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NSEU
4th Jul 2004, 05:35
Just wondering if the 737NG goes into Thrust Hold mode if the N1 button is used for takeoff rather than the TOGA switch/s.

Can't seem to find an answer in the Boeing Maintenance Manual D&O.

Thanks in advance.

Rgds.
NSEU.

P.S. Does it matter what position the F/D switches are in?

Dehavillanddriver
5th Jul 2004, 00:36
I must admit that I have never tried it but without having a look in the books I wouldn't have thought that the N1 button would have any effect on the ground.

Cornish Jack
5th Jul 2004, 12:06
Not at all au fait with the 37NG, but taking a read across from the 47 400, and relying on memory ( :( ),the Thrust Switch is inhibited on take-off until reaching 400', so a press on the ground shouldn't do anything.

NSEU
6th Jul 2004, 13:07
"Not at all au fait with the 37NG, but taking a read across from the 47 400, and relying on memory ( ),the Thrust Switch is inhibited on take-off until reaching 400', so a press on the ground shouldn't do anything."

You are, of course, correct, CJ (re the 744). Unfortunately, the 7437NG brings lots of surprises. Having worked on 767's and 747-400's for many years, I thought I knew a little about Boeings ...until, that is, I came across the 737NG.

E.g. FD's which don't appear when you select the FD ON (on the ground), electromagnetically held A/T switch and Wing Anti-Ice switch... and non-electromagnetically held Autobrake switch. Everything topsy turvey, so no assumptions can be made here :} .. but looking forward to quizzing you on the 744 when the Precision Manual Development Group starts beta testing their MSFS 744 add-on.

Cheers.
NSEU.

P.S. Nice to see you back on the Tech forum :ok:

avioniker
6th Jul 2004, 21:54
From AMM 22-31-00

FMC
The FMC calculates thrust N1 limits and N1 targets for each flight phase. The data goes to the DEUs. The DEUs show the N1 limits on the engine display. The DEUs send the N1 targets to the EECs which calculate equivalent TRA targets to send to the A/T to set thrust. The FMC also sends N1 targets directly to the A/T. During takeoff and max thrust go-around, the A/T uses EEC TRA targets and FMC N1 targets to set thrust. During takeoff, climb, and max thrust go-around, the FMC N1 targets are the same as the N1 limits. During reduced thrust climb and cruise operations, the FMC N1 targets are less than the N1 limits. The FMC calculates gross weight and sends it to the A/T to use in calculation of thrust and T/L rate commands. The FMC has an interface for A/T for BITE.

