Low Fuel
Thread Starter
Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 72
Posts: 4,156
Likes: 0
Received 29 Likes
on
14 Posts
Low Fuel
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
Answers on a postcard please.....
Phil
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
Answers on a postcard please.....
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
Last edited by helopat; 8th Jan 2007 at 08:11. Reason: I think I spelled 'lecturizing' wrong
Join Date: Jul 2001
Location: ...in view of the 'Southern Cross' ...
Posts: 1,383
Received 0 Likes
on
0 Posts
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
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
Join Date: Dec 2001
Location: Gold Coast, Queensland
Posts: 943
Likes: 0
Received 0 Likes
on
0 Posts
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?
Or have I misunderstood the question?
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.
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.
Thread Starter
Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 72
Posts: 4,156
Likes: 0
Received 29 Likes
on
14 Posts
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
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
Join Date: Nov 2001
Location: Land of the Brave
Posts: 73
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: Jan 2007
Location: UK/Australia
Posts: 73
Likes: 0
Received 0 Likes
on
0 Posts
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
they used to close the airport if it was more than a shower
Join Date: Dec 2001
Location: Philadelphia PA
Age: 73
Posts: 1,835
Likes: 0
Received 2 Likes
on
2 Posts
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.
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.
"Just a pilot"
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.
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.
Join Date: Jul 2005
Location: North
Posts: 124
Likes: 0
Received 0 Likes
on
0 Posts
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.
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
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
Join Date: Aug 2006
Location: Below Escape Velocity
Posts: 416
Likes: 0
Received 0 Likes
on
0 Posts
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!
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!
Last edited by Um... lifting...; 8th Jan 2007 at 20:03.
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.
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.
Join Date: Aug 2006
Location: Below Escape Velocity
Posts: 416
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Aug 2000
Location: England/Arizona
Posts: 118
Likes: 0
Received 0 Likes
on
0 Posts
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
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!
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!
Join Date: Apr 2003
Location: USA
Age: 75
Posts: 3,012
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.