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Driver170

Radio/baro readout

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Stupid question it may-be but got me thinking!

 

When on departure and climbing out, do you read the RADIO ALT or the BARO ALT ? I have been going off the RADIO ALT untill after 2500 AGL then switch to the BARO ALT to read my AA 3000 but this won't be 3000AGL like whats set in the FMC? ( i should mention climb thrust 1500AGL and Acc Alt 3000AGL) so if i read the BARO ALT 3000 this won't be 3000AGL or exactly above ground like whats set in the FMC so i'm actually going to Acc more or less than the true alt.

 

Anyone see my confusion lol


Vernon Howells

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Anyone see my confusion lol

 

I don't, no. Unless it explicitly states AGL, then you use altitude MSL. The whole time, you use your regular altimeter, regardless of the altitude.

 

For IAD, an accel alt of 3000 (which is AGL) would be 3320. If you just let the FMC take care of it, it'll automatically make the change at the appropriate altitude that you've set on TAKEOFF REF page 2 (the altitudes there are AGL, and I believe that it states this clearly on the page - I could be wrong).


Kyle Rodgers

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I cannot think of a scenario where the radar altimeter would be used on departure.  IFR departures are designed (by folks in a dark room without windows that we call the TERPS) to provide terrain and obstacle clearance if you follow the procedure. I've never seen a departure procedure that required radar altitude.


Dan Downs KCRP

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Its just on that initial dept where you have RADIO ALT and of course BARO ALT do you go off the radio or baro? Thats where i'm a bit insure on what to use.


Vernon Howells

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Its just on that initial dept where you have RADIO ALT and of course BARO ALT do you go off the radio or baro? Thats where i'm a bit insure on what to use.

 

There's no reason to even bother with the radio altimeter unless you're on an approach that calls for it.

 

Ignore it at all times until then. I feel like you're overcomplicating things again.


Kyle Rodgers

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If you are refering to the baro/radio setting on the glareshield, it  is only used for decision altitude/height for landing.

 

For takeoff you should set it for the landing runway at the airport you are departing, in case you need to turn back and land. So if it is an ILS approach you will set baro.

 

Before approaching the arrival airport you should select baro or radio according to the approach plates, but it will be baro most of the time.


Peter Schluter

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Hi Vernon,

 

Everyone has pretty much hit the nail on the head already.

 

The use of a radio altimeter is only applicable to ILS CAT II/III operations and the DH is based on the Radio Altimeter. A standard ILS CAT I or Non Precision Approach DH/MDH is based on the pressure altimeter.

 

Sam Breese.

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We are talking about departure aswell? Ok so only time i'll use radio alt is when performing a precision approach. Got it :)


Vernon Howells

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I've read through the FCTM and it mentions AGL and RA when taking off so it looks like you do take the reading of the radio altimeter


Vernon Howells

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Rather disingenuous not to provide a reference, had to dig through it myself to find what you might be talking about.  The diagram on Pg 3.2 of the takeoff profile does surprisingly indicate RA 400 as point to select or verify roll mode; however, the text beginning on Pg 3.27 always refers to AGL altitudes and I didn't find a reference in the text to radar altitude.

 

Sure, it's handy with that big RA readout in the PFD to use that for selecting autopilot at 400 agl; however, some departures have rapid ground elevation changes at the end of the runway and best to recognize that you do not use RA for departure procedures but AGL or baro altitude.

 

Man you are like a bulldog when it comes to topics.


Dan Downs KCRP

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AGL is still affectively RA?

Haha i like to know ;)

 

AGL (above ground level) is effectively RA.

 

However, most departure procedures are referenced to AAL (above aerodrome level) which may or may not be the same as radio altitude (for an extreme example: if the runway is located on top of a 400ft high cliff, as soon as you go past the end of the runway the radio altimeter will indicate 400+ft, whereas your height above the aerodome may only be a few tens of feet). This is the same reason why radio minimums must only be used on approach where explicitly stated by the chart; the terrain along the approach path has to be surveyed to determine the correct radio minima and missed approach point.

 

It may be (and an NG systems guru would need to verify this) that the 737NG systems take inputs from the radio altimeter to determine minimum autopilot mode engagement heights. I can categorically say this is not the case on the 747-400, which records barometric altitude as the aircraft accelerates through 100kts on the takeoff roll and uses this to determine VNAV/LNAV engagement, acceleration and thrust reduction altitudes. However, not all Boeings are created equal!

 

Certainly in my experience the only two instruments you should need on departure are the barometric altimeter and the VSI (both increasing = positive rate). Many airlines use the minimums selector prior to departure to bug acceleration altitude (on the barometric altimeter).


Simon Kelsey

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