. In addition, when you have asked pilots for advice on subjects that your simulator manuals don't cover, they have helped you by giving their real world experience. Do you have any grattitude for that time given?
Framer

[yanrair]

You raise the start lever at 25% and there is no need to wait for more. Captains who do this are breaching SOPs. and indulging in personalised techniques. Max motoring means more than 20% (below that you may get a hot/hung start) and that the engine has stopped accelerating towards 25%. So, this condition will be between 20-25% N2.
on the -800 this hardly ever occurs due to a powerful APU but on the -300 it was quite common and you would put in the fuel at maybe 23% due to having reached "max motoring".
Yanrair

[Denti]

Depends on company SOP i suppose. We changed it quite some time back to max motoring every engine start (advised by boeing), which meant usually around 30% N1 on the NG. Recently CFM did an audit within our company and was quite flabbergasted about this SOP and demanded it to be changed back to the old 25% N1 or max motoring whichever occurs first.

[yanrair]

Are you saying that Boeing changed VOL 1 starting procedures to suggest that you try to get max rpm before introducing fuel? I am not surprised that CFM went ape ****. Wear and tear on the starter gear over thousands of start cycles would be very high.

[barit1]

Going back 30+ years -

Boeing has pushed the N2 Fuel On point well beyond what some engine mfrs recommend. It was true on 742s where Boeing wanted a common procedure for 3 engines (JT9D, CF6, and RB211) even though only one type required it.

In the present case, only the CFM is involved, but perhaps some of the old mentality persists.

[adanhamidu]

For me, maximum motoring is when N2 does not accelerate anymore because of the 30psi duct pressure spinning the starter motor has spun the starter motor and consequently, the N2 to the maximum speed that it can spin both; at this point, you don't see an increase in N2 rpm because of the pneumatic start effort reaching its maximum capability. This is when you are sure that letting fuel into the engine won't result in a hot start (or a wet start).

[Dave Therhino]

Correct, although it can be said more simply. "Max motoring" is the maximum motoring speed you can achieve with the starter from the air source being used.

In general, you are given a target minimum motoring speed at which fuel should be introduced in order to avoid compressor stall, lack of acceleration, and high EGT. If your air source can't motor the engine to that normal minimum speed for fuel on, then introduce fuel when the engine has accelerated to the (lower) maximum motoring speed. You may not get a successful start in that case. From what's written above for the 737/CFM56, Boeing doesn't want a start attempted if max motoring is below 20% N2.

Also, what the heck is an "IDG fuel spar valve?" I know of no such thing on any Boeing model.

[adanhamidu]

IDG = Integrated Drive Generator. It's an alternator which is better known as "engine generator"; that device that provides electric power when the running engine turns it fast enough.
As for the fuel spar valve, it is upstream of the engine. It is a DC motor Hot Battery Bus-powered. The engine mounted EFSOV is fuel operated, solenoid controlled Battery Bus-powered. Fuel is delivered under pressure from fuel pumps located in the fuel tanks. The fuel flows through a fuel spar shutoff valve located at the engine mounting wing stations. The fuel passes through the first stage engine fuel pump where pressure is increased. It then passes through two fuel/oil heat exchangers where IDG oil and main engine oil heat the fuel. A fuel filter then removes contaminants. Fuel automatically bypasses the filter if the filter becomes saturated. Before the fuel bypass occurs, the fuel FILTER BYPASS alert illuminates on the fuel control panel.
The second stage engine fuel pump adds more pressure before the fuel reaches the hydro-mechanical unit (HMU). To meet thrust requirements, the EEC meters fuel through the HMU. The spar fuel shutoff valve and engine fuel shutoff valve allow fuel flow to the engine when both valves are open. The valves are open when the engine fire warning switch is in and the start lever is in IDLE. Both valves close when either the start lever is in CUTOFF or the engine fire warning switch is out. SPAR VALVE CLOSED and ENG VALVE CLOSED lights located on the overhead panel indicate valve position.
Fuel flow is measured after passing through the engine fuel shutoff valve and is displayed on the display unit. Fuel flow information is also provided to the FMS.

[Dave Therhino]

Thanks - I'm sure that took several minutes to type. Sorry if my question misled. I know what an IDG is and I am very familiar with the fuel systems and engine control systems on all the Boeing jets. I was curious what the poster who used the term "IDG spar valve" thought that valve was doing because the spar valve has nothing directly to do with the IDG.

[Dave Therhino]

Quote feature isn't working for some reason ...

Quote: Doesn’t the EEC monitor the engine and send signals to the HMFCU to provide a uniform rise in EGT which is designed to avoid hot starts?

The EEC on the CFM56-7B series engine during start is fueling the engine to achieve a scheduled target rate of N2 acceleration, as limited by minimum and maximum fuel flow vs burner pressure schedules (minimum to prevent lean blowout, and maximum to prevent compressor stall). Unlike the autostart features of the engines on the wide body Boeing airplanes, it does not reduce fuel flow based on high EGT as far as I know. Maybe tdracer can confirm this.

[tdracer]

The EEC on the CFM56-7B series engine during start is fueling the engine to achieve a scheduled target rate of N2 acceleration, as limited by minimum and maximum fuel flow vs burner pressure schedules (minimum to prevent lean blowout, and maximum to prevent compressor stall). Unlike the autostart features of the engines on the wide body Boeing airplanes, it does not reduce fuel flow based on high EGT as far as I know. Maybe tdracer can confirm this.
Targeting a specific rate of increase in N2 ("N2 dot") is pretty typical starting logic for all FADEC engines (N3 for Rolls) once lightoff has occurred, with various algorithms to deal with hung and or hot starts (optimizing lightoff is an art all to itself). The details vary between the engine manufacturers, and too a lesser extent between engine types. It's also gotten more elaborate over the years as the FADECs have become more sophisticated and powerful, but the basics are pretty much the same. EGT is not a target parameter during start on any system I'm familiar with.
I never spent significant time working the NG so this is second hand info, but my understanding is that while the CFM56-7B doesn't have full blown autostart, it does have some start protections, including EGT hot start protection. Hot start protections will cut fuel before reaching the start EGT redline in an effort to keep the EGT transient overshoot from exceeding the redline. Some autostart systems will cut fuel well below the EGT redline if the rate of increase of EGT exceeds some limit.
BTW, IMHO, GE has always had the best autostart logic, and Pratt has the worst (disclaimer - no knowledge of the Pratt GTF autostart), with Rolls somewhere between GE and Pratt.

[cashash]

Engine Hot Start Automatic Ignition

The EGT digital display flashes when the EEC finds there is a
possible hot start. If the EGT starting limit is exceeded, the EEC
stops fuel flow and ignition. The EGT digital display continues to
flash until you move the engine start lever to the cutoff position.

Ch 80-00-00