Rotorcraft Flying Handbook - Normal / Steep Approach

[Bristol]

yes, but? Superman? yer 'spossed to fly the approach in a helicopter, not yer blue & red suit !

 

Hahahahaa That's funny.

[relyon]

Well, I don't know about all the math you are doing, but I went and flew the approaches as you havelisted. crossing at 176 feet, 60 knots slowing and descending to the 1000' markers

Ok, so what was your impression of the angle you were flying? Did it feel shallow, normal, or steep? I fly out of an airport that has a 4200' runway, so if I cross over one end at 741' AGL, the approach angle to the other end is 10 degrees. In my book that's pretty steep, but the RFH defines that as a normal approach.

 

re: the math. It's simple trig: multiply the tangent of the approach angle times the distance or speed to get altitude or rate of descent, respectively. As was pointed out, the speed is groundspeed, so airspeed only applies in calm conditions. For reference: tan(10 degrees) = .176 and tan(15 degrees) = .258.

 

So, if you're following a 10 degree approach angle and your spot is 2000 feet away, you'll be (.176)(2000) = 352 feet above the touchdown point. Any lower and the angle is less than 10 degrees; any higher and it's greater than 10 degrees. Similarly, if you're flying at 60 knots in calm conditions the rate of descent at that speed will be (.176)(60) = 10.56 knots = 1056 feet/minute.

 

Bob

[delorean]

Ah yes, the ole' green "Basic Helicopter Handbook"! Yep, still got mine......all the helicopter pictures and illustrations were Bell 47Js if memory serves. I think the "B" revision was in the 1970s because mine actually had a paragraph about rigid rotor systems--nothing about Fenestrons though.

[Superman]

OK, let me try this again, sorry but the computer took on a mind of its own for a while

 

The angle did seem to be on the steep side on the normal profile, not alot though, the steep approach profile seemed about right to me

 

 

What I did was zeroed the altimeter, entered the approach from 300' agl / 60 kts to the 1000' markers, and when I crossed the runway threshold on the normal profile, we were at 150' and 50 kts, 250 fpm descent. On the steep approach we crossed at 250' and 30kts, 250 fpm, flying into a 10 kt headwind. Not too far off from the numbers that you have calculated. I'm not sure that we need to rewrite the handbook.....I think the Mythbusters would bust this one

 

Anyway, this is a good assignment, next time your out, fly it and see for yourself, gives you a good idea of what a 10 or 15 degree angle really looks like. We took some pictures, but I'm having trouble getting them to upload, I'll keep working on it.

 

Fly Safe

Clark

 

Oh ya......sorry to let you down Pokey, but the suit and cape are at the cleaners this weekend

[HH60Pilot]

In the Air Force we called a steep approach an "apparent 45 degree angle", while in reality it was really a 15 degree angle. That said, we would also make a near vertical approach if that is what the situation called for. Who needs an H/V chart when you've got two big turbines!

 

Doug

[Superman]

OK, I finally got the pics resized and uploaded

 

steep profile crossing threshold

 

Normal profile crossing threshold

 

I know that my numbers aren't exact.....Just wanted to give you an idea of what the profile looks like out the window

 

Fly Safe

Clark

[justfly]

Hmmm... will you look at that! Intended spot of touchdown right at the bottom of the trim strings for the "normal", and about half way between that and the top of the panel for the "steep", JUST LIKE WE TEACH 'EM! Good work Superman, and thanks for the great reference shots!

 

Thank you too Gomer for illustrating the "90 degree angle" approach (extreme) example. I pondered that asymptotic analysis as well when considering how approach angle has NOTHING to do with ROD, unless you insist on maintaining a constant speed all the way down the approach! Which makes referencing the Rate of Descent tables in instrument approach chart booklets, and "stabilized approaches" (in the context of their application to airplane approaches) irrelevant and spurious! <_>

 

...but wdik, I apparently eat lead paint and "obviously don't understand" the math!

[joker]

OK,

 

I'm a little lost as to what those pictures are saying!

 

Are they saying that the numbers in the RFM are actually not too far off? Or are they saying what collective down thought, that the numbers should be much less?

 

They look pretty shallow to me.

 

Anyway, it looks like a nice airfield though...

 

Joker

[C of G]

I would concur with Joker, the "Steep Approach" looks to be on the shallow side of normal, but it could very well be the picture perspective. The math suggesting a max ROD of about 1100'/min seems reasonable to me as that would be the max, and it wouldn't even have time to register that high on the VSI (if you have one) because you would be decelerating and decreasing the ROD as well.

