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Mainly a question for the SMEs: Why the blend of units?


frostycab
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I'm curious as to why the Apache using metric units for range and speed measurements but still uses imperial for altitude and fuel flow. AFAIK all the other modern Western aircraft (except  maybe the Gazelle if my memory isn't totally awful?) use nautical miles and knots.

Please enlighten me so that I may become worthy. 

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Because the Apache was designed to operate as an Army "maneuver" unit, just as other ground forces like tanks. These ground forces use MGRS, which is a metric based system.

This is why it has a blend of nautical mile/feet based measurements for speed, altitude and distance, but performs targeting in metric, with the option to also toggle distance calculations for navigation to metric as well.

The flexibility to integrate with multiple types of air and ground forces is also apparent with its integrated use of Lat/Long in its point file system.

As for fuel, I imagine it just makes it easier to manage munitions and fuel loads when they are all in a mass based measurement of pounds, versus having the fuel measured in volume in gallons.


Edited by Raptor9
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Afterburners are for wussies...hang around the battlefield and dodge tracers like a man.
DCS Rotor-Head

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You are worthy, don't let anyone tell you different. 

Truth is it's based on a blend of who we're talking to and what we are trying to accomplish.  I was neither alive nor familiar with the airframe when the a model was being conceived so these are my best guesses. 

Altitude - A large amount of airframes utilize feet over metric, I am guessing is due how predominant the western countries became with altitude in feet and there's 3.28 feet in a meter they chose feet, but I know there's several aircraft that successfully use the metric system for altitude.

Distance from something - Almost all ground units in the world use the metric system for distance, the maps are that way, that's why you hear roger bravo 1-1 i'm 8 klicks out in action films. So I would presume it's that way because it makes it easy to integrate in, especially for the distances we cover in comparison to fixed wing. 

Airspace and integration with ATC largely relies on nautical miles though. So it's nice to have an option to switch to be able to communicate with them since doing math of 1 x 1.8 every time I'm trying to tell someone how far I'm from an airfield in FAA land would suck. 

Speed. We use knots. what are you talking about? Granted KPH is probably more refined but with crappy winds I imagine they're guessing yeah cruising at 150-200. JK I'm sure it's fine and I'm sure its just an issue of me liking knots because it's what I know. 

Elevation.  Back to most ground forces in the world use elevation in metric.  Easy communication.  

I would presume during the development process and the idea the army had for the AH64A which carried forward through the some b's, some c's, and more predominant D up to the D that your playing that someone somewhere said I would like this and then it ended up being in.

To me it is the way it is and there are things I like, and there are things that I wish were better, and there are things I don't like.  

nah nah give it to @Raptor9 he answered first @frostycab

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Cool! Yeah, sorry about the knots thing. I was thinking about the other helis and I think I got the Russian birds stuck in my mind.

I hadn't considered the integration with ground units, which I expect can be a major part in the daily life of an Apache pilot. having though about it now I wonder why there wasn't some concession made with the A-10 in that respect.

Altitude I have no problem with. Years of flying airliners in sims makes feet second-nature for me, and I struggle a bit with the DCS birds that use metres. Same for speed in knots. 

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Can I ask another question on a bit of a tangent?

Does the Apache have any way of making offsets? For example, you get a call from a ground unit, saying something like "Enemy units entrenched 800m north of the water tower." Do you need to eyeball it, or can you mark the water tower and then plot a position relative to that mark?

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2 hours ago, frostycab said:

Cool! Yeah, sorry about the knots thing. I was thinking about the other helis and I think I got the Russian birds stuck in my mind.

I hadn't considered the integration with ground units, which I expect can be a major part in the daily life of an Apache pilot. having though about it now I wonder why there wasn't some concession made with the A-10 in that respect.

Altitude I have no problem with. Years of flying airliners in sims makes feet second-nature for me, and I struggle a bit with the DCS birds that use metres. Same for speed in knots. 

There was. The A-10 can be set to kilometers too.

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17 hours ago, Raptor9 said:

As for fuel, I imagine it just makes it easier to manage munitions and fuel loads when they are all in a mass based measurement of pounds, versus having the fuel measured in volume in gallons.

 

AFAIK fuel is always measured in weight, because weight doesn't change with altitude, (pressure). Volume does for liquids and gases. 

Think I heard this on the Fighter Pilot Podcast . 

Cheers! 


Edited by MAXsenna
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Usually, yes. There are a few caveats to that if you expand it beyond DCS. I believe most light GA aircraft have fuel gauges in volume. It can get a little bit confusing when you come to commercial aviation too. Fuel is bought and paid for by volume, but for obvious reasons the crew and flight management computers reference weight. This means that any time they uplift fuel they need to do a set of calculations based on things such as the tested specific density of the fuel being supplied at that airport on that day.

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29 minutes ago, frostycab said:

Usually, yes. There are a few caveats to that if you expand it beyond DCS. I believe most light GA aircraft have fuel gauges in volume. It can get a little bit confusing when you come to commercial aviation too. Fuel is bought and paid for by volume, but for obvious reasons the crew and flight management computers reference weight. This means that any time they uplift fuel they need to do a set of calculations based on things such as the tested specific density of the fuel being supplied at that airport on that day.

That's what happened to that Canadian Air that sideslipped "into" a drag race, on an abandoned airfield IIRC.

Incorrect calculations by the ground crew or something. 🤷🏼‍♂️ 

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27 minutes ago, MAXsenna said:

That's what happened to that Canadian Air that sideslipped "into" a drag race, on an abandoned airfield IIRC.

Incorrect calculations by the ground crew or something. 🤷🏼‍♂️ 

Yeah. I believe what happened there was confusion regarding units. They'd worked out figures in kg, but the actual fuel was loaded according to lbs. I'd need to look up the incident to go into detail about it (or see the pretty good "made for tv" film again.) I do remember the PIC being hailed in the same way that Sully was.

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The Canadian Army, US Army and many other units use metric like KM (Kilometre, literally 1000 meters), meters and so on, largely because of the early days of the NATO treaty in the 1949.  We had to be able to work directly alongside, cooperatively, with dozens of European armies, seamlessly upon landing.

We'd be needing their maps too. The radio comms to call in artillery strikes, or to report enemy tank formations accurately that everyone could be certain of, SUPER important. The MLRS rocket systems are often referred to as "grid square removers"... a grid square is basically a square kilometre. 

So North American armies learned the Metric system and adopted the whole thing. Except that flying is still kinda mostly Imperial units! 

...

So MAX and Frosty mentioned the airliner... Back in 1983, an Air Canada airliner, Boeing 767, was on one of it's early flights, and the fuel unit measurements were in Kilograms instead of the North American common use of pounds of fuel. There was one or a few mistakes in converting the numbers, as the fuel provider could only read pounds, and also a system failure on the Boeing. 

The result? only 45% of the fuel needed for the flight. Ran out of fuel. Fly a loaded 767 with no engines... glide to land!  Read about it in the link below.

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

Funny part is, the plane was later fixed, and spent a very long career until it houred-out for retirement! 


Edited by Rick50
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3 hours ago, MAXsenna said:

That's what happened to that Canadian Air that sideslipped "into" a drag race, on an abandoned airfield IIRC.

Incorrect calculations by the ground crew or something. 🤷🏼‍♂️ 

The airfield they chose was and is still in use, just not the runway they picked.

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