Question re: a certain display on the PFD

[XCLTM3 112]

Just for my own purposes, I was curious if someone could please definitively answer this question re the PFD:

 

If you refer to the Tutorial #1 manual, page 0.00.64 - there is a screenshot of the PFD. On it, you'll notice a small rectangular black box with a "-2" in it (lower portion of the artificial horizon in the bottom of the brown/earth area) - I've noticed this status each time prior to start-up and wandered what (if anything) it actually means/indicates? 

 

As I understand it, it is something relevant to the Radio Altimeter - not sure if it is a default startup indication or what?

 

Appreciate the clarification.

[PHXMD-11 86]

 

 

As I understand it, it is something relevant to the Radio Altimeter

 

It's relevant to the radio altimeter because it is the radio altimeter.

[XCLTM3 112]

It's relevant to the radio altimeter because it is the radio altimeter.

 

Kenny,

 

as I pointed out in my post - I'd like clarification on the "-2" status.........

 

enough said.

[FLEX1978 373]

It is completely normal, the RA antenna is calibrated for the aircraft being completely level, when the aircraft is on the ground it is sitting nose down, this causes the RA to give you a negative number

 

On my way out but will have a check of the docs to confirm the exact position of the antenna on the 777 later, should be in front or near the nose wheel? Anyway you can check it yourself! Or someone else can confirm

 

Regards

[Flamin_Squirrel 71]

It is completely normal, the RA antenna is calibrated for the aircraft being completely level, when the aircraft is on the ground it is sitting nose down, this causes the RA to give you a negative number.

Almost; it's calibrated to read zero at main wheel touch down. At this point the nose wheel is still quite a way off the ground, hence the -2 when level.

[FLEX1978 373]

Would that account for the -2? You are correct it does read 0 at main gear touchdown however the antenna is pretty high of the ground when derotation begins.

 

Am pretty sure the -2 is because the aircraft is not level. Take a look at some other types like the 757 or A330, the nose down is more pronounced and the -RA reading is lot higher

 

Cheers

[Flamin_Squirrel 71]

Would that account for the -2? You are correct it does read 0 at main gear touchdown however the antenna is pretty high of the ground when derotation begins.

 

Am pretty sure the -2 is because the aircraft is not level. Take a look at some other types like the 757 or A330, the nose down is more pronounced and the -RA reading is lot higher

 

Cheers

 

My nose wheel comment was slightly misleading. Simply put, we're interested in the difference in height of the antenna at touch down compared to that with weight on wheels.

 

The most simple factor is how much the gear compresses compared to weight on and off wheels - self explanatory.

 

The other factor is the difference in height of the antenna above the ground compared to during the flare and with weight on (all) wheels. There are two variables in this case: one is the difference in deck angle between these two scenarios. The other is the distance of the antenna forward of the main gear. The further forward the antenna is, the greater the arc it'll travel though during roll out and the greater the difference in readout.

 

The actual deck angle plays a part but doesn't directly correlate to the -RA reading. For example, the 777 and 737 are both fairly level on the ground (at least, very little in it) but while there's a huge difference in their sizes, the RA readout on the 777 is -2, it's -8 on the 737.

[Richard McDonald Woods 168]

Perhaps all these variables are used in the Boeing measurements so that the value shown is the most accurate achievable? :huh:

[kevinh 409]

Would that account for the -2? You are correct it does read 0 at main gear touchdown however the antenna is pretty high of the ground when derotation begins.

 

Am pretty sure the -2 is because the aircraft is not level. Take a look at some other types like the 757 or A330, the nose down is more pronounced and the -RA reading is lot higher

 

Cheers

The reason it's -2 is not because it's not level, it's because it is not at touch down attitude. The RA is calibrated to read 0 when the main wheels touch at a typical touchdown attitude. Because the antenna is forward of the main gear, the reading reduces (i.e. goes negative) as the aircraft de-rotates. Even if the fuselage was level on ground the RA indication would still be negative.

[Flamin_Squirrel 71]

Perhaps all these variables are used in the Boeing measurements so that the value shown is the most accurate achievable? :huh:

 

Not sure what you mean? These variables affect what the RA readout is when you're weight on (all) wheels, but what that figure actually is isn't actually important; the readout at touch down is.

 

The reason it's -2 is not because it's not level, it's because it is not at touch down attitude. The RA is calibrated to read 0 when the main wheels touch at a typical touchdown attitude. Because the antenna is forward of the main gear, the reading reduces (i.e. goes negative) as the aircraft de-rotates. Even if the fuselage was level on ground the RA indication would still be negative.

 

Thanks, put much better than I managed.

[FLEX1978 373]

The reason it's -2 is not because it's not level, it's because it is not at touch down attitude. The RA is calibrated to read 0 when the main wheels touch at a typical touchdown attitude. Because the antenna is forward of the main gear, the reading reduces (i.e. goes negative) as the aircraft de-rotates. Even if the fuselage was level on ground the RA indication would still be negative.

Yep, I had a read up on it a little earlier today. Still a little confused but that is in reference to what was explained to me by someone doing work on an A330 freighter during a D check, it's not as simple as some of you are making out. Will need to have a word with him to clarify.

 

Cheers

[Flamin_Squirrel 71]

Yep, I had a read up on it a little earlier today. Still a little confused but that is in reference to what was explained to me by someone doing work on an A330 freighter during a D check, it's not as simple as some of you are making out. Will need to have a word with him to clarify.

 

Cheers

 

Well, as you pointed out the A330 (passenger) has a nose down deck angle. This isn't good for freight, so the A330 freighter has a modified nose gear to level the deck. This maybe the complication?

[FLEX1978 373]

Yeah, likely has nothing to do with the 777. Thanks anyway, your post was an interesting read and I'm sure the OP has his questioned answered.

 

Cheers!

[XCLTM3 112]

Thank you to all for the positive contributions. I can say that I am now clearer on the "-2" reading/status on the RA pre startup. :rolleyes: