View Full Version : B737 Start Valve, Minimum Time Between 1 And 2...

9th Nov 2004, 18:34
Hi all, a trend is spreading in our company to save APU's life :rolleyes: :
the goal is to not allow big fluctuations in the APU Egt while starting the engines: instead of doing standard good procedures, it goes as follows:

You call "Before start cklist below the line except packs".
You check starting pressure above 30, but not the real unloaded pressure, then you turn the pack off and immediately open the start valve.
Also, immediately after starter cutout, you open the other engine's start valve. Immediately, at the speed of light.

So the APU delivers a constant flow and the Egt doesn't fluctuate.

I'd like to know if others do this, as I have some concerns regarding this procedure:
I have always watched carefully the unloaded pressure to crosscheck and confirm the closure of the start valve.

Doing this procedure so quickly doesn't allow to make a good crosscheck.

Also, I'd like to know if the START VALVE OPEN is an indication of valve position alone, or if it gets information from a pressure switch, like somebody says.

Comments? Thankyou

11th Nov 2004, 12:52
Lem. Sounds like you are seeing yet another myth in action. It never fails to surprise me how after 35 years(?) of 737 operation, someone in an A team comes up with a new myth such as the one you described. I wonder if that new procedure has the approval of the APU manufacturer.

11th Nov 2004, 17:49
I think this procedure has no base,and also it's not safe.
If you'll check the air press during start,you'll notice that it fluctuates,not much though.
Also you'll have starter cutout at 46% ,but you must monitor the engine param until eng stabilised. What if,after eng2 starter cutout,while you're starting eng 1 (monitoring valve ,oil press,N2,N1,...) ,you'll have a rapid increase in temp at already started eng nr2 (but not yet stabilised)?Or no oil pressure at stabilised eng? Who is monitoring that?
It's a stupid thing to do,my guess.And you'll see this at the first engine start problem they'll have.
START VALVE OPEN light indicates valve is open and air is supplied to the starter.

11th Nov 2004, 18:12
Exactly, that's why I'd like to collect as many advices as possible against this procedure.

START VALVE OPEN light indicates valve is open and air is supplied to the starter.

Yes, that's what the manual states, but is the sensor just a position sensor or also a pressure sensor?

11th Nov 2004, 18:39
my guess it's a position sensor.It will be on if you open manually the valve,as I remember seeing once.But I maybe wrong,I'll check

22nd Nov 2004, 00:11
This has been a common technique among some 737 operators for years.
If memory serves me right Boeing discussed this in a bulletin years ago. Obviously well hidden from many seasoned 737 pilots, I know of a lot of experienced 737 guys who don’t do this. I can dig out the bulletin if anyone needs the number. The essence was:
Recommended to have minimum time between packs off and engine start and between start valve close and next valve open. This to prevent large turbine wheel temp fluctuations which has lead to premature changes of 737-300/400/500 APUs due to rotor problems.

Has been removed from the future before start checklist draft. It will be part of the start procedure so you can do this “by the book” in the future LEM.

22nd Nov 2004, 09:53
START VALVE OPEN light only indicates valve is not in the closed position, sensed by a position sensor.


22nd Nov 2004, 11:39
START VALVE OPEN light only indicates valve is not in the closed position, sensed by a position sensor.
Interesting information IFixPlanes and this is for the classic and not the NG - I think. Many pilots will back the start valve open light up with a quick glance at the duct pressure. If duct pressure does not return to prestart value then you might have a start valve open problem. Abnormal duct pressure is not as easy to detect when you move quickly from one start to the other but I think it is ok as long as you don’t have a MEL write up of the start valve open light.

22nd Nov 2004, 12:37
1st I must correct my last Post:
I wrote ...sensed by a position sensor...
but it is a position switch.
Sorry for this false definition.

The only difference of Classic and NG is, that the "not closed" signal on the NG goes to the DEU.
The DEU now supply start valve position information to the CDS.
Also starter cutout on NGs is 55% N2.

...abnormal duct pressure is not as easy to detect when you move quickly from one start to the other...
Like alexban wrote before : "...you must monitor the engine param until eng stabilised..." .
So you have plenty of time to look for abnormal duct pressure.


