nap of the earth, how low is low? [Archive]

NickLappos

19th Jan 2007, 02:06

Ouch! If the terrain has small and large features, like grass and brush, the job is fairly easy, but requires flawless attention to the task. A foolish and hazardous thing with passengers, IMHO. At 50 to 100 feet, it would be very impressive and much less hazardous. At 500 feet, it would be cake, and even allowed by law.

True NOE is tactical, and saves lives in combat, when bad guys want to shoot you. Otherwise, we don't call it NOE, we call it foolishness, and the guys who issue licenses call it illegal.

helipilot82

19th Jan 2007, 02:28

5' agl + high speed + single engine = dissaster. Even in a twin you would only fly like that if abosolutley necessary.

I am sure all of us have gone low from time to time for a thrill but not with passengers.

Perro Rojo

19th Jan 2007, 02:59

As Nick said, NOE is a military tactic. It is employed when the risks of being above 50 ft is greater than the risk of being very low.

SASless

19th Jan 2007, 03:20

Back in the time of Dinosaurs, Ol' Nick and I went through the US Army Aviation School where they told us we were helicopter pilots. At that time we all learned everything about the "Dead Man's Curve" commonly known to civilian pilots as the Height Velocity Curve or H/V to Bell drivers. We were well versed in flying "Low Level" or by means of a method called "Contour Flying". Both methods were designed to allow us to avoid the dreaded "Dead Man curve" even if it prevented us from being able to get to a safe forced landing area if we had an engine failure. (think single engine aircraft here).

While we were away on our Senior Trip....we were told to forget about Low Level and Contour flying and start flying at heights of 1500' or more and to avoid being repetitious and committing sins like following terrain features and roads and such.

After that period of unpleasantness between our employer and some Southeast Asian gentlemen, our employers took on some old business with folks who liked Bears. It seemed the Bear Boys had lots of tanks, mobile anti-aircraft guns and the like and seemed very willing and capable of using them.

Our employer realizing the old SEA method of flying at specified heights above the ground for administrative and seperation of aircraft types would not work well in the now rather hostile threat enviroment.

We then were told to fly "Nap of the Earth" known by the working class as NOE. There we learned to vary our airspeed, speed across the ground and height from obstacles and such as required to remain out of sight like our bosses did whenever there was work to be done. Amazingly enough....there was no more mention of the H/V curve or Dead Man's Curve lest one be flogged for Heresy and Back Sliding.

Thus....assuming what Ol' Nick and I were taught there is either no safe height to fly or no unsafe height to fly....but one thing is certain....bend your birdie and you will be wrong.

Whirlygig

19th Jan 2007, 09:37

What was the speed? I generally hover-taxi at 5 foot :}

Cheers

Whirls

Blackhawk9

19th Jan 2007, 10:07

In the good old days,(mid to late 80's) 5-10 ft on rad alt at 110 kn up rainbow beach in RAAF UH-1's, or 140kn in Blackhawks, popping over 4x4's and fishermen

Flying Lawyer

19th Jan 2007, 10:41

At 500 feet, it would be cake, and even allowed by law.

Otherwise ....................... the guys who issue licenses call it illegal.Nick

I assume you gave 500 feet as an example safe height, not as a legal minimum, and meant illegal under FAR 91.13 'Careless or reckless operation' which doesn't specify any minimum height but does cover endangering people in the aircraft.

Helicopters flying across the Alaskan tundra would (I assume) be legally allowed to fly lower than the 500 feet minimum height specified in FAR 91.119(c) by virtue of the exemption in 91.119(d) - provided they comply with the conditions of the exemption.

(We don't have that very sensible exemption in Europe.)

Tudor

NickLappos

19th Jan 2007, 12:37

Mr. Lawyer, you are correct, the US law is 500 feet away from things, not necessarily the ground.

Bravo73

19th Jan 2007, 13:22

the US law is 500 feet away from things, not necessarily the ground.

Huh? :confused: I thought that that was (effectively) the UK ruling...

My (very limited) understanding of the FAA ruling was:

Sec. 91.119 - Minimum safe altitudes: General.
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

BUT:

(d) Helicopters. Helicopters may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section if the operation is conducted without hazard to persons or property on the surface. In addition, each person operating a helicopter shall comply with any routes or altitudes specifically prescribed for helicopters by the Administrator.

So, in this Alaska/B206 scenario, he wasn't contravening any FARs but he was just a 'sneeze' away from disaster... :eek:

Apologies in advance for the egg-sucking lesson. :O

Flying Lawyer

19th Jan 2007, 14:13

So, in this Alaska/B206 scenario, he wasn't contravening any FARs but he was just a 'sneeze' away from disaster... :eek:
He wasn't contravening 91.119 but, as Nick said, the flight would be illegal if it endangered any person (in or out of the helicopter) in contravention of FAR 91.13 'Careless or reckless operation, endangering etc.'

