Multi Engine Height-Velocity Avoid Curve
 

Section 5 (Performance Data) of a typical (lets use the EC 135) RFM gives data on the Critical Height-Velocity envelope. It allows calculation of an 'avoid' area which is critical for helicopter operation in the event of a single engine failure during take-off, landing or other operations near the ground.

The area is a combination of height (agl) and IAS as a function of gross mass, PA and OAT.

As an ex Single Engine QHI of some experience I can cope with this, as well as getting my 'bloggs' to grasp it too, in the knowledge that if the engine fails within the avoid curve the likelyhood is that I/he/she will crash due to a lack of height (time) or speed to do something with an autorotation.

Can someone advise the applicability of such an avoid curve in a ME Helicopter that is operating to CAT A - Class 1 performance standards?

Am I close is suggesting that this data is provided for either:

Non-Class A operations - such as the military where they load and lift so to speak where losing one engine in this regime could hurt, despite the other one doing the business as best it can, (not a convincing argument but you never know!!)

or

Is to advise a pilot who ends up OEI that he should avoid or at least minimise exposure time on landing (methinks) as he reverts back to the understandable use of the Avoid Curve for SE helicopters.


212man (from Flight Testing forum) confirms that this data has to be given in the RFM under regulatory requirements - but any thoughts/advise/guidance etc.??

tbc,

The avoid area is provided to guide you as to expected response. It is somewhat independant of Cat A procedures, which also separately test response, but use different entry conditions, weights and delay times, so they are not directly comparible. Some elaborations:

Cat A uses different weights, so response can be better than the avoid area's chose weight (almost always max gross).

Cat A has dyanmic conditions during acceleration especially. Avoid areas are set in steady flight.

Once OEI in a Cat A machine, the avoid curve isof no use at all, it will not predict how much trouble you will be in if the remaining engine quits.

Cat A uses lesser delay times, generally set to .3 seconds. Avoid area uses 1.3 second above the knee. .3 below.

I have been of the opinion for a long time that the avoid area HV curve is of very limited use for a twin turbine, in practical application, and with ever more reliable engines, is not of primary importance. I have now fastened my seat belt for the fairly typical pprune onslaught where engine failure is the Cause of All Evil, and with hard Cat A, six engined helos, our accident propensity will drop to nil! ;)

Nick,
a more reasoned response might be: "why don't more manufacturers provide sets of graphs so that the HV curve can be derived for any given set of weight, altitude and temperature, such as that found in the Bell 212 FLM?"

The graphs most FLMs have are simply there for the regulations and not directly relatable to day to day conditions.

Interestingly, according to a conversation I once had with an ECF test pilot (now head of Flight Test), there was a move to try and miss out the testing altogether on large expensive multi engine types, as it invariably resulted in a 'crash' at some point in the proceeding with lots of bills and delays.

212man,

An even more reasoned approach might be to actually test and approve those elements that can eliminate crashes. Look up the stats and tell us the last time a twin had an engine failure where the HV curve (or full Cat A) would have made any difference. You will not find a useful HV curve, because HV OEI failures produce NIL accidents.

I sound like a broken record, but here goes! We think, practive, study and learn engine failure from our first piloting breath. It is so ingrained into us that we believe engine failure tolerance, procedures and performance are synonymous with safety. This pilot-cultural conditioning is like the lemmings who rush from some danger and end up drowning.

We have accidents in twins because we do not have the right warning kit (EGPWS, TCAS) because we do not have the right instrument equipment (SAS, GPS) because we do not have the right helicopter instrument routes let-downs (Heliport precision approaches) and becuase we try to fix what feels good (engine failure response) rather than what kills us.

We get a F in self improvement, and it gets worse every time we again debate engine failures, and not accident causes.

Nick,
I don't dispute that statistcally engine failures are not a big proportion of accident causes (the NASA report into all rotorcraft accidents in the US from 1964 to 1997 puts it at about 10% for twins, I recall) and I applaud the introduction of devices such as EGPWS etc.

