Low Fuel

[paco]

Trying to get up a discussion subject for a CRM course - if you were having to nurse a helicopter back on low fuel, are you better off reducing power and beeping the rotor RPM down to the bottom of the green range or pulling as much power as you can, on the basis that the (turbine) engine's best power is in the high range anyway and that you get a higher percentage of groundspeed against airspeed?

Answers on a postcard please..... :)

Phil

[oldbeefer]

Helicopter? Low fuel? Why not land??

[helopat]

Trying to get up a discussion subject for a CRM course - if you were having to nurse a helicopter back on low fuel, are you better off reducing power and beeping the rotor RPM down to the bottom of the green range or pulling as much power as you can, on the basis that the (turbine) engine's best power is in the high range anyway and that you get a higher percentage of groundspeed against airspeed?
Answers on a postcard please..... :)
Phil

Gidday Phil,

Been there, done that, so here's my 2 cents worth. I think our pal OLDBEEFER has the right idea, but in my case (halfway between Kalgoorlie and Warburton) or ANY case in the 90% of Australia which is going to KILL you if you land there, I think other options are in order.

In my case, winds were exactly opposite those forecast...got to the PNR for Kal and had enough fuel to continue (just) with reserves intact...almost immediately the winds got worse...calculated a last point of diversion...got there, all good, crack on! AGAIN the winds get worse. If you've never been in that part of the country then let me tell you, landing is an option of last resort. We eased the aircraft up to max range speed (way back when fuel first started looking iffy) but that wasn't enough. Eased the altitude up looking for better winds...a little better. My copilots fingers were flying on the whiz wheel...with the slightly better winds our fuel looked to be right on the margin...perhaps breaking the reserve fuel, but still getting there with SOME fuel. Long story short, we ended up getting a little help with better winds on the last hour of the leg and got in with enough fuel to be comfortable with...however, a long and creepy leg all the same.

Now, to your question. In the Seahawk there is only one normal variable (besides turning back) and that is speed...for us, using max range speed. We discussed singling up, but the significant reduction of speed whilst operating single engine was prohibitive so that idea was scrapped. We CAN adjust Nr up or down (up to 101% or down to 96%)...the rotor system of the Seahawk operates more efficiently at 96% but its an option that we aren't really all that well versed at so I wasn't really keen to experiment in the real case...I suspect that if we had beeped Nr down to 96 we would've gained a slight advantage.

We did a few other things that might have just got us over the line (might not matter depending on the type you are flying at the time...for example, we made sure our APU was shutdown (burns 150lb/hr)...we also shut down the ECS (environmental control system...air conditioner)...this uses bleed air and we wanted ALL the power of those engines for flying. I'd suggest that if you can afford for engine anti ice, or any other bleed air option to be off, thats the place for it to be in the 'tight fuel' scenario.

In the case where you're going to a ship (as in the Navy option) then the sooner you let 'mother' know you're in dire straits the sooner she can start making way toward you.

I'll just add that the maintainers we had riding in the back were blissfully unaware that we were sweating our green suited asses off in front...the only point here is that those guys put complete faith in us when they hop in...I guess we need to consider CAREFULLY why we're going and if there is any urgency to get there...in my case, we did all the proper flight planning, had plenty of fuel to go and the scenario just kept deteriorating and deteriorating...it seems that we kept plowing on and on, but at every 'checkpiont' along the way we had our required fuel plus reserves intact...it was only when we were PAST those critical points that things further degenerated...the point, I guess, is that despite all the best planning there are factors that ARE beyond our control.

Anyway, enough of my lecturizing. Hope this was helpful in some way...even if just to give you a feeling of moral superiority over my dumass self.
Bottom line is that you MUST have a story to tell to generate discussion at these CRM courses...use mine if it is any help but if you've got one of your own (or if one of the students have one even better) get that discussion going and see what kind of CRM points you and the class can generate from the discussion.

Happy to discuss further.

HP

[spinwing]

G'day Paco,

Kinda depends on the aircraft your considering ....

Offshore ...... 212 ??? I guess depends on the aircraft configuration but work out the specific consumption .... prob no point winding one of the donks down as things can possibly get worse ... beep Nr down to 97 or so max Tq for cruise and cross fingers ....

Believe RAF used ferry the Pumas with one donk at idle to get a significantly better specific consumption and have done the same myself so I know that works!

Onshore ...well I guess you land when the pucker factor gets excessive!

