What's a "maximum performance takeoff abort - backwards." ?
Joined: Feb 2005
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From: Florida & UK
Shawn:
Fair point about the low air speed accuracy of the ASI.
I should have been less lazy and typed "best angle of climb" as opposed VX, and "best rate of climb" instead of VY.
Best ROC, as you corrected, is zero if the power is sufficient, and I believe that at limited power (insufficient power levels for vertical) it is the point where the tangent touches on the back side of the power required Vs air speed curve, drawn from the "given available power limit of the day. However there is definitely going to be a margin of error involved in establishing that flight performance. What I was trying to get at, is that "best angle of climb" will be at a slower air speed than "best rate of climb".
Question: Do you think that given an accurate power required curve table, that "best angle of climb" speed could be judged reasonably well for different max available power figures.Referenced from disk attitude, set by experience coupled with GPS ground speed on a calm day?
Fair point about the low air speed accuracy of the ASI.
I should have been less lazy and typed "best angle of climb" as opposed VX, and "best rate of climb" instead of VY.
Best ROC, as you corrected, is zero if the power is sufficient, and I believe that at limited power (insufficient power levels for vertical) it is the point where the tangent touches on the back side of the power required Vs air speed curve, drawn from the "given available power limit of the day. However there is definitely going to be a margin of error involved in establishing that flight performance. What I was trying to get at, is that "best angle of climb" will be at a slower air speed than "best rate of climb".
Question: Do you think that given an accurate power required curve table, that "best angle of climb" speed could be judged reasonably well for different max available power figures.Referenced from disk attitude, set by experience coupled with GPS ground speed on a calm day?
Joined: Apr 2000
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From: EGDC
Brian I think for most helicopters that 'best angle of climb with limited power' speed is going to be between 30 and 40 kts, but few helis have an ASI accurate enough to indicate it. If you know the wind speed accurately and have a method of measuring groundspeed (doppler/GPS etc) then you could get close but disc attitude is not a measurable factor. If you feel the aircraft attain ETL and can maintain it then that is your best chance of managing a steep climb out but I am firmly with SASless and the JAA regarding the use of a vertical climb until above obstacles and then transitioning. If you don't have enough power to do this you need to ask yourself if you should have less pax or fuel on board and how desperate are you to get out of the site before trying the FAA/US Army 'lets fly forward and hope we clear the obstacles' technique.
I know the Australian Army favour a technique where they try to accelerate to a suitable climbing speed but they identify and brief an abort point so that the aircraft can be safely brought back to the hover before reaching (impacting) the obstacles.
Back to the original post - this seems like an instructor trying to demonstrate something he doesn't really understand - a rearwards descending abort for God's sake - it doesn't get more dangerous than that (much).
I know the Australian Army favour a technique where they try to accelerate to a suitable climbing speed but they identify and brief an abort point so that the aircraft can be safely brought back to the hover before reaching (impacting) the obstacles.
Back to the original post - this seems like an instructor trying to demonstrate something he doesn't really understand - a rearwards descending abort for God's sake - it doesn't get more dangerous than that (much).
Joined: Jul 2001
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From: the cockpit
The best system I have come across so far for limited power take offs was the UH-1H techniques developed by the Australian Airforce. They divided all departures into 4 categories:
Category 1 can only be attempted on a runway style environment and is used with no requirement for power margin, ie you may not even have enough power to hover IGE.
Category 2 is used in open confined areas, and is what crab refers to. Before departure, you determine the last point of safe hover in the pad, then about half way between you and that point, you determine a decision point (abort or continue). The technique is to draw a line from your hover to the top of the obstacle you intend to clear. Simply apply max power available whilst adjusting cyclic to fly that imaginary line. At the decision point, it is easy: if you are achieving the line and will clear the obstacles, continue. If you have fallen below the line, abort to your last point of safe hover. You need a minimum of IGE power plus 3 psi (6%) to attempt this up to a max of a 6 degree departure angle (this number may be wrong: my memory is fading), and there is a table that determines margin required for steeper angles, up to about 18 degrees.
Category 3 is what the JAA style technique is. A verticle departure is made pulling max power available. The abort point is directly below you, or slightly to your front if the LZ allows, and a transition can only be made by lowering the nose if either the rate of climb is significant, or for heavier aircraft once attaining about 100 ft above obstacles. Obviously, terrain and obstacle shapes are also considered. No power margin is required for attempting this manoeuvre, and it is acceptable to try this if you do not have a Cat 2 margin available.
Category 4 means not limited by power, and is required for all hoist operations, NOE, and OGE required manoeuvres such as an OGE arrival requirement. You must have OGE hover power plus 3 psi (6%) to be "Category 4", ie basically unrestricted by power availability.
The charge at the trees and cyclic pull up style manoeuvre is considered a wartime manoeuvre only, where the high risk of tree/obstacle strike is less than the risk of not going!
Category 1 can only be attempted on a runway style environment and is used with no requirement for power margin, ie you may not even have enough power to hover IGE.
