Judging height over terrain

[Hobie]

What tricks do you guys use to know your height over terrain?

 

For example, you are flying up a canyon at 500' and the ground below you is rising. Radar altimeters excluded;)

[aeroscout]

I must say I am quite leery of the question. I suspect more than meets the eye.

[DS_HMMR]

Unless you have a target (in this case, on the ground), that is of known size/height/or width; then your range estimation is going to be highly suspect.

 

It takes Years of dedicated range estimation to be able to get it right.

With 30 years experience as a scout sniper, I've got it down to about 2 feet, but that's with complete focus on the task at hand. So, I'd have to say the correct answer in this case is going to be RADAR ALTIMETER.

 

besides, you cant range jack squat if your vision is obscured, your attention is shifted, its night (NVG's or not).

 

or to answer your question as stated: maintain your 500' interval with a climb rate matching the terrain: 500' on the vsi should do nicely.

[Velocity173]

Without a RAD ALT you're gonna need to compare your MSL altitude to an area of known terrain altitude. Outside of that it just comes down to experience. Kinda like traffic distance estimation.

 

Once you've seen what 500 ft looks like several times you start to develope a sight picture. Obviously without good terrain contrast it'll make it harder but with good high contrast objects it becomes easier. These days I can pick an altitude to cross a ridge line and usually I'm within 50 ft of what the RAD ALT indicates. Just takes time.

[apiaguy]

There is an OSR-10C rivet on the side on the skid... I focus on that rivet and with my peipherial vision look at the terrain... if the ground clears that rivet I'm golden pony boy.

[Hobie]

Thx DS and Velo. Newbe here and you confirmed my thinking. Experience, seat time. Especially at night.

[pilot#476398]

Thx DS and Velo. Newbe here and you confirmed my thinking. Experience, seat time. Especially at night.

 

Actually, at night you're better off planning/knowing your route and using your altimeter, lest that sudden, SURPRISE!

[Little Red 22]

Rocks can be very deceiving. Unless you have reference to man-made structures; i.e. powerlines, small buildings, roads, etc. the next best option is to look for animals and trees. Cows, deer, vultures, trees, are your next best reference for scale and distance determination.

[Wally]

What tricks do you guys use to know your height over terrain?

 

For example, you are flying up a canyon at 500' and the ground below you is rising. Radar altimeters excluded;)

 

If you don't have cultural features to scale from, it's hard to do.

If you have houses, cars, etc. that you have experience viewing from 500' (my minimum ever, ever cruise altitude) then the absolute altitude is easy. If those are too big, you're too low. Without those known size references, it gets tougher because rocks, trees, and all that outdoors stuff comes in widely varying sizes. That rock might be 3 feet vertical or 300 feet (slight exaggeration), do you have 500' or 5' clearance? Trees, vegetation, with some woodsman-ship help add scale.

 

"Why are you in the canyon" needs to be considered as well, my opinion. Terminating the leg in the canyon is different than using it as a pass, especially with altitude limitation issues. Start a climb, and check the front- if there's a point that's not moving down the windscreen, then one needs to consider carefully while approaching that point, especially if the check climb is at or near power limit. If your terminal point is beyond that...

 

Canyons, valleys, draws, are all dangerous places to be. Flying up the floor might be fun, and between the verticals picturesque, but winds can be a real hazard, some downdrafts being faster than you can climb. Places to land, or even turn around, can suddenly be suddenly important but hard to find. You will crash the aircraft if there's not a suitable place in reach when you need it, often survivable crashes are all you can hope for. So, why are you there?

 

Not to mention that it's not unusual to find big wires crossing valleys here in the East. If a wire is big enough to cross a valley that I'm comfortable flying up, my wire cutters are only going to scratch them a little.

Edited April 4, 2013 by Wally

[Gomer Pylot]

There is only one answer for this. A radar altimeter. Nothing else works reliably.

[avbug]

A radio altimeter isn't reliable over uneven terrain; it's not predictive, and it only states what's directly beneath the aircraft, perpendicular to the underside of the aircraft. While maintaining a constant altitude with rising and falling terrain beneath, the radio altitude varies, sometimes rapidly and significantly. Bank, and the reading changes.

 

The tool for determining AGL altitude varies with the circumstance. Radio altitude is one tool, but isn't reliable in many circumstances. Visual means is subjective to experience, lighting, and the means of viewing the terrain.

 

Yesterday during a local observed flight, I had a problem with my airspeed and static instrumentation. After landing and coming to a stop, my airspeed still indicated 80 knots, and my altimeter showed 200' high. Part of the flight involved a descent to 60-100 AGL and a release from the aircraft. As I descended I noted the altimeter showing 200' high, and continued the descent. I arrested the descent approaching what I determined to be 100', and descended to approximately 60', with the understanding that the altimeter was inaccurate. That was in daylight conditions. The altitudes and circumstances would have been experienced and perceived differently at night.

[aeroscout]

There is only one answer for this. A radar altimeter. Nothing else works reliably.

Well said, especially during overwater ops. I assume from your online name you have more than enough overwater experience to have a very credible reply.

[aeroscout]

There is only one answer for this. A radar altimeter. Nothing else works reliably.

 

 

 

 

 

 

 

 

edit for duplicate post.

Edited April 5, 2013 by aeroscout

[Gomer Pylot]

Certainly the RA isn't predictive, and only shows what is directly below the aircraft. But it can show a trend, as the valley slowly climbs up. Nothing will help in the case of a very steep wall except eyeballs. There is supposedly reliable TAWS coming, but I haven't seen one. We do have a semi-TAWS, in the Garmin 396, which has a terrain database and will warn you when you reach what it thinks is 500' AGL, as well as warn of obstacles such as towers, and the background turns red when you get too low, if you have the correct page showing. It's not perfect, but it works pretty well. Nothing I know of is 100% reliable at keeping the aircraft out of all obstacles other than flying well above them and keeping one's eyes open.