I remember (when on the way to the Antarctic a long time ago) seeing an RAAF Orion appear out of the stratus trying madly to get the 2 shutdown engines online ... so I guess they have an SOP to shut down donks to increase sfc while on patrol.

Cheers

[Nigel Osborn]

I would have thought the answer was simple. If you want to go the max range, you fly at max range conditions. If you want to stay airborne as long as possible, you fly at max endurance conditions. These settings should be in the RFM. Finally of course you land & wait for someone to bring you fuel.
Or have I misunderstood the question?:confused:

[the coyote]

If you're trying to squeeze every last drop by taking one engine out of the equation, then having that engine at idle is pointless, burning fuel for no torque, so shut it off completely.

Best range configuration from the RFM is the best place to start, but if you have groundspeed and fuel flow data available, then work the actual numbers. Quantity of fuel burnt per mile, whatever altitude/power/wind/Nr/OEI combination gives the least has to give the most range.

[paco]

Yes, of course, just landing is an option, but sh*t happens sometimes (thanks helopat - your example is just what I was thinking about!) If you were over tundra or water and wanted to get as near as possible to a place where you could get fuel from, or be recovered, how would you nurse the old girl along?

One guy here who has been in such a position said that he beeped the rotor RPM right down and reduced power. Another said that the engine is producing x amount of power anyway and it's only a little extra to pull max. Which is right? I think the former is more to do with endurance, but what does the panel think?

Phil

[CareBear]

As far as I am aware you need to spend as little time as possible in the air for your given fuel. This means you need to get to where you are going as quickly as possible, therefore you want to be flying at the max speed possible for the min fuel burn. I believe that for most helicopters this is usually Vmax or very close to it. Some additional benefits can be gained in some helos by beeping down the Nr.

[paco]

Yes, that's my thinking also.

Phil

[tacr2man]

Just as an aside when I was flying an R22 at Jandakot if they declared adverse weather( I think it was) you could not carry enough fuel to actually go anywhere due reserves required (Ozzie res req a bit excessive)
they used to close the airport if it was more than a shower :}

[Shawn Coyle]

Max range airspeed is only valid in a no-wind situation. If you're going into a headwind, you want to go faster (you'll spend less time being affected by the headwind).
As far as beeping the rotor - power equals torque times RPM (times some constant, which is of no concern for this discussion). Fuel flow relates to power, and not torque.
If the rotor is more efficient at a lower RPM in the cruise, then you will be using slightly less overall power. Not sure that you could measure that in most helicopters with the accuracy of the torque and rotor RPM gauges - best way to tell is by N1 and TOT changes in my experience.
Altitude will make a big difference if the winds are the same - in the 212 (UH-1N) I used to fly the fuel flow went from 600pph at sea level to 500pph at 10,000' - that's a 20% decrease. There's a good reason airliners fly very high!
Look at sideslip as well if you can - zero sideslip should improve range over the 3-4 degrees inherent in most single rotor helicopters.

[Devil 49]

I have to second Nigel Osborn's suggestion, know what your RFM tells you. Adapt the data in the RFM to the variables present, especially wind. If you're flying into a wind, increase power from max range towards max power, and if you're downwind, decrease towards max endurance.
Personal rule of thumb, based on single engine, which usually have minimal RFM info, and single pilot, when I've had no free hands to calculate, experience: More altitude is a good thing, up to a limit, specifically, density altitude. Climbing to the DA that's the point at which one has to limit load gets the best range, more or less. If the climb to that DA decreases ground speed, then lower is better. GPS minimizes pilot stress so much!
When I've been minimal fuel on a cross country leg, which is always a headwind situation, it seemed getting the pig up to the DA at which I have to decrease load, and getting there as fast as possible worked best. If the wind was considerable- which again, always seemed to be true- then pulling max continuous power got me there with the most fuel remaining- at least accoding to available gaugeing, which in those aircraft is poor.
I don't use that RoT in multis that have real single-engine capability, but it's a good approximation in a old light twins, like the AS 355F1.

[996]

Trying to get up a discussion subject for a CRM course - if you were having to nurse a helicopter back on low fuel, are you better off
Phil

PPPPPP......."you are beter off"........going back to basics. You should never be in that sittuation in the first place. However, as an academic exercise using nil wind, work out the fuel flow per minute at max burn acheivable at vne for a given distance and do it per minute using the reduced weight as a consequence of fuel burn. Calculate time/track/fuel flow/ for a climb to a given altitude selected so that you can acheive a 200' per minute at vne with a reduced power setting to arrive at your destination at a safe height from which to auto.