SASless

19th Jan 2007, 14:33

Key point folks......without undue hazard to persons or property on the surface.

As to the other rule...."Careless and Reckless" ...usually winds up being whatever the FAA cares to call it but in court there must be some sort of violation. An example was the head lopping done by a 412 pilot hauling skydivers in Illinois a few years back.

He chopped off a photographer's head with the main rotor blades of a 412 while on takeoff over a corn field while hauling a full load of sky divers. He was found not to be in violation of any rule, regulation, or law.

For those of you that have never been to Alaska....there are places up there where one could fly all week and never see a person, structure, or vehicle except at your base of operations.

slowrotor

19th Jan 2007, 15:47

A few more details...
This was in '74, so memory is cloudy, I was working for an exploration company that had three jetrangers at a base camp in Umiat Alaska. My regular ship was down so my crew flew that day with this "other" pilot when the low flight incident occurred. Glad I never had to fly with him again!
We must have been going near cruise speed (100 to 120mph) would be my guess as we were enroute to the work site. But it may have been something more like 50 or 60, to answer Whirlygig's question.
Sasless is correct, nothing to hit for miles except maybe a bear and they were usually good about getting away (they hate helos), that's another story.

I wasn't thinking of an engine failure in the HV curve, I am well aware of that risk. But rather just how risky it is to fly low over open terrain. What would happen if the skids hit the ground in cruise? Would it rip the skids off?
I have seen airplanes touch wheels on water in cruise, no problem.

B Sousa

19th Jan 2007, 16:34

Nic hit the nail. NOE is for Military and anyting else places lives in danger.
Bet your flight was on a contract with some youngster wearing Ray Bans.
Probably out of FAI.
You cant see anything that low anyway so its nicer to fly higher and enjoy the scenery. Believe it or not a Bear will take a swat at you if you come that close.

nigelh

19th Jan 2007, 17:15

I am confused ,,,what is the relevence of the avoid curve with relation to NOE flying ? At 120 kn in a 206 you could flare up to 500ft to initiate your landing. I am not saying that particular flight was safe but there is no doubt that you CAN fly safely at 10ft and 100kt over flat ground safely, even in a single.

Bravo73

19th Jan 2007, 17:37

I am not saying that particular flight was safe but there is no doubt that you CAN fly safely at 10ft and 100kt over flat ground safely, even in a single.

nigelh,

I suggest that you have another look at your flight manual. (Specifically section 4-17 for the B206).

100kts @ 5ft puts you right in the middle of the 'high speed' section of the H/V curve. ie in the event of an engine failure, you won't have time to react before the aircraft has contacted the ground. :{

And that obviously doesn't take into account the 'sneeze factor'... :ugh:

nigelh

19th Jan 2007, 17:57

It also says that a 200 ft hovver puts you in the middle but in reality you can quite happily put it down safely from this altitude. Dont confuse the book with FACT. I am not saying it is advisable , just that it does not mean you crash. As for the low level stuff, i know of dozens of engine failures at around 10 ft and not one of them hit the ground as you say , the pilots had time to **** it up a hundred yards later after climbing !!:D

Bravo73

19th Jan 2007, 18:18

As for the low level stuff, i know of dozens of engine failures at around 10 ft and not one of them hit the ground as you say

Of course you do, nigel. And of course I'm going to take your 'word' over that of the Bell test pilots and the findings of certification authorities. :rolleyes:

Flying Lawyer

19th Jan 2007, 18:43

Bravo73

Is it possible there's a difference between what can actually be achieved with an aircraft by experienced pilots and the limitations imposed for safety reasons by certification authorities in conjunction with the manufacturer?

Surely the limitations are pitched at a 'safety level' which will be more conservative than what the test pilots actually achieved in testing?

Evalu8ter

19th Jan 2007, 19:07

Tried NoE over Lafonia in the Falklands with a pair of F3s chasing me. 200kts GS, noise to 10' on the Rad Alt (with the occaisional sheep setting it off). B******d if I could find any cover.....! Agree totally with the other far more erudite and experienced aviators on this site; true NoE is a balance between threat and flight safety. We stooged around at 50'agl at night during GWII right up to the point when the Cdrs accepted the RF SAM threat had gone away. That tipped the risk equation and we bumped up to 100' day / night for flight safety reasons. The ground has a PH of one and a PK of nearly one, honour the highest threat.

As for infringing Licences & civil laws, well I suppose rule 1 applies. Just don't endanger anybody else with your shenanigans!

