After engine start amperage check B737NG

[PitotTube]

dear forum,

when checking TR1,TR2,TR3 amperage on the AC/DC metering panel after engine start, the T3 amperage is always about half the value compared to TR1 and TR2. How come?

thanks. pt

[LAHSO 06/03]

Hello PitotTube,

Without pretending to be an electrical expert here (i chose to be a pilot instead) i'll give awnsering your question a go.

A low amperage, as i'm sure u know, indicates that there is little electrical load. As you can read in the Boeing manual: "Under normal conditions, DC bus 1 and DC bus 2 are connected in parallel via the TR3 disconnect relay. In this condition, TR1 and TR2 are each powering DC bus 1 and DC bus 2. TR3 powers the batterybus and serves as a backup power source for TR1 and TR2"

As you see, TR3 doesn't really come into play in a normal situation. The reason why the current indicates half of the TR1 and TR2 values i'm afraid i am unable to awnser. Maybe someone more proficient knows?
Hope to have helped you a bit..


Per ardua ad astra

[blackmail]

hello pitot tube & others,
to add to lahso 's post. for TR1 & TR2, you read DC bus voltage/amps. for TR3, you read actual volt/amps of the device. so, when either TR1 or TR2 fails, by selecting volts/amps, you will still read DC bus1 or DC bus2 volt/amps. on the otherhand, if TR3 fails you will read 0volt/0amps. for this reason, after start, we always select TR3 on the DC meter. there is also on B737NG, a TR amber caution light, with a different logic on ground or airborne. on ground the caution light will illuminate for any TR failure, whilst airborne either TR1 or TR2+TR3 must fail for the light to illuminate. so, to answer your question, the different values come because the measured pickup points are different & going back to basic electricity : Ohm's law states that E= IxR. E= volts, I= amps, R= resistence(ohm). E being roughly constant (28volt), then, if TR3 amps are half the values of TR2 or TR2, then resistence of TR3 must be twice the value of resistence of DC bus 1 or 2.
hope this helps.
kind regards,
bm

[PitotTube]

thank you for great replies. Very much appreciated! /pt

[A37575]

My old Boeing Seattle instructor told me not to waste my time checking generator cycles, volts and amps after engine start as the generators will not go on line unless the everything is normal.

Re checking of TR's which is part of some operator's procedures. Is that also a waste of time as well? It does not form part of Boeing recommended procedures in either FCTM or FCOM. In short, some pilots are by nature multi-fingered switch flickers while others take a more sanguine view. Surely the operation of TR's is part of a maintenance schedule and therefore superfluous as a pilot cockpit check?

[GearDown&Locked]

A37575, instruments are there for a reason, to assess your machines' health I believe. One of these days some multi-fingered switch flicker pilot will save his/hers sanguinical mates' arse just because he/she took the liberty to check some secondary instrument reading even if it wasn't present at any checklist, SOP, FCTM or FCOM.

Fly safe.
GD&L:ok:

[blackmail]

hello 37575,
to give you a concrete hands on experience: a long time ago, when performing an approach to linate airport(italy) in a b737 classic, suddenly, without any reason, the autopilot disengaged & neither autopilot, A or B could be re-engaged. we had absolutely no clue whatsoever to what was the cause of this problem, no amber lights, no master caution, nothing, until i glanced at the overhead panel to check the generators output & there the answer unfolded : generator n°1 cycles showed only 200hz instead of the normal 400(+/-)10hz & was still connected to its generator bus. so we started the apu & connected the apu generator to the generator bus nr.1. from then on the autopilot could again be engaged & performed flawlessly. just to say, if you "flick" a switch, make the effort to see the result, even if it takes a few seconds.
kind regards,
bm

[A37575]

The design of the Boeing 737 series right from the original 737-100 was planned as a two pilot operation that did not need a flight engineer. Various cockpit system switches were automated because, by the then certification rules, if the cockpit design meant that more than (say) 100 manual actions including eye scan was needed, the rules required a flight engineer. Hence the 727 needed a flight engineer. One small example of the automation in the 737 was the no smoking/seat belt signs. Set them at auto and forget about them. Automatic changeover from auto-pressurisation to standby mode is another example. Automation has just saved perhaps 8 switch and /or eye scan items.