You might discover that the total theoretical increase in range/fuel saving based on a start with max capacity is so inconsequential that its not worth the effort.

[bb in ca]

If you have a fuel flow gauge then a useful method I was shown is to divide Fuel Flow (pounds per hour) by Groundspeed (nautical miles per hour) thus giving you Pounds per Nautical Mile. The lower number you can achieve the better.

With so many variables with your aircraft configuration and the atmosphere you should do a little experimenting to find what works best under the current conditions.

Rotor RPM, collective setting, trim, altitude, shutting down a donk or bringing it to idle are some of the options worth considering.

If you don't have a fuel flow gauge then it's probably best to use the charts in the RFM.

cheers,
bb

[Um... lifting...]

Of course you're better off never getting into that situation (if we take it to the extreme, you're better off staying in bed), but if you're offshore for SAR or other reasons it certainly happens, and landing isn't always an option. In several thousand hours of SAR flying, approximately once per 1000 hours I had a mission go precisely as planned.
So there you are, off the S. Carolina coast, middle of winter, doing your job, looking for 3 guys who were swept off their boat in crap weather in declining daylight. You find two, winch them aboard, reset the bingo fuel in the mission computer down from the standard 40 minutes down to 30, and then to 20, which is as low as the computer will let you go and as low as the boss will let you anyway. You punch into the computer the nearest airfield to your current position, which is maybe a couple miles in from the beach (you're 30 minutes offshore). The bingo light flashes on, it's time to go, you tell the crew chief to close up the sliding door, we're going back (what else you might do at this point, I'll keep to myself for now). Right then, as he reaches for the door he says... "I see him!"
So what do you do? This isn't theory, this isn't a whiz wheel problem, this is CRM, which is ethics, professionalism, piloting skills, aircraft performance and the whole shooting match rolled into one.
There are some rules of thumb out there that say to increase max range cruise speed by anywhere from 30-40% of windspeed when bucking a headwind (used to have some performance curves that addressed the matter, but can't seem to find them). In general though, in a helicopter, if the wind is substantial (say 20+ knots), you're probably going to want to go just about as fast as the machine will go if into the wind is your only choice to maximize range. The curve on page two of the document at this link gives you the general idea.
http://web.usna.navy.mil/~dfr/flying/wind.pdf
996, I don't wish to be unkind, but I expect you've never flown out of sight of land in your life. Autorotate to what? Plus the additional fuel burn to climb from an 'operational' altitude to somewhere you can get a 200fpm descent @ Vne will burn up everything you have aboard long before you cross the beach. And believe me, the 'might be inconsequential' is very much worth the effort.
bb, yes, groundspeed/fuel flow is what you're looking for... but one additional factor is how much fuel is unusable in the end... shutting down an engine will often change that depending upon your machine. But your general thesis of checking all the variables is right on the money.
(This is a genuine situation that occurred with a friend of mine about 10 years ago... I know how it came out... but at this point, my question stands...) Oh yeah, a cold front just came through, winds to get back home are 35 knots on the nose and you don't have the option of shutting down an engine because there is insufficient fuel aboard to transfer between systems to feed the one you keep running.
What do you do? Have fun!:ok:

[helopat]

Ya know, this is quite a good topic. I have to reiterate that sometimes planning just isn't enough...when the weather goes to cock or you're in an operational situation (like the SAR described by Umm) then all the planning in the world doesn't help...knowledge of your aircraft and the things you can do to improve your chances to get back DOES.

The situation described by Umm Lifting, where the bingo light goes off and THEN the missing man turns up is classic...not exactly the same, but I've had that very thing happen myself...you're right, there are a lot of things rolled up into one...at the end of the day, sometimes leaving that guy out there (maybe with a raft and a long burning smoke closeby) is the best option...going in the drink with your aircraft, crew and the first two survivours does nobody any good (I'm not criticising you mate...believe me!).

Anyway, hope we can carry on with further useful additions to this topic.

Gentlemen.

[Um... lifting...]