(I think the USCG may have a different take on that one though!)

It still doesn't make you feel any more comfortable landing or taking off from a swamp barge in the middle of a jungle clearing, knowing that if an engine does fail there is very little your training will do to prevent an accident, and a bad one at that.

Of course, the irony is that in 5-10 years time when we are all whizzing around in a/c with EGPWS, de-icing, TCAS2 and coupled approaches to the ground, the engine failure accidents will become a large proportion of the total! (assuming no change in failure rate, which is likely as people are saying they don't fail so why change anything?)

We can then re-debate the relative merits of Cat A then!

212man,
The 10% is inaccurate, to say the least. The data I have seen from OGP says that the accident rate for Cat A twins (cat A enroute) is absolute zero due to engine failure.

the 'Irony" that we would solve all the crew error and maintenance error accidents, then the other mechanical failures, and the mid air collisions, and then we would be behind in fixing the engine failures is really stretching! In the approximately 2 million hours logged by those in the OGP list, with about 1/2 million hours in twins, zero is a number that doesn't need fixing, let alone worrying about how we wasted our time driving down the 80% of all accidents when we built a full IFR precision approach network with routes, equipped all helos with TCAS and EGPWS, created full error-free maintenance systems and then said "Gosh! We screwed up, now the engines will start failing!"

I don't think we really know how much energy we spend trying to fix the unbroken, while the real accidents go on, labeled 'crew error" and thus no real problem.

If I might submit a gentle input to this debate. My organisation has SPIFR EMS 412's (definitely non Cat A!), and has a strong Check & Training program during which we practice all forms of engine failure every 6 months. We also spend thousands of dollars (Oz) on sending our pilots to a 412 sim to practice even more OEI emergencies. We have been doing this for many years. I hear what you say regarding record of OEI in twins (I personally know of 5 in the last 10 years) however the other week a pilot had an engine chip in which the company procedure was to shut down the engine. No engine failure, but OEI non the less. He was very glad of all the OEI practice landings and approaches he had done over the years. I suggest there are many reasons why one might end up OEI other than outright engine failure.
We also carry out Cat A profile rearwards takeoffs when the power is available, as this is the safest flight profile from a restricted area or rooftop pad.
We use the Cat A HV as a guide. As we all know, HV's have been around a long long time. even the Sycamore had them. Their concept hasn't changed. Low IAS at some heights agl is not conducive to a safe landing follwing an engine failure, multi or single doesn't matter. The only difference is the multi HV area is much smaller due to some power being available to either fly away or to cushion the landing.
I look forward to the iminent generation of truly Cat A helicopters (AB139 at el) that can hover OEI at MAUW at 25 deg C. And like computers and NVG's, we will wonder how we ever flew without them.

(Source: NASA report into civil US rotorcraft accidents)

7.4.1 Loss of Engine Power (39 Accidents)

As table 30 shows, introduction of twin-turbine helicopters to the civil fleet dramatically reduced the percent of loss of engine power accidents from 31% to 13%. However, table 32 suggests that a very disturbing trend began when larger helicopters capable of carrying more people were introduced: any serious accident affects more people and likely receives greater attention by the public. This trend exactly parallels the situation faced by the fixed-wing industry as they moved from the 1920s Ford Tri-motor to modern day, large jet airliners, such as the Boeing 747.

Sorry, I was wrong it wasn't 10%

As Katfish says, you don't have to have an engine failure to end up on one engine: engine chip, false fire warning, loss of oil pressure, are all events I've seen in the recent past.

Cat A
 

Don't confuse the HV diagram with Cat A certifications. Cat A means a multi engine rotorcraft designed with engine and system isolation features specified in Part 29 and utilizing scheduled takeoff and landing operations under a critical engine feature consept which assures adaquate designated surface area "land back" and adaquate performance capability for continued safe flight in the event of an engine failure.

HV assigns an area "shaded" where an average pilot with adaquate warning will be not able to make a safe touchdown once an engine fails.