 

And yes, I have a few thousand hours of flying over water. The radalt works very well for that, and you don't often have to worry about rising terrain or mountains out there.

Edited April 5, 2013 by Gomer Pylot

[avbug]

There is supposedly reliable TAWS coming, but I haven't seen one. We do have a semi-TAWS, in the Garmin 396, which has a terrain database and will warn you when you reach what it thinks is 500' AGL, as well as warn of obstacles such as towers, and the background turns red when you get too low, if you have the correct page showing. It's not perfect, but it works pretty well. Nothing I know of is 100% reliable at keeping the aircraft out of all obstacles other than flying well above them and keeping one's eyes open.

 

A Garmin handheld isn't TAWS or EGPWS . I've used those extensively. The terrain features on the Garmin are a joke; I turn those off.

 

EGPWS works to some extent if one is planning on cruising above terrain (not down among it), but it's little to no help in close to terrain. It deals more with closure rates, particularly during an approach, which is where it has some value. It's greatest value is in faster airplanes, with higher closure rates on the terrain.

 

A radio altimeter has some value over flat terrain, and only when straight and level. Beyond that, not so much.

[Velocity173]

I don't think the OP is asking how to determine if you have the altitude to clear a ridge line, he simply wants to know how to judge his present altitude. Yes a rad alt will work. Obviously unless going over water it will be going up and down. Simple, just average it out. Without it then you need to compare your current MSL alt with a known terrain alt on a sectional or what ever type chart you're using. TAWS isn't going to give you an exact alt, it's just going to give colors and alerts.

[Spike]

What tricks do you guys use to know your height over terrain?

 

For example, you are flying up a canyon at 500' and the ground below you is rising. Radar altimeters excluded;)

 

In my opinion….

 

Flying by Visual Flight Rules (VFR) means just that, operating the machine by visual reference to the outside world. Over time, i.e. experience, pilots gain a keen sense of altitude and/or height. Some better than others and, the “better” ones usually fully understand what VFR means. With that, no matter the terrain, I constantly keep my eyes moving and purposely utilize my peripheral vision. I also constantly maneuver the machine to change the sight picture causing my brain to reevaluate the situation continually. I move my head as well. As a VFR pilot, I utilize the instruments to confirm what I should already know. If I need to rely on a particular instrument, then I’m probably not where I should be…..

 

 

 

[Velocity173]

 

In my opinion.

 

Flying by Visual Flight Rules (VFR) means just that, operating the machine by visual reference to the outside world. Over time, i.e. experience, pilots gain a keen sense of altitude and/or height. Some better than others and, the better ones usually fully understand what VFR means. With that, no matter the terrain, I constantly keep my eyes moving and purposely utilize my peripheral vision. I also constantly maneuver the machine to change the sight picture causing my brain to reevaluate the situation continually. I move my head as well. As a VFR pilot, I utilize the instruments to confirm what I should already know. If I need to rely on a particular instrument, then Im probably not where I should be..

 

 

 

That's one of the best statements on VFR flight. "I utilize instruments to confirm what I already should know." Too often people are concentrated inside when they should be using a good outside scan to stay oriented. In the Army, commanders try and force HUD down our throats. HUD has no application when flying at night during terrain flight. It's not going to keep me from hitting terrain or wires, all it's doing is telling me what I already know.

 

Now under low contrast (open water / desert), I'm going to put more emphasis on instruments than outside scan. That's a rare ocassion though.

[DS_HMMR]

 

 

Now under low contrast (open water / desert), I'm going to put more emphasis on instruments than outside scan. That's a rare ocassion though.

You just described 50% of my flight environment...............

[Velocity173]

You just described 50% of my flight environment...............

Lol! If you're an oil & gas guy then I would think instruments play an important role even while flying VFR. I know the FAA's interpritation of flying over open water at night with no moon is considered loggable instrument time. I'd log it.

[aeroscout]

Lol! If you're an oil & gas guy then I would think instruments play an important role even while flying VFR. I know the FAA's interpritation of flying over open water at night with no moon is considered loggable instrument time. I'd log it.

On clear nights with calm water the stars and the moon can reflect off the water and completely disguise the perception of where the horizon is. Pitch and or roll and or yaw attitudes that are not small and entered into slowly in those conditions can easily lead to total disorientation.

[Wally]

Previous postings mentioning rad alts piqued my curiosity:

Radalt emits signal in a "cone";

uses reflected signal from nearest reflective surface to measure altitude;

updates a couple of times a second;

some models have fairly substantial acceptable error;

should be tested before takeoff to determine proper functioning.

[Velocity173]

Previous postings mentioning rad alts piqued my curiosity:

Radalt emits signal in a "cone";

uses reflected signal from nearest reflective surface to measure altitude;

updates a couple of times a second;

some models have fairly substantial acceptable error;

should be tested before takeoff to determine proper functioning.

I suppose acceptable error depends on the model. In the 407 I haven't really played around with it at altitude to see how accurate it really is. I can say from experience that the one we had in the Black Hawk was dead on. I've shot many instrument approaches with it and the rad alt almost always matched the HAT when on glidpath (flat terrain). Some where taught that the signal goes through trees, hits the ground and comes back to the aircraft. Wrong. I would do a demo by flying over a field with one tree in it and it would pick up that single tree. Very accurate.

 

So depending on model I suppose accuracy could be off a few feet. Still, if you average out the variances in terrain and subtract some feet for tree height, a rad alt should give a decent picture of what 500 ft looks like.

[ridethisbike]

My guesses so far have been consistently 200-300' off. Now my rule of thumb is to subtract 200' from my guess. Over time I'm sure that will change...