NoE at trg weights in a powerful helo, ie CH47, isn't particularly dangerous in terms of pwr loss when you can hover OEI OGE! It is also important to stress that the majority of true NoE military flying is conducted at a fast hover taxy during advance to fire posns and Concealed Approaches & Departures (CADs).

Can't deny that it's fun though!

NickLappos

19th Jan 2007, 20:20

FL,
Good question, and the answer is No, there is no cushion.

The pilots who know of dozens of others who took failures inside the forbidden zone and walked away are not reporting the hundreds who died while doing so ( I am using the same skewed math, proportionately.)

If you are near max gross weight, and deep inside the HV curve (say at 200 feet when the top is 300) you will make an oil stain on the runway in nil wind. I have been there, made a living doing it, and there is no such margin, folks.

Regarding the high speed, low altitude zone that hugs the ground out above 50 knots:
At 5 feet and 100 knots, a surprise cut in a single engine helo will wrap those skids around your arse so fast you won't have time to know what happened. Those who think they are at 5 feet are actually at 15 feet, and they drop 5 feet in the first few fractions of a second. When they then zoom they assume another mighty boast, the stored energy in the decel from 100k to 50 knots done with zero energy loss is 300 feet, done by humans is about 200 to 250 feet.

Do they zoom from 120 knots to 500 feet in a Jet ranger? On steriods, maybe....

Bravo73

19th Jan 2007, 20:26

Is it possible there's a difference between what can actually be achieved with an aircraft by experienced pilots and the limitations imposed for safety reasons by certification authorities in conjunction with the manufacturer?

Of course there is, FL. I'm also aware of the '1 sec' delay criteria for certification.

I mentioned the 'high speed' section of the H/V curve because nigelh appeared to be ignorant of it's existence. (See the dual references in his post about the 'safety' of flying at 100kts at 10ft).

He asked "what is the relevence(sic) of the avoid curve with relation to NOE flying ?" I just showed him the relevance (which he then refuted in his subsequent post, BTW!)

FWIW, when I'm down that low and fast, I'm much happier at 20ft where I know that if I do have an engine failure then the aircraft has been demonstrated to be survivable. However, if I'm down at 10ft, I have significantly raised the level of risk. Can this lower level then be considered 'safe'? Is it safe to be 'IN' the H/V curve when it can be so easily avoided by flying a few feet higher?

Surely the limitations are pitched at a 'safety level' which will be more conservative than what the test pilots actually achieved in testing?

I would hope so. Otherwise surely we would all need the skills of a TP in order to be 'protected' by H/V curve?

Edited to add: Just cross-posted with Mr Lappos. I guess that there isn't quite so much 'protection' for us 'average Joe' pilots as I'd first hoped...

NickLappos

19th Jan 2007, 20:50

Bravo73,
I can't disagree that we take a 1 to 1.3 second delay in lowering the collective pitch, but frankly, the all the sticks are being handled by a test pilot who can't help but do his/her best, so I don't think there is any real cushion at all when a typical pilot does the maneuver, by surprise, for real. On some HV cuts that make the edge of the curve, I have hit the collective stops just at touchdown, 80 to 90% of the Do Not Exceed stress levels on the landing gear, and 75% rotor speed, simultaneously. That is clearly the definition of "nothing left in the machine."

Bravo73

19th Jan 2007, 20:54

:eek:

Thanks for the clarification, Nick. All the more reason for me to respect the H/V curve in future... :ok:

Flying Lawyer

19th Jan 2007, 20:55

Thanks Nick. I've learnt something new today.

Agreed B73. :ok:

FL

eagle 86

19th Jan 2007, 23:26

My experience of military nap of earth flying is that it isn't 110kias/10 feet - that is low flying. Military nap of earth is low/slow/terrain hugging/concealment/masking/unmasking scout type work.
GAGS
E86

Brian Abraham

20th Jan 2007, 02:31

just how risky it is to fly low over open terrain. What would happen if the skids hit the ground in cruise? Would it rip the skids off?

Risky and riskier.

Saw a Huey making a landing, well attempted landing as it turned out, as the ground was a little closer than he thought and tore the skids out. Kept it in the air, returned home where upon the mechs fitted a new set while he remained in the hover.

Another case was a 206 who dinged the ground while low flying. Pilot had helmet but chin strap not done up (doesn't look cool). The glancing blow with mother earth caused the helmet to rotate forward to cover the pilots eyes, where upon the second impact was not glancing but terminal to the airframe. Pilot survived to relate what happened. Had his vision not been obscured he could have had the same outcome as in the first tale.

topendtorque

20th Jan 2007, 13:22

Risky and riskier.
Saw a Huey making a landing, well attempted landing as it turned out, as the ground was a little closer than he thought and tore the skids out. Kept it in the air, returned home where upon the mechs fitted a new set while he remained in the hover.

and perhaps slightly off shore in one of those mornings where the fog (morning glory) rolls in up in the gulf of carpentaria, i believe it was an event repeated there.