EEC
The DEUs send FMC N1 targets to the EECs. The EECs use the data to calculate equivalent TRA targets. The A/T uses the EEC TRA targets to set thrust during takeoff, climb, and max thrust go-around. For takeoff and max thrust go-around, the A/T initially uses EEC TRA targets to advance the T/Ls. As the T/Ls get to within 4 to 6 degrees of the FMC N1 limit, the A/T then uses FMC N1 targets to make final T/L adjustments to the FMC N1 limit.

~~~~~~~~~~~~~~~~~~~~~~~~~~
And here's the lots-o-words version

A/T SYSTEM - FUNCTIONAL DESCRIPTION - MODE SELECTION
General
The A/T computer receives digital and analog data from different systems and sensors to determine operational modes. This data comes into the A/T computer through the input devices.

The A/T has two primary modes of operation, speed control and N1 control. These are the only two A/T modes that are selectable on the MCP. Depending on the flight phase and mode selected, the A/T can enter these additional modes:
Arm
Retard
Throttle hold
Go-around
Test (on the ground).
After the A/T enters a mode, it puts mode selection data on the general ARINC output bus.
These systems use the mode selection data:
DEUs to show A/T modes
FDAU for data download and storage to the FDR
FCCs for A/T mode status.
The A/T controls power to the ASMs.
The A/T removes power from the ASMs during throttle hold mode operation during takeoff.

This section discusses each A/T mode of operation.
N1 Mode
The N1 mode is used during these flight phases:
Takeoff
Climb
Maximum thrust go-around.
In N1 mode, the A/T controls thrust to the EEC TRA target, calculated by the EEC for the equivalent FMC N1 target.
The N1 mode can be selected in these four ways:
Pilot manually selects N1 mode from the MCP
DFCS requests N1 mode when DFCS is engaged
TO/GA switch is pushed for takeoff (on ground)
TO/GA switch is pushed a second time during reduced thrust go-around (in the air).
When DFCS is engaged in VNAV climb or LVL CHG climb, the FCCs command the A/T to N1 mode.

When the DFCS is not engaged, the pilot can push the N1 selector switch on the MCP to manually select the N1 mode. Throttle Hold Mode

The throttle hold mode is automatic and the A/T goes into this mode during the takeoff ground roll. In this mode, the A/T removes power to the ASMs to prevent the A/T from moving the T/Ls during the takeoff roll and initial climbout.
The A/T uses two separate functions to remove power from the ASMs. One is a software function and the other is a hardware function. When both throttle hold functions agree and remove power to the servos, the A/T mode shows THR HLD on the FMA.

Arm Mode
ARM mode means that no active A/T mode has been selected. In the ARM mode, the A/T is enabled and ready to receive commands. The ASMs are powered but the A/T control logic prevents the servo motors from moving the throttles.
The A/T goes to ARM mode during these conditions:
On the ground when A/T is armed during preflight
After THR HOLD mode above 800 feet barometric alt
During Descent RETARD when T/Ls reach aft stops.

Speed Mode
In the speed mode, the A/T controls engine thrust to control airplane speed. It does this by comparing the actual computed airspeed (CAS) from the ADIRU, to the target speed from the MCP.
These are the two A/T Speed modes:
FMC SPD from the FMC target speed
MCP SPD from the speed selected on the MCP.
In FMC SPD mode, the A/T controls thrust to control airplane speed to the FMC flight plan target speed. When the A/T is in MCP SPD mode, it controls thrust to control airplane speed to the target speed selected on the MCP.
The A/T Speed mode can be selected automatically or manually. If the DFCS is engaged, the DFCS selects the A/T speed mode automatically, either FMC SPD or MCP SPD, consistent with the active DFCS pitch mode. A/T MCP SPD mode may also be selected manually by a push of the A/T speed mode selector switch on the MCP.

Retard Mode
In the RETARD mode, the A/T moves the T/Ls to the aft stops.

These are the two retard modes.
Descent RETARD
Flare RETARD.
Descent RETARD occurs during descent from altitude.
Flare RETARD occurs during flare to landing.
Both modes show as RETARD on the FMA on CDS.
Descent RETARD occurs during DFCS VNAV SPD decent, or when LVL CHG descent is selected on the MCP. During DFCS VNAV SPD descent, the A/T usually starts to retard the T/Ls to idle at the FMC top of descent (TOD) point. When the T/Ls get to the aft stops, the A/T mode changes from RETARD to ARM.
The A/T stays in the ARM mode until a new mode is selected.

Flare RETARD occurs during landing flare. During flare retard, the A/T retards the T/Ls to idle and RETARD shows on the FMA. The T/Ls move back to the aft stops as the airplane flares for landing and touchdown. The A/T disengages 2 seconds after touchdown.

Go-Around Mode
During approach when you push a TO/GA switch once, the A/T commands a reduced thrust go-around. The A/T mode on the FMA shows GA.
During go-around, if you push a TO/GA switch a second time, the A/T commands maximum thrust go-around to the FMC go-around N1 limit. The A/T mode on the FMA changes from GA to N1. The go-around mode resets if the N1 or the speed mode is selected on the MCP.

Man I gotta learn to type faster...

NSEU
7th Jul 2004, 01:04
I didn't mean to put you to so much bother, Avioniker. This is exactly what I have in my manuals. It says N1 can be used for "takeoff", but what does this mean? (Can takeoff include early climb?). Will it go into HOLD mode from N1 mode or just from TOGA mode.

"When the DFCS is not engaged, the pilot can push the N1 selector switch on the MCP to manually select the N1 mode."

Is the DFCS considered engaged when the FD is ON? (or are they talking purely about A/P engagement).

I have already received a reply on another respected pilots' forum (from a Fleet Technical Captain) telling me it won't, so I still don't know for sure. Hence my confusion.

Rgds.
NSEU

avioniker
7th Jul 2004, 16:30
First you must get the antiquated ideas out of your system.