This suggests that the more additional switch operation the pilots choose or are required to undertake for local SOP reasons (and by definition superfluous to Boeing design philosophy), the more is the two-pilot certification compromised. You can flick the volts and amps as much as you like - whatever turns you on I suppose - but the fact that these actions are not mandated in the Boeing manuals as a specific part of cockpit drills, suggests (IMHO) certain switch checking is superfluous to the operation of the aircraft. Every action, however minor, causes a work-load increase - hence the two pilot certification rule that limits the number of actions before the flight engineer claims back his position in society.

[blackmail]

hello A3757,

i totally agree with you & mr. Boeing philosophy : " if it ain' t broke, don' t fix it".
but, if during a normal flow or scan pattern, you have to reposition a switch or valve, it is common sense to x-check the result, you don' t need a flight engineer to do that, nor for that matter, have the aeroplane recertificated.

[160knots]

Nice to know in case I ever get to fly a 737.

[rubik101]

I have been flying B737s for 25 or more years. When I first started flying them, we were required to check outputs. Since about 1986, Boeing removed the requirement due to increased reliability and feedback over many years of monitoring system failures. I have recently joined and airline that thinks it is flying -200s, in spite of the fact that all they operate is -800s. They require me to check these outputs after start, regardless of Boeing's procedures. As mentioned already, if the outputs are bad, Generators won't come on line. TRs, if not working correctly, will show all sorts of other problems.
they also insist on the A Hyd. pumps being off for pushback, in spite of the mods Boeing did to the system about 20 years ago.
Such is progress.

[stator vane]

rubi--
you must be flying the one's i fly.
and yes, it is a waste of time. but when the man with the ink pen is on board we must play the game.
whilst we are at it, i have never understood why "start levers" has always been on the after start check list either!!!!
and of course, AFTER something happens, then we will be able to see something on the meters, and use the information to determine exactly had happened and what to do next, but during the change overs, it will not connect if something isn't right.

this same airline will require us to "BRIEF" for half of the flight!!! actually pull out the low enroute chart going into your own base!!!! or other airports where you know the names of the trees!!! honestly, my last check ride---and i was required to determine the turn around take off weight limit whilst descending for the approach!!!

[FCS Explorer]

my outfit recently changed proceedures. and since the book no longer requires me to check voltage etc after eng start i will drop the habit:ok:. (i will recover the lost block time discussing with the boss why i didn't do what i didn't do):}

[CaptainSandL]

SV
i have never understood why "start levers" has always been on the after start check list either!!!!

At the risk of thread creep I have twice seen Capts not putting the start lever fully in the idle detent – just up to idle. The checklist caused them to notice that the lever was not in the detent and could have prevented it being accidentally knocked out of idle.

[relax.jet]

As already mentioned, make a difference between NG and Classic. I guess it make sense to check TRs on Classics but it it not necessary anymore on NGs (there is TR light). :ok:

[stator vane]

and when checking the TR's, you must check amps---that is the only real indication that the individual TR is working---amps. volts can come from another TR--that was hammered from the beginning when Noah brought the first pair of rabbits onto the B737-100!

[hoschi]

Talking about the 737NG. When you have different ampere readings on TR1 and TR2( TR1 showing 28V/30A and TR2 28V/22A) after eng. start on ground, could it be a sign of unclosed Bus Tie Breaker ?

[flash2002]

The TR light will only light up in the air if TR1 or TR2 AND TR3 fails.

On the ground the TR light will illuminate on any TR failure.

[vwreggie]

here in aus we only check the tr1 as part of the before descent procedure on a classic. Came about as a result of loss of autopilot on g/s when the tr1 had failed some time before.