Exactly right helopat. Your scenario of dropping a raft and a smoke is one they considered (I won't yet say if that's what they actually did).
We were taught this scenario (and I taught it once or twice) and it's not a question of right or wrong (though if at the end of the day everybody's home and dry you'd be a bloody fool to not shout a round for the room and slap everyone on the back), it's a question of making the best decision based on the available facts. The various groups in CRM class often came out with different answers, but quickly exploring the available options and then acting upon them... that's a recipe for success.

[Banjo]

If you're trying to squeeze every last drop by taking one engine out of the equation, then having that engine at idle is pointless, burning fuel for no torque, so shut it off completely.

I'm not advocating pulling back one engine but it should save on fuel and if you are light enough (after all you are running low on fuel) chances are performance will not be to bad.
The reason for not shutting the engine down Coyote, but only pull it back is simple....shut it down and you can bet murphy will raise his head and the second engine will fail. At least if the other is at idle once in auto you can reach up, open the fcl and then fly away. As mentioned this option will depend on your fuel system set up and ability to pump between tanks (dont forget CofG)
Banjo

[the coyote]

Banjo, I'm well aware of the murphy factor.

If the fuel situation is such that you must consider shutting an engine down, then you are well into using reserves to get to a safe landing area, and concerned about total fuel exhaustion prior to reaching it. In this situation, who cares about the chance of an engine failure while the other is shut down?

There is no point in such a situation having an engine burn fuel without contributing to drive the rotors, just because you are worried about murphy's law.

Just as long as you don't run the only engine out of fuel while crossfeeding, transferring etc.....then yeah, it'd be great to have the other one at idle! :E

[NickLappos]

Nick's rules on squeezing max range:

To fly the farthest, get to max range speed, which is so easy to find you don't need a flight manual. Just take fuel flow and divide it by ground speed. Hunt around for 3 minutes and find the best speed, this way:

take a shot at 430 pph/130knots = 4.07 pounds per nautical mile. Now vary the speed by 15 knots or so and take another shot. 600pph/145knots = 4.13 lbs/nm. OOPS, wrong way! Then go to 120 knots, 504 pph = 4.2 lb/nm. Back to 130 knots!! Now vary it downward by 5 knots a shot, and find the very best.

Once you get the best place, then calculate the range available in your tanks (total fuel / fuel per mile) 400 lbs / 4.07 lb/nm= 98 nm to burnout.

If the answer is not good enough (you cant make it to a safe landing), then do this:

Pull one engine to idle, get to max OEI speed and take a snapshot of that condition. 115 knots, 350 pph = 3.04 lb/nm fuel remaining 400 lbs/2.6 lb/nm = 131 nm to burnout. If this is enough range, climb to 2000 feet, shut down one engine and fly home, get ready to start that puppy if the first one quits! It NEVER is better to leave the engine in idle, it will burn whatever advantage it gave you in the first place. Just do the calculation to see.

BTW, if you wonder why I get so snickety about pilots who ask "How many hours of fuel do you have in the Belch Fire 355?" now you know. RANGE is miles, not hours, and ALL helos differ greatly in speed and fuel flow, so hours of fuel means nothing.

[Shawn Coyle]

One of the main problems in all of this discussion is that a lot of machines don't have a fuel flow indication in the cockpit, and the fuel gauges aren't really accurate enough to use over a 3 minute period for determining fuel flow.

[NickLappos]

Nick's comments on Shawn's:

If you bought a helo without fuel flow, it is beacuse you are one of those people who think hours of fuel is sufficient, so this whole thread is moot! I am kidding but only a little. When I insisted on fuel low gages being an option on S76's, I was told nobody will buy them. I believe we never delivered an S76 without them. Pilots, buy FF gages, they are worth it (unless you think hours of fuel is a unit of measure, in which case, nevermind!)

Regarding accuracy, Shawn, you are (for the first time??) wrong. The relative accuracy is the issue, and that is not an issue. Even the absolute accuracy of FF gages is quite nice, about 5%, in my experience. When I set a wrold record once, I used a fuel hygrometer to measure the density of the gas I bought for the 20 hours prior to the record, and I calibrated the gages - bothe were within 3%!

[Shawn Coyle]

Nick:
Nice to see that someone insisted on the fuel flow gauges as an option. Why aren't they mandated by regulation?
There is no requirement to provide fuel flow, range or endurance information to the pilot - some manufacturers do, but some don't. Yet we hang people who run out of gas (if they survive).
So why aren't fuel flow meters required equipment? (hmmm - sounds like the subject for another column...)