212Man:

It is generally a good idea to read the report, since you then know how to support your arguments from its information. If you had read it, you would see that, indeed, you were simply wrong. Here is what the report actually says:

1) The report says that 13% of all twin engine helo accidents occur from "loss of power" but you failed to tell us that about 2/3 of these "power" losses involved both engines failing due to fuel starvation and the like. Exactly how does this support your argument for Cat A standards?

2) The actual losses where we can account for an engine failure in this data is 16 out of 302 accidents, for 5% of all accidents involve twins where an engine fails. In other words, you wish to solve the 5% problem. The report you quote states that the number of engine failure accidents is "relatively few" and "not statistically meaningful" in twins.

3) the recommendations from the 35 year study state the following areas of concern for twins (and no mention of engine failure improvements is made):

A) Solve loss of control problems, especially in bad weather.
B) Employ an alert system to warn pilots of approaching limits
C) Improve crew selection and training and insure that operational info is available to the pilot
D)Improve criteria for time change components, based on sound materials science
E) Require HUMS

Does this list look like the one I gave above, albeit with somewhat different wording?

Here is the report:
http://safecopter.arc.nasa.gov/Pages...P%20209597.pdf

tbc:
If you fly i.a.w. CAT 1/Gp A in the EC135 atleast, then you will not enter the avoid curve (Fig 5-9 FLM) under any take off or landing circumstances. In fact it confirms this at: FLM: 9.1-1 - 37

That's one of the reasons why they invented this profile.
The 'Deadman's Curve' is of no relevance to normal Cat 1 ops. Even (as you state) under OEI.

But I would agree that for non CAT 1 ops then you would have to consider the curve.

Response to Thomas Coupling
 

You are correct only if your takeoff distance including a land back area meet the requirements outlined in the operators manual.

Nick,
I wasn't doubting the rest of the report: I was simply backing up my original quote of "about 10%" which you appeared to dispute.

Surely, though, the very fact that these figures are so low is a direct argument for twins in the first place? And it is the fact that engines have improved in their reliability and excess power that has improved things further. These figures are for accidents, by what factor can we multiply these figures to get the number of safe outcomes from failures? Quite a large number I suspect. However. It is no use having a twin and operating at a weight in excess of what the single engine can handle, as you just end up with longer before you hit the ground.

Ending up single engined is not as uncommon as you seem to imply, I think.

I flew Super Pumas for just over 4 years and in that time there were two engine seizures on the fleet (caused by tie bolt failure and subsequent ingestion of the bits that dropped off). One on the approach to a ship, the other in the cruise. Another lost its engine oil and had an inflight shutdown. Safe outcomes for the crews involved.

When I was doing my 212 conversion I had a P3 tube union fracture which meant loss of governor air and the engine ran down to idle. No problem as we were at a 'training' weight. Not much fun in the GoM having come off a rig at 11,200lb using the 100% Tq technique we have read about elsewhere, though!

More recently, I've escorted a 76 back to shore that had an engine failure on T/O from a rig. Actually, a loss of N2 signal to the overspeed protection box, so it shutdown as the N1 went up. Sensible weight, so no problem.

I can think of half a dozen 212s flying back single engine following chip warnings. No problem, all at Cat A weights.

Spurious fire warnings in two types in the cruise, safe landings single engine at sensible weights.

Numerous chip warnings in the 155 requiring the engine to be placed in idle, but never a problem as never at MGW.

Not one of these was an accident. In each case, had the aircraft been at the max Cat B weight the outcome would most likely have been different. Some would have ended up in the water or forced landing onshore. Others would have had engines with chips running at normal power, or remaining engines running above continuous OEI ratings ("hey Bill, we're going down at OEI continuous, what do you think; stick it in the jungle canopy, or pull 2 minute rating for another 15 minutes, or relight the engine we just shut down with a fire warning that we now know was false?")

