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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

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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).

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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.

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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.

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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.

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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.

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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 :)

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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

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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.

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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).

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You don't give up do you.... try taking off from KCRW Yager Field Charleston WVA and use RA... right at the end of the runway the hill that the airport is build on drops into the Kanawha Valley and your RA will almost instantly increase to over 1000 ft... this is why several of us have told you to ignore RA on departure.

 

Also, I have no idea where you get your reference material.  The FCTM included with PMDG B77X is licensed by Boeing.  True that the PMDG/Boeing FCOM has the same graphic on a different page, please read the text in the FCTM regarding takeoff procedures. No where in the text does it mention RA... only AGL.

 

Yield Vernon.

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Also, I have no idea where you get your reference material.  The FCTM included with PMDG B77X is licensed by Boeing.  True that the PMDG/Boeing FCOM has the same graphic on a different page, please read the text in the FCTM regarding takeoff procedures. No where in the text does it mention RA... only AGL.

 

That text was from a new edition i got from a RW pilot i'm only stating what it says in that FCOM text. Above 400 feet radio altitude engage a roll mode.

 

About KCRW and not using RA, that'll be airport specific and procedures!

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There's a trap of solely reference to RA only. Imagine there s a cliff shortly after the departure end of the runway. Or you are approaching from the sea into an airport where the terrain rises up as you descend. Under these two scenarios the RA will give you a false sense of climbing/ descending if solely reference to it, because it only shows your actual height right underneath he airplane, not with reference to the airport.

 

In the former case, if you look at your RA it will show increasing but you can still be able to do level flight while having the RA showing you that are climbing and vice verse when on the approach in the later case.

 

Therefore most people I know including myself scan all three parameters, RA, ALT and v/s. Because they all compliment each other to improve the situation awareness.

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Therefore most people I know including myself scan all three parameters, RA, ALT and v/s. Because they all compliment each other to improve the situation awareness.

Finally.

 

I'm guessing the AF447 and probably the AirAsia crew would have been better off had they looked at instruments "they weren't supposed to be."

 

 

If my GPWS and RA are screaming but my Alt is happy, I'm gonna be concerned.

 

And I probably do scan the RA on departure for heading mode. It's all in the mix.

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Looked at the books, radio altimeter is used by the Digital Flight Control System on takeoff (and other stages of flight). Examples:

- Both F/D MAster lights illuminate when TOGA is pushed. After 400' RA, the F/D MAster light extinguishes for the F/D switch that was switched on last (usually pnf/pm side).

- LNAV engages at 50' RA.

 

Your FCTM is no longer current, but is still correct, Vernon.

 

Airmanship should prevail, as discussed by Matt et al.

 

Brian Nellis.

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Therefore most people I know including myself scan all three parameters, RA, ALT and v/s. Because they all compliment each other to improve the situation awareness.

I'm guessing the AF447 and probably the AirAsia crew would have been better off had they looked at instruments "they weren't supposed to be."

 

Agreed, but as usual, I think he's getting lost in the minutia again here and losing the bigger picture. There's no formulaic, cut and dry answer. The prevailing method should be your MSL, as that's what most all altitudes on the charts are referenced to (unless the chart specifically says AGL) because, as mentioned, terrain can make the RA useless.

 

Of course, as mentioned, true SA means using all sources available (but relevant) to operate most appropriately. The altitude judgment could obviously be supplemented by the sight picture out the window provided that was an option.

 

Still, for someone who seems to be permanently lost in the details, and questioning the common wisdom with "well some random pilot said," I think it's most appropriate to say that the primary method is barometric altitude for departure. Later on, when he's fine-tuning his flying and trying to be a better aviator instead of a formulaic one (not to be judgy - I used to be that way, too), that's when he can concentrate on using additional sources for SA.

 

Why? In your first flying lessons, you're taught to fly a traffic pattern. Are you using a radio altimeter to maintain the standard 1000' AGL? Nope. You use the standard baro altimeter and a little math. Sure, if you have it, use it, but for someone who's learning, it's best to trim back unnecessary sources of information until the core concept is understood.

 

At the moment, trying to concentrate on all that is like drinking from a firehose.

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Agreed, but as usual, I think he's getting lost in the minutia again here and losing the bigger picture. There's no formulaic, cut and dry answer.

 

Bingo.  Sometimes you actually have to be a pilot and interpret all the information presented to you.

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Bingo. Sometimes you actually have to be a pilot and interpret all the information presented to you.

If he was my student, we'd be following the syllabus, which would have more measured info. Now, where did I leave his progress chart?

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400 feet radio alt is stated in my FCOM

 

Are they with the ones with the words "Boeing Proprietory. Copyright ©  Boeing " on them?

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