Negative DH Rad Alt display.

[BoeingBoy]

As a current 757/767 Captain with a lot of 737 experience, I was recently discussing the fact that some Boeing installations allow the display of negative decision heights on the rad alt, with the reset switch setting the value back to a display of 0. This would seem to be the case in Qantas and the Pacific.

Conversly every installation I have flown on all three types in Europe does not allow the display of negative DH's and the reset switch just resets the colour of the DH.

Can I ask for a straw poll on how many of us have aircraft in either config as the argument centred on what is the default setting as supplied by Boeing and which is the company mod. We also wondered which is the more common.

Thanks.

BB


Edited to keep the pedants happy

[Eff Oh]

In every B757 I have flown you can set the DH to -20.

[BoeingBoy]

Hi Eff Oh,

Just to clarify this, I was talking about the actual EADI display not the numbers on the selector panel. On all mine when you get to 0' the green display dissapears, on others it shows all the way down to -20'.

It caused an interesting discussion as we wondered if you have the -20' displayed on the EADI, what do you use it for? After all a DH is a decision height, and if it's in negative values then surely you have already hit the ground!

Would appreciate some views and polls on this as I am about to lose a beer or three!



;)

[alf5071h]

Why can the rad alt bug be set to -20 ft?
A thought; if the GPWS ‘minimums’ call is tied to the rad alt setting, which is normal, then setting a negative value is one method of disabling the call. Many operators disliked the ‘minimums’ terminology, particularly on NPAs where rad alt, hence the rad alt bug, is not normally used for decision making. I don’t know of any aircraft with a ‘minimums’ inhibit switch, so the -20 ft could be a cheat solution and installed as a customer option.

Don’t forget that rad alt on all NPAs is a very good situation awareness instrument and should be monitored at all times. I set the bug at 250 ft because on all NPAs I should be visual by then.
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Unless specifically authorized everything else is forbidden.

[alexban]

my guess
0 RA reperesents distance calculated at touchdown,LG extended,from the antena in use .After landing ,the struts are compressed,also the antena position is lower (more or less depending of its position on the fusselage forward or aft) ,so that's why the negative measurement.At touchdown,due to landing angle of the fusselage,the antena is a little bit higher than after landing,fusselage aprox horizontal.
On a 737 is around -2 /-4 ft.
On the 757 (which is longer) if the antena is close to the forward part of the fusselage,the deviation can be up to -20 ft,as said by BB.(I don't fly the 757 ,it may be less than this.It can be calculated if you know the angle at touchdown and distance from main gear to the antena,I think.)
Brgds Alex

[Eff Oh]

Ah!! Gotcha. Well on all B757's I have flown anything below 0 on the RDH causes the numbers on the EADI to dissappear! ;) I would suggest that a negative setting on the EADI would be pointless, as surely you would have landed gear up before it called anything!!??!! :D
Sorry Alex, but I have never seen the actual radalt read -20! I have seen -6 or so, but you have a BIG problem if it is showing -20 on the ground.
Eff Oh :ok:

[alf5071h]

Alex, the Rad Alt cert requirements are for a calibration so that zero is indicated at main wheel touch down. In most aircraft the oleo compression is relatively small and / or any difference between extended and compressed position is off set by de-rotation, and / or change of attitude with the nose wheel on the ground – all subject to aerial location.

Some Rad Alts give unusual indications on the ground when taxiing over water puddles due to double reflection of the radiated beam.

[mondriver]

What fun....!

Tricky little topic this.

The figure that BoeingBoy is referring to (as I read his post), is the "actual" radio altitude sensed by the RAD ALT system. This, as has been previously posted is calibrated to read 0 (ZERO) feet at main wheel touch down during any landing with minimums below Cat I.

The reason why a negative value is displayed on the ground (and this can vary between -6 and -20ft for various technical and quite boring reasons) is that the relative position of the main wheels and the rad alt assembly, which is located just aft of the forward nose gear strut, together with the aircraft body pitch attitude on touch down, means that a correction is needed if you want a zero rad alt value on touchdown.

The small "RESET" button on the ADI selector panel, is probably the most pointless (I think) switch on the 757 !

It can be pressed (if you so desire!) once you have descended through the selected RAD ALT minimum as set on the ADI panel. By pressing the reset switch, it merely changes the now amber coloured RA display back to its normal white colour !!
(This is also done automatically upon touchdown or climbing above the selected DH value +75ft)

Clear as mud eh?!!

[BoeingBoy]

Hi all,

Thanks for the replies.

I guess that no one actually has the DH displaying negative values so my beer is safe. I agree that the actual rad alt reading may go to negative values to cover the aerial wheel separation distance.

Thanks again for your inputs.

BB