And I meant that the fuel tanks indications were inaccurate in most cases, not fuel flow. Over a very long period, you can get a reliable indication of fuel flow from the fuel tank gauges, but not over a short period.

[Um... lifting...]

I think a point that needs to be made is if you're flying something older or cheaper (and plenty of fules fancy themselves equipped to fly overwater in R22s with four dials on the panel...), that doesn't have FF gauges, you should ALWAYS, ALWAYS, ALWAYS use the most conservative range and consumption estimates you have. As an example, flying in a 206B in the US, use 30 gph (actual burn is between 27 & 29). As far as groundspeed goes, now that GPS is ubiquitous, there's pretty much no excuse for inaccurate groundspeed. No-wind estimates are an extremely unsafe idea unless you KNOW you have a tailwind component... and even then...
I would never, never EVER go fooling around someplace I couldn't land and get anywhere near a fuel margin if I didn't know my fuel flow, didn't write down the time and quantity figures in case the electronic gizmos went south, and didn't know cold the unusable fuel in my machine in case of any and all fuel or engine-related maladies. To do otherwise is just begging for trouble.
To use a furniture-making analogy, cutting fine dovetails is a futile exercise when your marking and measuring is done with a stick and piece of chalk and your chisel and saw are made out of flint. I believe Nick is advocating a fine marking knife and razor-sharp tools.

Question for Nick... now that all avionics gizmos are getting better and cheaper, what kind of new generation fuel measuring systems exist that provide some fair amount of precision and could be retrofitted into lower-cost machines yet still be cost-efficient (flow and quantity). Many of the systems currently on existing aircraft ain't much better than these:
1st generation "I cain't see how much gas is innit... gimme match..." (this one wasn't very successful...)
2nd generation "Gimme stick..."
3rd generation "Gimme stick with marks on it..."
4th generation "Stick with a float on it sticks out the top..."
5th generation "Toilet float thingie what moves a needle an' mebbe turns on a light"
and so on... into various electronic level sensors, impedance systems, and who knows what else... what's next?

[helopat]

Nick's comments on Shawn's:

If you bought a helo without fuel flow, it is beacuse you are one of those people who think hours of fuel is sufficient, so this whole thread is moot! I am kidding but only a little. When I insisted on fuel low gages being an option on S76's, I was told nobody will buy them. I believe we never delivered an S76 without them. Pilots, buy FF gages, they are worth it (unless you think hours of fuel is a unit of measure, in which case, nevermind!)

Regarding accuracy, Shawn, you are (for the first time??) wrong. The relative accuracy is the issue, and that is not an issue. Even the absolute accuracy of FF gages is quite nice, about 5%, in my experience. When I set a wrold record once, I used a fuel hygrometer to measure the density of the gas I bought for the 20 hours prior to the record, and I calibrated the gages - bothe were within 3%!

Well, it'd be a first if SOMEONE didn't disagree.

Range is good and I copy what you're saying about this...HOWEVER, if you're a navy guy like me, sometimes (like when you're on task hunting a submarine rather than going from point A to point B) endurance (in hours, minutes, or whatever time unit you desire) is the name of the game. So, we go out to the operating area at max range speed, and then kick into max endurance mode so we can stay on task for the longest time. For this reason, Seahawks do NOT have fuel flow gauges and, except for a few times on long transits, I've never missed them. If, however, I got into the proverbial poo with fuel (which is why we're here talking) then of course I'd like to have one.

So, an alternate point of view AGAIN.:}

Cheers,

HP

[NickLappos]

HP, you are absolutely right, endurance is a whole nuther thing, and a property of helos that is mission-important.

But 99.99% of the guys who ask "How much fuel?" and expect "2.6 hours" as an appropriate answer are not thinking about hunting subs or survivors!

Regarding fuel flow gages on a Sea Hawk, anyone with 4 hours of gas in normal mode has already solved his fuel problem!!

