A320 purpose of THS on MCDU

[MD83FO]

hello.. at my new job the MCDU displays THS on the perf takeoff page flaps setting line, I never saw this on my previous airline,
what is the purpose of this when all i have to do is take the load sheet takeoff CG and dial it on the trim wheel.

[tubby linton]

It is there as a reminder. Some airlines do not enter any value into it.All of the fmgc I have seen Honeywell and Thales have this field.

[vilas]

MD
Sure you would dial it on the trim wheel but you cannot till the engines are started so you write it in MCDU along with flap setting instead of trying to memorise and make a mistake. It is like a scratch pad entry.

[MD83FO]

vilas my SOP has always shown to select the fuel prediction page to set the CG after engine start.

the trim units on the back of the QRH were deleted with an advisory circular informing that this procedure was unnecessary for this airplane and only to be used on the company A300's

[vilas]

MD83FO
Some companies follow that method, however airbus does not teach it in their training. You calculate Conf and CG before engine start but without hydraulics you cannot set it. So instead of scribbling on a piece of paper you write it in MCDU. After start read and set it so you don't make a mistake.

[sonicbum]

Vilas, there is no need to scribble anything. The primary way of setting the THS as you know is thru the percentage of MAC on the trim wheel to avoid confusion between THS up or down. Writing the THS position in the perf page is a nice reminder but nothing more than that. The main action is to check the % of MACTOW during the after start from fuel pred. Airbus teaches it in the various PDP / normal sop.

[vilas]

sonicbum
What about the flap? Pilots have selected wrong flap settings than calculated and taken off. Also I am using the latest airbus training DVD and in last 5 years airbus has changed it 3 times but none of them or airbus procedures say anything about fuel prediction page. It must be your company procedure.

[Denti]

In the PDPs we had available during the typerating (and after) there was no mention of using the fuel prediction page for that. Performance page is all we need. The loadsheet provides both MAC and trim position, the trim position is entered together with the flap setting into the performance page and used to set the trim after engine start and crosschecked against the MAC.

[tubby linton]

The stab setting on an A300 varies according to the flap setting.When we used to operate them we had a bug card to write speeds, flap , stab etc on.

[Wingswinger]

Years ago we used the THS reminder to set the trim. That was the SOP - until an LH crew set 1.2 NU instead of 1.2 ND. Or was it the other way round? Anyway the take-off was "interesting". It changed to % MAC thereafter.

[Metro man]

Units can be a better alternative with mixed types as the A319 usually trims nose up and the A320 usually trims nose down. Pilots have set the correct amount of trim but in the wrong direction.:hmm:

Using the Fuel Prediction page comes with the caution that you must enter the ZFW C of G correctly on the INIT page. How many times do we enter the correct ZFW because it is available first and forget to enter the C of G after the load sheet has been completed ?

I check both after engine start.

[tubby linton]

This one happened a long time agoASN Aircraft accident Airbus A320-212 N357NW Detroit-Metropolitan Wayne County Airport, MI (DTW) (http://aviation-safety.net/database/record.php?id=20010317-1)

[MD83FO]

Airbus Instructor Support states the airplane is certified for takeoff with full nose up or down trim. makes you wonder in this accident.

this is what it looks like in the sand pit After Start

PITCH TRIM handwheel................................................... ......................................SET PNF
Crosscheck load sheet T/O C.G against FMGC T/O C.G, In case there is a discrepancy of more than 2 % check that the ZFW and ZFWCG have been correctly inserted in the MCDU, and use the T/O CG value from the fuel prediction page.
PNF is to re-select FPLN page on MCDU.

New Company

PITCH TRIM handwheel................................................... ......................................SET
Set takeoff CG on pitch trim hand wheel.


the first one reflects many years of operating mistakes, the second one,
new company, standard SOP. though not mention of trim units.

[Gysbreght]

Airbus Instructor Support states the airplane is certified for takeoff with full nose up or down trim. Does it really say that? Are the take-off distances based on out-of-trim conditions?


This is the relevant FAR/JAR rule:

(4) Reasonably expected variations in
service from the established take-off
procedures for the operation of the aeroplane
(such as over-rotation of the aeroplane and
out-of-trim conditions) may not result in
unsafe flight characteristics or in marked
increases in the scheduled take-off distances
established in accordance with CS 25.113(a).

[MD83FO]

Instructor Support page 195

Consequences in case of erroneous ZFWCG / ZFW entry:
Operational consequences:
At take-off, the pitch trim setting might be wrong. This is a minor mistake, since the take-off has been certified in full forward GG with full aft pitch trim and vice versa.
- VS1G and VLS may be slightly affected (± 2 kt) as CG is taken into account in their calculation.

[Gysbreght]

Thank you MD83FO

[sonicbum]

You guys are right the docs I was referring to are my company tailored ones, I was pretty sure it was Airbus but... :8

[mono]

Actually guys, the MCDU THS position is used to generate a warning depending on the FAC and FWC standard installed.

Below is an extract from the MOI.

• Pitch Trim/MCDU/CG disagree:
In the frame of "take off securing", a Pitch Trim/MCDU/Center of Gravity (CG) disagree caution is implemented in FAC B0621. FAC B0621 computes a theoretical Trimmable Horizontal Stabilizer (THS) position using the MCDU CG. This theoretical position is sent to the FWC which compares it with the
actual THS position. In case of discrepancy (threshold defined in FWC logics) the "pitch trim/MCDU/CG disagree" alert is triggered. This function requires FWC standard H2F6.

In FAC STD BAM620 the alert is triggered only if MCDU and actual THS position disagree. (i.e. the FAC doesn't compute the THS position)

There are also slight differences depending on which standard of FMGC is installed.

[vilas]

mono
The MCDUs so far takes only nose up/down is it also changed to % CG? What is the date of this MOI? I know H2F6 is for A320 can you confirm it please.

[sonicbum]

Hi mono,

I just checked and we have this MOI too. F/CTL PITCH TRIM/MCDU/CG DISAGREE

the system detects that any of the following disagree :
- the real pitch trim value
- the pitch trim value calculated by the FAC, based on the CG
- the pitch trim value entered in the MCDU

Can you give an example of a scenario where that would be triggered ?
Thanks

P.s. This is a hot topic especially for MFF 320/330 where the system has a different logic (FCMC and envelope computations)

[vilas]

This will only be applicable to FWC H2F6. The aircraft having this FWC also have a 30 seconds count down from engine failure detection for relight.

[MD83FO]

The NTSB issued safety recommendation A-02-006

the item reads "acceptable alternate action" but i cant find that action.

http://www.ntsb.gov/_layouts/ntsb.recsearch/Recommendation.aspx?Rec=A-02-006

[vilas]

Airbus changed the method of setting from NU/DN to % MAC that must be the AAA.

[tubby linton]

I remember an old fleet manager's notice summarising a foreign incident of miss-set trim. The advice was "anywhere in the green band and it will fly"

[MD83FO]

Hi vilas, SOP still states on FMGC cockpit preparation to insert flaps and THS on takeoff perf page,
the on after start it states "Pith trim hand whell set takeoff CG", rendering the THS units previously set useless. I feel we shouldnt be given those units nor should they be set

[vilas]

MD
Yes. At the moment the MCDU software requires NU/DN input. But after start you set % on the wheel which you can convert from NU/DN to %.Also in A320 it is only as a reminder doesn't give warning. Therefore some airlines after start just take it from Fuel Pred page and set it.

[Capt Scribble]

If you mis set the ZfwCoG on the init page, but then set the correctly calculated trim setting, you will get the warning.