The DFCS is always "on". Engaged takes on a new meaning with the 737NG.
While the system may not be actively controlling the flight controls for navigation purposes things like Speed Trim and Track calculations are never disabled. The Flight Director is actually telling you what the Auto Flight system would be doing if the Autopilot were engaged or coupled. It is always doing the calculations unless you pull the breakers. When the Autopilot is coupled in any mode the Flight Director is showing you what the system is doing to the flight controls. They are no longer separate systems.
The Auto Throttle is a separate computer but it is fully integrated into the DFCS in a way that allows it to command Pitch changed (Through the FCC's) as well as throttle position. If you select N1 you are limiting it to commanding throttle position. In speed modes it actually "coordinates" with the FCC's to manage the best Throttle/Elevator/Stab positions to achieve the desired result.
Therefore your question is not relevant to the system as designed and engineered. Thrust hold is more of a misnomer used by many to explain a variety of modes depending on previous experience. Some use the term interchangably with what is called "Clamp" in other aircraft and some use the term to mean EPR management.
Thrust hold simply means that the Autothrottle will maintain the desired N1, moving the throttles as appropriate to do so.
Any time the switch is in the Arm position and any mode selected the computer will do what's needed in coordination with the FCC's to make your wishes come true.
I'm glad you got another reply from another respected pilot's forum. I'm sorry you couldn't get the answer you liked from the manuals.
Now if we could just get Boeing off of the ATA 104 "Simplified English" kick and give us manuals that make sense...:O

NSEU
7th Jul 2004, 17:25
"Thrust hold is more of a misnomer used by many to explain a variety of modes depending on previous experience. Some use the term interchangably with what is called "Clamp" in other aircraft and some use the term to mean EPR management.
Thrust hold simply means that the Autothrottle will maintain the desired N1, moving the throttles as appropriate to do so."

What you are saying here disagrees with what you quoted in your manual, Avioniker. You said earlier that power was removed from the ASM's in Hold mode, therefore, one would assume they couldn't move the throttles (if that's what you mean by thrust levers?).

Our 737NG Hold mode is the same as our 767-200/-300 and 747-400 hold mode.... i.e. power is removed from the A/T servomotors (for safety) and it will not do anything at all to the throttles. Our thrust hold mode allows the pilot to move the thrust levers freely without obstruction from the servos, or have the thrust change, uncommanded by faults in the A/T system, until the aircraft reaches a certain altitude above the runway.

I'm still not clear on (your understanding of) what happens on your aircraft if you push the N1 button on the ground with the DFCS system powered (but A/P not engaged). Are you saying it will or won't go into HOLD mode at 80-something knots?

Thanks.
Rgds.
NSEU>

80/20
10th Jul 2004, 09:38
Here is how I understand it:
Try to push N1 before you push TO/GA and nothing happens – it is not compatible.
Push TO/GO and N1 will display of FMA and the N1 switch illuminates.

If you push N1 after TO/GA but before 80 kts you will get FMA ARM (you should not do this).

After 80 kts you will get THR HLD (just as with TO/GA) and the light in the N1 MCP panel button extinguishes and you are stuck with this until you get ARM during climbout.

The difference between FMA ARM and THR HOLD is ARM’s underspeed protection which we don’t want in case of an abort – hence the THR HLD mode.

Sorry NSEU - but I have a feeling that I did not talk about what you asked. In which manual did you find the text "When the DFCS is not engaged, the pilot can push the N1 selector switch on the MCP to manually select the N1 mode." ? Are you talking about the N1 SET knob on the center forward panel which can be used to manually set the N1 limit on the thrust mode display (which has no effect on the autothrottle operation)?

NSEU
10th Jul 2004, 11:03
"In which manual did you find the text "When the DFCS is not engaged, the pilot can push the N1 selector switch on the MCP to manually select the N1 mode." "

It's in the 737NG Boeing Maintenance Manual (D&O section). the quote was immediately preceded by...


"When DFCS is engaged in VNAV climb or LVL CHG climb, the FCC's command the A/T to N1 mode. When the DFCS is not engaged....."

I understood this to mean that the N1 button could not be manually pushed when the DFCS was engaged(in general).

Anyway, thanks for the feedback. BTW, when you say "Here is how I understand it", do you mean that this is what happened when you tried it in the sim? Do you recall whether or not the FD was on?

Thanks.
Rgds.
NSEU.

80/20
11th Jul 2004, 15:58
The statement “the pilot can push the N1 selector switch on the MCP to manually select the N1 mode” can be misleading because the A/T switch must be on, there will always be A/T FMA annunciation and actually the N1 switch will change rather then engage A/T mode, if compatible with existing mode.. For example, to change between ARM & N1.

Fat Dog
13th Jul 2004, 11:07
Tried pressing N1 with the A/T armed on the ground in a -700 yesterday. Nothing happens.

Regards
Fat Dog

B73567AMT
15th Jul 2004, 04:08
The B737s (Classic and Next Gen) do not work in the same manner as let's say the B757, B767 and B777. Those airplanes have go-around switches, not TOGA switches. Therefore, it is necessary to hit N1/EPR on the MCP to initiate takeoff mode.
The B737s have the TOGA switches used for Take-off and G/A. Hitting N1 mode on the gnd, as has been stated WILL NOT SET TAKEOFF POWER.

If everyone knew this already, my apologies for chiming in. I just saw that NSEU wrote using the N1 button INSTEAD OF TOGA. There is no substitute for the TOGA switches for initiating a takeoff.