[ShyTorque]

I disagree with the notion that the RAF used to bring an engine back to idle for ferry flying the Puma. Certainly not in my time on that type, either on a squadron or instructing for two tours on the OCU.
If the freewheel isn't engaged, then the engine is compressing air and burning your precious fuel reserve for no output whatsoever. I think there was even something in the pilot's notes to that effect.
All turbines are inefficient at low power, the Turmo 3C4 was amongst the worst of them in that respect, albeit a superbly reliable engine.
Also flew the S-76 in a SAR role on what was a relatively new unit, over sea, long range - as far as we could go, because there was no other SAR cover in that area. We used to fly with the Specific Fuel Consumption page up on the FMS (shows nm/lb) and experiment on the day with cruise Tq settings and altitude changes, which also took into account the wind. My experience was that the best profile was to make a gentle cruise climb to about 6,000 feet and make a steeper descent at the business end, subject to the job in hand (radar search of the oggin, or whatever was required). The wind was most often the deciding factor, especially when a typhoon was inbound. Once got back with only VFR reserves but the Wx had gone bad (very - cloud below 200 feet). After one abortive attempt to get in visually, we didn't have enough to do a radar with the pattern, but good old ATC fitted us in against the flow, which was nice :ooh:
NEVER flew with one engine throttled back, though, complete waste. In a 76 with Arriel engines it's about 100-150 lbs an hour of waste at idle, IIRC.

[Hippolite]

Nick

Not often we can correct you but Bristow S76A G-BJVZ S/N 760084 was delivered with fuel pressure gauges not the Foxboro fuel flow gauges. It was based on the Southern North Sea for many years before being transferred to Nigeria as 5N-SKY?.

[NickLappos]

Hippo, I stand corrected, but then again Alan Bristow was a business genius and a very cheap one at that! I recall that he wouldn't put rad alts into his 61's for a very long time!

[Hippolite]

Nick

760084 was the only one which was delivered like that so I don't think it was for reasons of economy.

When I started flying S61s in 1980, they all had Rad Alts so I can't comment on that aspect.

Yes, Alan was a business genius but with the help of some good people around him. He is still around and in OK health, sailing at the moment I believe.

[SASless]

Hippo....

In the mid-70's, we were flogging the oggin in the Ekofisk using our three BarAlts while the Helicopter Services folks were using RadAlts for infield transit heights.

It made for some interesting times...Bristow on BarAlts thus using MSL heights and the HS guys using AGL heights. All altimeter settings were in QNH there.

I reckon they just did not understand the importance of QFE altimeter settings and the fun one could have fiddling with three BarAlts during approaches and missed approaches.

I remember a very, very senior Bristow Manager showing me my new horse....and being told how well instrumented the bird was.....Decca, three BarAlts, two channel SAS (one doesn't need Yaw SAS you know old chap!)

It was all good fun however....loved flying the 58T...and enjoyed the folks working that part of the company.

By the way, I assume you have heard Garth Parfitt has passed away while at his retirement home in the South of France. I will miss him and his wonderful sense of humour.

[arismount]

Back in the Huey Days...

In the UH-1 of course we could beep down. The N2 range was 6600 to 6200 and we usually flew around at 6400 and about 30 to 35 pounds of torque for a "normal" cruise of 90 to 100 KIAS.

Flew with an old DAC once and he said to beep it down to 6200 and pull 40 pounds and that would get you 30 to 40 miles further down the road on a tank of gas. Tried it several times, he was right. This would also give you about 110 to 115 KIAS.

I may have the numbers wrong but this is the best I can remember.

The results have to do with the engine being more efficient at closer to its temp limit, and also the reduction in drag from the rotor at lower RPM. Maybe I have this wrong, Mr. Lappos feel free to chime in, I am just a pilot but I know this worked.

Now I can no longer beep down the rotor, FADEC is in command. Progress.

[NickLappos]

arismount,
I do believe it, and I agree, except I think the major effect was on the rotor not the engine. The tip speed of a Huey is way too high for efficiency, especially at 110 knots, so beeping the rotor down would get it more efficient (but I would believe only about 5 to 10%, vice the 40 miles out of 220 max range in a Huey.)

[eagle 86]

If I recall my Huey days beeping down/selecting a certain altitude was to find a combination that produced the most efficient angle of attack. None of the types I've flown since have the ability to beep RRPM over such a range if indeed the RRPM could be altered at all.
GAGS
E86
Of course PPPPPP applies!

[jellycopter]

My two penneth worth.......

There are plenty of type-specific techniques that have been helpfully posted, but I'm not sure that's the point of this thread.

In my opinion, the wind is by far the greatest performance variable when discussing max range. We all know wind generally is stronger at height and veers slightly (in the N hemisphere). Therefore, with more than about 10kts of surface wind, you always fly high with a tailwind component and low with a headwind component.