We've talked about autos in this environment before, and I remember some criticism on the technique of a quick flick up of the collective first, then down, then into the usual quick stop to termination. Really if you are trained for it it is a piece of cake as long as you realize that the collective is your primary control, hang onto it at all times. this bit about falling five feet in those circumstances is horse manure.

That is of course, only IF you are in this environment with the training, a licence and reason for it.

for others, the bit about five feet is still horse manure, more like ten feet and panic.

slowrotor

20th Jan 2007, 16:13

Brian,
Your story of a 206 that hit the ground and bounced answers my question.
I am doing an extensive study of low flight and what type of aircraft would be best suited for low flight. (helo, stol plane, ultralight, airship, wing in ground effect)

The helo seems like the obvious choice, but considering the problems with the HV avoid zone and other things, the choice is not clear.

thank you
slowrotor

Graviman

20th Jan 2007, 17:08

Slowrotor, again don't want to say too much because i don't wish to compromise Bug's chances. This strikes me as another area of benefit for auto collective. Get the system right and the lever and stick (in that order) have already done what is required before the pilot even realises low Nr - ie no reaction time delay. For this flight regime you may be talking FBW, because the simple reactive control may now need to be a short program.

My car has ABS, and on the one unseen ice occasion i needed it i was glad it was there...

Mart

SASless

20th Jan 2007, 17:58

Any red blooded Helicopter pilot knows the best choice for flying low is a Shelby Cobra (Red with white racing stripes) with about a thousand shaft horsepower in its supercharged engine! Fuel the guidance system up with high octane whisky and watch out!;)

i4iq

20th Jan 2007, 18:05

nigelh

20th Jan 2007, 19:08

Nick L Thanks for your input and i bow to your experience. I was not , for once, trying to be contentious and i accept that there should be no cushion otherwise people will obviously push it !! I do however stand by my statements which are hard fact
1) Many pilots , some working with me in the past, have had engine failures at low level crop spraying . The ones that i have spoken to said that the failure was just like a large airpocket or downdraft which you naturally correct with collective within a split second, and i do repeat ..believe it or not they all managed to not hit the ground....explain this please.
2) There are many ex mil instructors out there regularly demoing EOL,s in the 206 from all different hovver altitudes and in the last checkride that i did with a very experienced one he got me to do one from 200ft but not i agree at MAU.!
For the benefit of doubt i am not advocating disregarding all the work by people like Nick and agree it is safer to remain as much as possible outside these avoid curves , but as we all know there are times when needs must.

B Sousa

20th Jan 2007, 20:31

Sasless, maybe you havent been here. www.superformance.com

Phil77

20th Jan 2007, 23:38

nigel: my reply is not an attempt to get into a piss contest with the big boys (I would pee on my shoes :ooh: ) but...

possible explanation: my understanding is most H/V curves are beeing developed at max gross weight and zero wind (+ a flat surface) - was that the case when your friends had their incidents when flying crops?

I'm not trying to be smart ass, really!

Colonal Mustard

21st Jan 2007, 07:49

In order to post your first thread you ask "and was it extremely stupid?"
,that implies that you must have thought it was stupid at the time and therefore you had doubts at the time it was done, i was always taught "if there`s any doubt, theres no doubt", you have imho answered your own question, rather foolish if you didnt mention it to the pilot if you felt uncomfortable, :rolleyes:

slowrotor

21st Jan 2007, 16:35

Colonal,
Yes, you are correct but.... I was 18 and on my first job away from home with a group of ex-vietnam pilots.... with almost none of the knowledge about flying I have now.....not very likely that I would question a pilots judgement.

If the experience in Vietnam had any effect on this pilots behavior, I do not hold that against him. I appreciate the jobs you guys did in the war.
But he clearly should not have done that low flight with passengers.

Actually, the job involved flight from one station to another about 1000ft apart, so I suppose there was risk in the normal daily flights. Can't remember how high they flew between stations. I think it was usually about 50 feet high.

Colonal Mustard

21st Jan 2007, 21:08

The post wasnt intended to suggest YOU were foolish btw, i suppose my first flight next to a pilot "i assumed" they knew what they were doing and i dare`nt complain, it was only after a period of time did i have reservations about small areas of low level flying, i only "felt unhappy" with one pilot who went low level yet by that time i had enough confidence to mention something "as a joke" which stirred his grey matter into gaining height....

i got the desired effect...