22nd Nov 2004, 15:55
Interesting discussion Ingo, lets compare notes – not win but to learn more – at least for me. Here are my sources:

Quickly: comes from Boeing Flight Operations Technical Bulletin 737-ALL 99-1, February 12, 1999 suggests minimum delay between starts.

"...you must monitor engine until stabilized..." Yes but this can be done while starting the last engine per FCOM Volume 1 NP 20.20 Engine start procedure: First Officer should monitor stabilization while Captain starts the last engine (Boeing training technique suggests that there is no need to wait for stabilization of the first engine before starting the last engine - all you need is start valve closed).

difference of Classic and NG
Steady – respective engine start valve open and air is supplied to starter
NG MAINTENANCE MANUAL: The engine start valves send a ground discrete to the DEUs when the valve is in the open position.
CLASSIC FCOM: START VALVE OPEN Light is Illuminated (amber) – related engine start valve is open and air is being supplied to the starter. Don’t have a classic maintenance manual handy right here but I suspect it would say what you point out Ingo which might be different from the NG? In addition the calssic does not have blinking – for uncommanded opening of start valve.

The 200 is different again I think - B737-200 FCOM: The START VALVE OPEN light monitors air pressure downstream of the starter valve.

My conclusion – Boeing suggest that we keep a minimum delay between starts and this is further facilitated through 737NG FCOM Normal Amplified Procedures/Normal Checklist draft 4/29/2004

22nd Nov 2004, 18:50
I don´t use FCOM because i am not a Pilot.
My source is the AMM and my experience as a Mechanic.

"...you must monitor engine until stabilized..."
I personally starts the 2nd Engine after 1st Engine has stabilized, because if you have trouble
with the first Engine after "cut out", better 2 Peoples working on that Problem.

difference of Classic and NG
FCOM.... OK, but (sorry) it is written for Pilots.
Before i get angry PMs, let me explain:
FCOM shows fundamental System Information, because no Pilot needs to know what´s going
on behind the Switches.
The Pilot must only know that the Illumination of START VALVE OPEN Light mean "Valve open".
The Background may be much more complex.

So, here the some detailed Information about my wording "not in the closed position" :
wiring = http://img1.uploadimages.net/thumbs/tn_151010Start_Valve_737CL.gif (http://www.uploadimages.net/show.php?img=151010Start_Valve_737CL.gif)

Valve =
http://img1.uploadimages.net/thumbs/tn_503423Start_Valve_737CL_2.gif (http://www.uploadimages.net/show.php?img=503423Start_Valve_737CL_2.gif)

In the NG the signal goes in the DEU. If the DEU sense "not closed" you get the indication
START VALVE OPEN on the Display.
Blinking indication is possible due to digital handling of the signals in the NG.

btw : your Profile says a little more than nothing. What is your Occupation.

(hoping my english is not too bad ;-) )

22nd Nov 2004, 18:54
The initial concern here was to extend the APU turbine life.

The earlier B737’s (-100 through –500) were fitted with a Garrett… AlliedSignal… Honeywell GTCP85-129 of some sort. Some airlines such as United, Lufthansa etc. selected the APS2000 for their later B737 (classic) deliveries. A very few operators also selected the Honeywell GTCP36-280 (total flop) APU.

There are two distinct types of turbine wheels used in the GTCP85-129 series APU's; a two-piece forged wheel and a single piece cast wheel. The single piece cast turbine wheel has always been the Achilles heel of this unit. Although the cast wheel had better performance, it was susceptive to what we called “saddle” cracking and turbine wheel separation. This cracking occurred in the vicinity where the shaft was molded to the turbine wheel casting. Saddle cracking was a direct result of thermal cycling and also difficulties in getting a uniform casting material in such a relatively large piece. Several iterations were attempted and the Mar-M wheel with a TAFCU (Time to Accelerate Fuel Control Unit) comes to my mind …?