As for best-range cruise speed (best specific fuel consumption), I don't fly anything with a fuel flow gauge, so would dearly like someone to post a rule of thumb that works for modifying the AFM best range speed for wind effects.

J

[B Sousa]

Along the lines but no answer for the question. I am curious though if OZ land is so remote why does the Government not have pre-positioned fuel caches maintained for emergencies along well traveled routes. Other than they may be stolen once in a while.
We used them in Alaska for certain missions and it helped.

[Shawn Coyle]

For most helicopters, whatever you get at maximum continuous power is going to give you pretty close to maximum range speed.

There is, however, another small issue to consider.
Specific air range (miles per pound of fuel) vs true airspeed will give you a curve that (in theory) should peak - this is the absolute maximum range airspeed. Then the curve will show a decrease in specific air range at a higher airspeed. If you draw a horizontal line at 99% of maximum specific air range, you will interest the curve at two points. If you use the higher of the two airspeeds, you will take a 1% hit from absolute maximum range, but you will arrive more quickly. When flying into a headwind, this will make a significant difference in time enroute.

[Matthew Parsons]

The way the wind effect made sense to me is to consider a max range speed of 100 knots while flying into a 100 knot wind. You'll go nowhere. If you want to go upwind, you'll have to fly faster.

Conversely, you'll have to fly slower with a tailwind.

The real question is how much faster/slower? With a fuel flow gauge we can do the math. I think SFC displayed in the cockpit trumps the fuel flow gauge because then we don't have to fly AND do math, but even better would be a doppler/INS based wind calculation and enough performance data programmed into your computer that the max range speed is calculated for you. We're now escaping the upgrade budget of most operators.

If you have a helicopter where the relationship between fuel flow and power is a straight line, or nearly so, then there is a way to determine how much faster/slower. If you look at the power required curve (power required versus true airspeed is best, but IAS will be fairly close), you can find the zero wind max range speed by drawing a line between the origin (0 airspeed and 0 power) that just touches the curve (the tangent). Someone posting a sketch of this will help me very much.

If you have 20 knots of headwind, then draw the line from the point that is zero power, but +20 knots of wind. Why? Because your hover power with a 20 knot headwind would be the same as flying at 20 knots. Now when you draw the tangent line it intersects the power required curve at a higher speed. This will be your best range speed with a 20 knot headwind.
If you have a 20 knot tailwind, then do the above but start drawing your line at zero power and -20 knots.

Note that you will never have a max range speed that is less than your max endurance speed, no matter how fast the tailwind is.

While this sounds easy, you might not have the power required curve, and even if you do, it's not just one curve but is many curves depending on temperature, pressure altitude, and aircraft weight. Also, the result may exceed a limitation, in this case just fly at the maximum continuous rating for whichever limitation is of concern.

Back to the original question. In general, beeping down, climbing, reducing bleed air and electrical loads, jettisoning external stores, and flying with zero sideslip will reduce your fuel flow rate. Shutting an engine down may help in an emergency, but its a risk because without performance charts you might be running out of fuel further from shore with less power to ditch. Idling an engine is less likely to give you a fuel flow advantage than shutting one down, but is less risky.

Summing up: consider the winds and don't do procedures that aren't authorized or recommended for your type.

Matthew.

[Woolf]

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Another interesting question arises from a low fuel situation:


Once you have established that your fuel state looks a bit tight, at which point would you consider telling someone about it or even declare an emergency? It would be interesting to see the different responses from different countries as well as compare what you should have done to what you actually did.

Here in the UK there is no such thing as a "Fuel emergency" - preferential treatment for routing/landing can only be achieved by declaring an emergency (PAN call). It will also trigger the usual events associated with a PAN call i.e. don't be surprised to see the fire engines waiting for you .... in any case questions will be asked!

Here is what JAR-OPS 3.375 says:

(a) An operator shall establish a procedure to
ensure that in-flight fuel checks and fuel
management are carried out.

(b) A commander shall ensure that the
amount of usable fuel remaining in flight is not
less than the fuel required to proceed to a heliport
where a safe landing can be made, with final
reserve fuel remaining.

(c) The commander shall declare an
emergency when the actual usable fuel on board is
less than final reserve fuel.


Thankfully I haven't been in this situation yet (even though it did look like it on one or two occasions early in the flight). I can see the temptation in not declaring an emergency and hoping for an expeditious clearance which on the other hand can make things much worse if it doesn't work out!

Any thoughts or experiences?


Woolf