So, I don't doubt the statistics that say accidents from engine failures are rare and not the largest cause. All I am saying is that based on personal experience (and a relatively short one at that compared to many) I can say that failures are not as rare as one might think, and there are plenty of other things that can go wrong which leave you single engined. The fact that they do not result in accidents is largely due to being at a weight appropriate to the conditions i.e. a PC1 or 2 weight.

I'd also like a TCAS and EGPWS too, by the way!

212man,

The base issue is where to place the next dollar spent on safety. If one starts with the premise that a Cat A enroute twin is the right baseline, then do we agree that the next safety dollar, and the next pprune wish list entry should not be about engines?

Well, that depends! If you take the premise that existing a/c can be operated with commercially viable payloads whilst remaining within the PC1 or PC2 constraints, then by all means. If you take the view that the only way to operate is at max Cat B weight, and that (for an example) it's perfectly acceptable to take off from rigs by waffling around IGE at 100% Tq waiting for a breath of wind to allow you to 'throw yourself' off the deck, then I'd disagree.

As I said above, ignoring any risk of failure at rotation, there is a still every chance things may go 'pear shaped' very rapidly with a relatively minor problem en-route, and for what? So one extra 300 lb driller could take the trip rather than the next one? (believe me they come in all shapes and sizes, though that would not be your 95th percentile I admit!)

So, spend the dollars on engines if you don't like dropping payload at 20 C (because you want to operate PC1/2) and know you could add the 2S2/2C2 and keep your loads till 29 C (ball park figures as an example). But don't keep the same engines and say "well, I don't want to drop load I'll just stay at max gross anyway and end up PC3."

Then concentrate on the other goodies, matched with quality simulator training with scenarios designed to reflect the high risk areas like CFIT. Quite possibly we agree but are wording the points in slightly different ways. Possibly not!

The MD900 does show a Height Velocity Diagram in Section 5 - 9. and is pertinent at weights above 6000lbs Gross Weight. It prescribes an area which starts at 35 feet agl and extends upwards to 75 feet agl and the tip of the 'nose' is at 17 knots IAS.

Therefore MD has evaluated the flight profile and discovered that a safe landing would not be guaranteed in these situations. So no hovering or slow speed ops in this area.

Below this weight (6000 lbs) there is no height velocity diagram.

What happens if the helicopter is being flown backwards? Or sidewards?

Or at 5,000 feet; or with OAT at 25 degrees above standard; or with an engine 5% more powerful than specification; or when the avoid area is entered in a 500 foot per minute rate of descent; or when you are 20 knots faster than the curve but in a 1000 ft/min climb; or.... I guess I could go on, but I hope you get the point.

The HV curve is one of those charts that looks great, and tells only one story, when you come down to brass tacks.

Geez Nick. I find these arguments rather weak. There are many ways to diagram an HV curve but most I have seen are corrected to density altitude and MGW. Are you saying a more powerful than specification engine would result in poorer HV or single engine performance? Am I reading this the wrong way around? Generally entering autorotation from descent equates to reduced pitch which means less rate of rotor decay and/or a more rapid transition to the autorotative rotor state - usually not a negative following engine failure. The last example has so many variables it is hardly worth comment. Certainly the use of better examples would add more weight to your argument.

But the two can't be separated. Cat-A certification requires that the scheduled procedures have sufficient margin to avoid the H-V.

And for Part-29, the H-V is a limitation, while for Part-27 it is considered "only" performance information. So a Part-29 aircraft operating under Cat-A has no business being in the H-V, nor should the procedures take you there...

What??? I might agree if you said, "Once OEI while operating under PC1 in a Cat-A machine..." If you were operating at Cat-B weights within the H-V, a Cat-A design won't be sufficient to keep you from smacking the ground once OEI. With no Cat-A ops rules in the US, I fear that too many are confusing "Cat-A design" with "Cat-A operations" (like mixing the terms "IFR" and "IMC").

Hullaballo:
Category A supplement will say that the HV curve is no longer valid (all that I know of say this).
HV curve is a limitation only with more than 9 passenger seats - very long story about this - see my book 'Cyclic and Collective' for more details.