Anyway to the point! The APU turbine deterioration is primarily due to “time at temperature”, in other words running at elevated temperature for sustained periods. This does not differ from any turbo machinery using nickel based alloy turbines. The next worst scenario is thermal cycling and here I mean from deep cold soak to full electrical and ECS (Environmental Control System) or MES (Main engine Start) mode. Someone at Honeywell might argue this, but after years of observation, we have enough data to support this. I personally feel, and I have the support from some more unbiased Honeywell engineers, that the relatively small EGT thermal cycling going form electrical load and full ECS load, down to electrical load only, and back to electrical load and MES mode will have an absolutely MINIMAL impact on the life of the turbine.

They (today’s Honeywell) have always had a design issue with their 1950’s design GTCP 85 series APU and particularly the turbine wheel. The operators have been on their case about this for years and they were attempting to alleviate this by shifting the focus to the operation instead of addressing the real issue!

Fortunately the newer Honeywell GTCP 131-9B fitted to the B737NG’s is a absolutely “marvel of engineering”… :ok:


22nd Nov 2004, 20:23
Thanks Ingo, very good explanations – that’s what I love about this forum. I agree 100% with what you say about the FCOMs. They are good for what they are designed for and not if you want to dive deeper into the systems. I am a pilot-instructor/examinator/evaluator and as you understand no engineer. I try to read the maintenance manuals to learn more but don’t have your background. Ironically even the maintenance manuals can be too simplified at times. With your example the illustration you enclosed show a slightly different function than the maintenance manual's text which I quoted above – trust your illustrations.

As far as LEM's - question is concerned will the avoidance of APU pressure drop extend APU life? In some cases yes – depends on APU model ref Boeing Bulletins and Dag’s details above.

How long time should we use for our flows and starts?
The overall Boeing philosophy is to be safe and time efficient. As an example a pilot would do some 330 items from memory before start and only check about 10% by checklist to save time. The extreme oposite - a typical airforce SOP is often 100% read and do which takes much longer and probably be a bit safer?
Procedure designers must keep in mind that if an average sized airline with 30 to 40 737s who flies shorthaul use one extra minute on each leg then they will need an additional aircraft to serve their schedule. With this in mind Boeing have installed the lights which when they work should allow one pilot to monitor while the other continues the next start. I do agree that your method is better especially if you need to clear fluids after maintenance work which increases the probability of abnormal starts.

23rd Nov 2004, 17:57
Hi all.
I agree minimum time between starts extends APU's life a liiiiitle bit, but I wouldn't sacrifice good operating practices in the name of (another) exasperated money saving attempt.

Some considerations: with a weak APU (I do operate Classics, not NG), how are you going to check 30 psi minimum if you are using a pack for airconditioning and very quickly turn it off and open the start valve?

In case of abnormal start of the first engine, if you need to open again to motor the engine you will have to remember cutting the second engine also...

Too many ifs, maybe, but ask yourself: would you use minimum time procedures while being checked in a sim?

There are so many factors which could save some money at the end of the year, but is it a reason for regularly flying with no extra fuel, or always flying fast approaches on idle till the minima, or requesting taxi while still starting the engines etc. etc.?

Also, what I don't like is people modifying procedures on their own...
If that's standard-official, why don't they write it in the sop and checklist?

.........By the way “AIR CONDITIONING PACKS……………………..OFF
Has been removed from the future before start checklist draft. It will be part of the start procedure so you can do this “by the book” in the future LEM...............

Hi 80/20, how d'you know? You mean they'll change the CLASSIC cklists?

I don't think so.

My morale: this is a dumb procedure, as people tend to do it faster and faster, like opening the second starter as soon as the first one pops, without even checking for the Start valve open light to extinguish.

They like to play with "No starter cutout" in the sim, then encourage dumb procedures at the speed of light...



24th Nov 2004, 20:08
Hi LEM, suggest that you get a draft copy of the new enhanced 737 QRH and normal procedures draft through your company, overall I think you will be pleased with them :-)
By the way I enjoy your good questions - keep them coming!
Cheers 80/20

24th Nov 2004, 21:33
Oh … and LEM I agree with you - avoid the hurry up syndrome, http://asrs.arc.nasa.gov/directline_issues/dl5_hurry.htm
(However I have seen many pilots start without too much APU pressure drop in a controlled manner).