 

JustFly,

 

I would caution the use of windscreen references as the airspeed varied greatly in those two pics, and attitude would have a lot of leeway in interpretation. Using the apparent shape of the Aiming points would prove more objective. Like the circle on a helideck for example.

 

On another note, are you suggesting to not use the recommended ROD for precision approaches or stabilized descents? Maybe I'm reading your text wrong. But it is a lot easier to keep an ILS to minimums using just that technique and breaking out at 200' or even 3-400' on a non-precision at 70 - 90 knots is not that taxing. Considering that it makes it much easier to go missed if you need to and would minimize attitude changes and lessen the chances of getting disoriented. Just my 2 cents.

[Gomer Pylot]

The ROD tables on approach plates are accurate, for the (ground)speeds listed. You fly an instrument approach at a constant airspeed, not with a constant deceleration. You cross the fence at some speed, certainly above Vmin (minimim IFR airspeed). This varies with models, but is generally about 60 knots. A standard ILS is 3 degrees, whether flown in a fixed-wing or a helicopter.

 

What I generally do is fly an approach at about 80 knots, sometimes higher, depending on the conditions. You have to be at or below 90 knots to cut the visibility in half, so if I'm doing that I'm slower, but if I want to get on down and out of the way, I may fly the ILS at 120 knots. There is plenty of time to slow down once you get the runway in sight. The final landing approach profile from there depends on your speed and altitude. If the weather is really down, say to 1/4 mile vis, I tend to fly the approach at about 80 knots, and then start a deceleration on the glideslope at just above DA, so it's easier to get it on the runway when you see it at 100'. But the decel isn't essential, you can slow down as you go down the runway. Just don't balloon back up into the clag.

 

In short, the published RODs on approach plates are there for your reference, and it helps to use them, although you may need to interpolate to get the ROD for the speed you get, and this is groundspeed, not airspeed. Just because you fly the approach at 90 knot indicated doesn't mean you use the 90 knot ROD, because you may only be doing 50 across the ground. Bottom line, keep the needle in the donut, whatever ROD it takes, and you'll do ok.

[relyon]

What I did was zeroed the altimeter ...

What do you mean by this? The steep approach picture shows an altitude of about 1210' @ 37 KIAS, while the normal approach picture around 1130' @ 53 KIAS; both show a ROD of 200 feet per minute. What was your altimeter set to?

 

... and when I crossed the runway threshold on the normal profile, we were at 150' and 50 kts, 250 fpm descent. On the steep approach we crossed at 250' and 30kts, 250 fpm, flying into a 10 kt headwind. ...

If your numbers are correct, the angles work out to 8.5 degrees for the normal and 14.0 degrees for the steep.

 

What's the runway length? Multiple that by .176 (10 degree normal) and .268 (15 degree steep) to get the AGL altitudes to fly over one end so that the other end is at the referenced angles. The runway at the airport I fly from is 4201' long. Flying over one end at 1126' AGL puts the other end at 15 degrees. That's pretty steep in my book.

 

Bob

[joker]

What a coincidence!

 

While we are on the subject of Rate of Descent tables, I wonder if anyone could point me to an online version of some tables...similar to the NOS charts...you know, the ones with different angles and different speeds. Or could someone post a scanned version somewhere?

 

I was looking for some today to put in my kneeboard. Yes, I too think these are invaluable tools for helicopters as well as fixed wing. In fact RoD, based on my airspeed and glideslope / target altitude and distance to go, is the first thing I look to establish my descent on an instrument approach.

 

Also, while I'm here....the stabilised approach concept is perfectly valid for helicopter operations too. OK you are not maintaining a constant speed, but a helicopter doesn't need to maintain a constant speed, in order to be stabilised. I would agree that the definition of that word 'stabilised' may differ from aircraft type, but the concept is the same.

 

 

Joker

[Superman]

QUOTE(Superman @ Sep 10 2006, 19:06 )

What I did was zeroed the altimeter ...

 

What do you mean by this? The steep approach picture shows an altitude of about 1210' @ 37 KIAS, while the normal approach picture around 1130' @ 53 KIAS; both show a ROD of 200 feet per minute. What was your altimeter set to?

 

Set the altimeter to 1000' on the ground, 1210 - 1000 = 210' agl, I'm old skool, they only taught us to add, subtract, multiply and divide....I'll let you whiz kids do the fancy math.

