jmdriskell

Radio Altimeter

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How 'bout adding a radio altimeter to our DC-6/C-118?  I suspect that the bird was equipped with one when they were active. 

Jim Driskell

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10 hours ago, jmdriskell said:

How 'bout adding a radio altimeter to our DC-6/C-118?  I suspect that the bird was equipped with one when they were active. 

Jim Driskell

Nope they did not have one. The F/E was the radio altimeter back then

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That's how we still do it in general aviation, set QFE on the altimeter (this means the altimeter reads zero on landing, so the altimeter gives you height above the airport)

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4 hours ago, ckyliu said:

That's how we still do it in general aviation, set QFE on the altimeter (this means the altimeter reads zero on landing, so the altimeter gives you height above the airport)

General aviation in what area? I've never done that, actually, but I haven't flown outside of the States.

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United Kingdom. Although it's not especially common here, some fields operate on "circuit height" rather than "circuit altitude" (that's "the pattern" to you!) and ATC will occasionally issue heights instead of altitude. QFE is usually included in ATIS here, so it's not difficult to set on a secondary altimeter if you consider you need height information.

It could be related to the fact our transition level historically as low as 3000ft, so you could conceivably be at FL35 or even lower depending on the air pressure. These days they're standardising on 6000 ft and pushing for 17/18000 ft across Europe, which is going to mean a lot of fiddling with the altimeter for non-jet traffic as we fly across different Regional Pressure Setting (RPS) regions. But that's a whole other discussion!

Edited by ckyliu

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Just now, ckyliu said:

United Kingdom. Although it's not especially common here, some fields operate on "circuit height" rather than "circuit altitude" and ATC will occasionally issue heights. QFE is usually included in ATIS here, so it's not difficult to set on a secondary altimeter.

Cool - I'd assumed so, since I recalled hearing something about it in the UK context, but wanted to be sure. Interesting - thanks!

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But none of the preceding discussion eliminates the value of a radio altimeter.  Setting field elevation works only for that area and doesn't take into account different areas where the pressure is changing, or even at the home field after a front has moved thru.  Reminds me of an old Ernest Gann trick when trying to get under an overcast while on route to Iceland.  He had the radio operator let out the trailing antenna.  The radio operator yelled when he felt the antenna weight hit the water, about 75 feet below the aircraft.  I suppose one could call this technique a radio altimeter!  Anyway, some DC-6s did have real radio altimeter installed:  https://propspistonsandoldairliners.blogspot.com/2010/02/navigating-pan-am-dc-6-new-york-shannon.html

Why doesn't our vintage DC-6 have one?

Jim Driskell

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AAL was the last major airline I know of to use QFE and that stopped in 1996 (?) I could be off a couple of years since I'm getting old. 🙂

I never used QFE in my general aviation and military flying which goes back over 50 years, 🙂

Grace and Peace, 

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47 minutes ago, jmdriskell said:

Why doesn't our vintage DC-6 have one?

RADAR in the 1950s was a big heavy piece of equipment.  Lots of tubes and big power supplies with iron core transformers.  I would be surprised if anybody used radio altimeters in those days... not even the military; however, you have information to the contrary I'd find that very interesting.

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Hi Dan,

This citation may be more that you want to know but https://www.aviationpros.com/article/10387134/radio-altitude-the-instrument-of-choice

It has an interesting discussion of a Radio (not RADAR) Altimeter.  As the article states, the device was probably available for aircraft use in about 1940.  It uses a rather simple FM system to measure height by bouncing a signal off the ground and measuring the frequency shift between the transmitted signal and the current receiving frequency of a continuously re-tuned transmitter receiver combination.  The system was probably not very large initially, and now it's probably contained in a couple of chips in a box attached to an underbelly antenna.  The previous article talks about a Pan Am DC-6 in 1955 equipped with most of our current system, and a radio altimeter. 

I also read somewhere that some agency is considering bringing back LORAN in some form because of the possibly that GPS satellites could be disabled under certain circumstances.  So I guess we had better break out our sextants and drift meters if we want to ba able to navigate.

Semper Fi,

Jim

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15 hours ago, jmdriskell said:

It has an interesting discussion of a Radio (not RADAR) Altimeter.  As the article states, the device was probably available for aircraft use in about 1940

Thank you very much.... I stand corrected.  Basically a Doppler device in the 4 GHz range... okay.  I'm sure that the devices in the 1930s were not a frequency anywhere close to that.  Back then an ultra high frequency was about the highest devices could obtain using tube technology.  So I could see something in the 1950's comprised of a half dozen vacuum tubes (for oscillator, amplification, reception and detection and the power regulator) and power supply capable of doing this.  It'd be the size and weight of one of the VHF radios.  It would be interesting to learn more, at 4 GHz the Doppler effect provides 40 Hz per foot frequency shift. Drop that carrier to 400 MHz (probably the top end at the time) and that Doppler effect is reduced by an order of magnitude to 4 Hz per foot and I wonder if that impacted the accuracy of the measurement.

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Hi Dan,

The following articles may provide more information about radio (radar) altimeters that you want, but I found them interesting as they explain the current usage of the instrument and some of its history.  I found the reference to the device's use in the WWII Stuka significant.  It provided an automatic dive pullout for pilots on a bombing run if they blacked out because of g forces.

https://en.wikipedia.org/wiki/Radar_altimeter

https://www.skybrary.aero/index.php/Radio_Altimeter

Jim Driskell

 

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When your airport/aerodrome is at 1750ft you don‘t want to set QFE... that will rather break your altimeter. And yeah, I was the dull 15 years old who did exaxtly that. Was quite expensive. Instead we usually set it to the next lower or higher thousand. In that case 2000ft. Quite handy too but of course only possible when you just do patterns. 

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