 

Runway length = 4401'

 

All I was trying to do was give everyone something to go by other than a bunch of numbers and formulas, so far the reaction has been that I'm shallow, so therefore the angles in the book would be correct. I think that is what the start of this thread was all about.

 

Fly Safe

Clark

[Gomer Pylot]

Joker, I don't know of any such tables, although that certainly doesn't mean they don't exist. I'm not sure why you need them for different angles, because an ILS is always the same, except for a few oddballs, and they're published on the approach plate. For nonprecision approaches, there is no set angle, and I generally just set 500'/min, all else being equal. I don't care about the exact angle, and you usually have plenty of time to get down to MDA. I agree that a stabilized approach is ideal, but it's difficult to get an exact ROD that will apply to every approach, because it depends on the distance from the FAF to the MAP, and those vary.

 

As an aside, the S76C+ & C++ have an excellent flight management system (FMS) which will allow you to fly any approach that has vertical nav data just like an ILS, with the GS needle giving guidance, and you get a perfectly stabilized approach to the MAP. However, the FAA won't let us use it, because it's not in our Ops Specs. There is nothing in the FARs that specifies how you must get from the minimum altitude over the FAF to the MDA, but we still can't couple the thing to give the safest approach profile, just because of bureaucracy.

[joker]

GomerPylot,

 

Thanks for the reply.

 

You're right, there aren't tables for all different angles....but I thought I remember seeing tables for RoDs vs Airspeed though for a (limited) range of glideslopes. You know, the ones that Jeppessen print in the chart...but NOS print them in one big table at the back...I thought.

 

I would also like to have at hand tables which would give me a descent rate for a given drop in altitude over a given distance at various approach speeds. I know that these are not usually used, but they would be useful for non-precision approachs, just to ensure that you do make it to MDH. Without TERPS to keep things straight, some of our copter procedures here can be...shall we say non-standard!

 

I guess I'll just have to knock these up on an excel spreadsheet and print them out!!!!

 

As for the C+ FMS, over here, we don't even have VNAV approaches yet, so no VNAV coupled approaches for us!

 

Still, a fully 3-cue coupled ILS approach with DECEL punched in as well (for good measure) is very pleasing, especially when ceiling is not far above MDA. Did one of those just today! How beautiful to arrive right where you want to be at 70kts without having to touch thing! Its worth all the DAFCS failures, wouldn't you say?

 

Joker

[C of G]

Joker,

 

I'm sure you remember the approx ROD for a 3deg glide slope is: GS x 5 + 25

 

For example 90x5+25= 475'/min

 

I know that's not perfect, but gets as close as I can actually hand fly anyway. In the mantime, I'll have a look for the charts and post them if I can find 'em. Later........

[relyon]

Set the altimeter to 1000' on the ground, 1210 - 1000 = 210' agl, I'm old skool, they only taught us to add, subtract, multiply and divide....I'll let you whiz kids do the fancy math.

That's what I thought, but didn't want to assume. In that case, you were shallower than 10 & 15 degrees (7.4 and 11.9 degrees, respectively).

 

Runway length = 4401'

Based on that length, overfly one end at 776' AGL for normal or 1179' AGL for steep; the spot is the other end.

 

Bob

[Gomer Pylot]

Yep. Although I'm not sure what you mean by 'all the DAFCS failures'. We haven't had any, that I know of. But although everything mechanical or electronic can, and will, fail, that's no reason not to use them. Engines fail, but we continue to use them for powered flight.

[joker]

GomerPylot,

 

Are you saying that you don't get a 1EXX or 2EXX or the odd 'Trim Fail Caution Light On' or 'AP Caution Light On' error at least once a week, that requires resetting the APs?! Wow! How old are your aircraft?

 

Anyhow, it doesn't stop us using all the systems.

 

Joker

[Collective Down!]

What a coincidence!

 

While we are on the subject of Rate of Descent tables, I wonder if anyone could point me to an online version of some tables...similar to the NOS charts...you know, the ones with different angles and different speeds. Or could someone post a scanned version somewhere?

 

http://www.naco.faa.gov/d-tpp/frntmatter.pdf

 

Go to the last page... It's a PDF version of the legend information on NOS approach charts

[Gomer Pylot]

I haven't seen any of those lights, or needed to reset the APs. Our aircraft are pretty new, most of the C+ a/c are a year or so, and now we're getting brand new C++ models. One thing we do get now and then is an inverter failure, sometimes dual inverter failures. Those are fun.