Category 2 is used in open confined areas, and is what crab refers to. Before departure, you determine the last point of safe hover in the pad, then about half way between you and that point, you determine a decision point (abort or continue). The technique is to draw a line from your hover to the top of the obstacle you intend to clear. Simply apply max power available whilst adjusting cyclic to fly that imaginary line. At the decision point, it is easy: if you are achieving the line and will clear the obstacles, continue. If you have fallen below the line, abort to your last point of safe hover. You need a minimum of IGE power plus 3 psi (6%) to attempt this up to a max of a 6 degree departure angle (this number may be wrong: my memory is fading), and there is a table that determines margin required for steeper angles, up to about 18 degrees.
Category 3 is what the JAA style technique is. A verticle departure is made pulling max power available. The abort point is directly below you, or slightly to your front if the LZ allows, and a transition can only be made by lowering the nose if either the rate of climb is significant, or for heavier aircraft once attaining about 100 ft above obstacles. Obviously, terrain and obstacle shapes are also considered. No power margin is required for attempting this manoeuvre, and it is acceptable to try this if you do not have a Cat 2 margin available.
Category 4 means not limited by power, and is required for all hoist operations, NOE, and OGE required manoeuvres such as an OGE arrival requirement. You must have OGE hover power plus 3 psi (6%) to be "Category 4", ie basically unrestricted by power availability.
The charge at the trees and cyclic pull up style manoeuvre is considered a wartime manoeuvre only, where the high risk of tree/obstacle strike is less than the risk of not going!
Joined: Dec 2001
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From: Philadelphia PA
When I was at NTPS, we had a very accurate GPS for post-processing takeoff data. We did some exercises in the UH-1N and had the students rotate at different airspeeds (as low as 35 KIAS if I remember correctly) and climb out 10 KIAS faster.
(We tried using GPS, but found that it didn't have a fast enough update rate to be useful in any way. it would read very low values for quite a while and then suddenly be close to what you were at in a steady climb. Not useful at all for anything dynamic.)
What we found was quite astonishing. The airspeed system simply could not keep up with the reality - (bear in mind the UH-1N has the airspeed boom on top of the roof, not out from the nose like the 212/412 so these results may not be appropriate for those models). If we cross plotted airspeed with the real speed (we did all these tests in a no wind day) we found that when the airspeed indicator was saying 35 Kts, you were really doing 50 Kts.
Sadly, the low airspeed indicating system we had was not serviceable, as I dearly wanted to prove that it might have a useful roll in getting better angle of climb performance at a very low and repeatable airspeed.
And the problem with defining one speed for the climb out is made worse by trying to define how much power you have available to make the tangent point from....
(We tried using GPS, but found that it didn't have a fast enough update rate to be useful in any way. it would read very low values for quite a while and then suddenly be close to what you were at in a steady climb. Not useful at all for anything dynamic.)
What we found was quite astonishing. The airspeed system simply could not keep up with the reality - (bear in mind the UH-1N has the airspeed boom on top of the roof, not out from the nose like the 212/412 so these results may not be appropriate for those models). If we cross plotted airspeed with the real speed (we did all these tests in a no wind day) we found that when the airspeed indicator was saying 35 Kts, you were really doing 50 Kts.
Sadly, the low airspeed indicating system we had was not serviceable, as I dearly wanted to prove that it might have a useful roll in getting better angle of climb performance at a very low and repeatable airspeed.
And the problem with defining one speed for the climb out is made worse by trying to define how much power you have available to make the tangent point from....
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This is all great stuff, particularly the difference between simulated and real max power take off, but it seems no-one really knows what this instructor was up to ?
It appears s/he either took off backwards in a Cat A type profile intending to abort forward (showing off and pretending to be flying to the rigs ?) or took off with the intention of aborting backwards to the take off point ?
I've only ever done two Cat A take offs so can't comment but would someone in the know care to explain the abort bit and/or whether it would ever be SOP to abort backwards ?
I ask the latter due to the reference to it being a mariner so maybe this is what you do on/off a boat or deck ?
It appears s/he either took off backwards in a Cat A type profile intending to abort forward (showing off and pretending to be flying to the rigs ?) or took off with the intention of aborting backwards to the take off point ?
I've only ever done two Cat A take offs so can't comment but would someone in the know care to explain the abort bit and/or whether it would ever be SOP to abort backwards ?
I ask the latter due to the reference to it being a mariner so maybe this is what you do on/off a boat or deck ?
Joined: Feb 2002
Posts: 83
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From: Northern Ireland
Aborted take off
Never heard the like of a backward aborted take off. How can you see a reject point.
A Group A take off if I remember correctly for the Puma is.
Take off with a slow rearward movement and gentle climb so that you can reject at any time to the take off point
At Decision Point of 100 feet call decision and rotate from this point you will have sufficient height and power single engine to go around should the other one fail.
Really restricts take off weight so does'nt tend to get used very often offshore. Probably only be used to come out of a confined area, hospital helipad, after dropping casualty brought in from a rig.
A Group A take off if I remember correctly for the Puma is.
Take off with a slow rearward movement and gentle climb so that you can reject at any time to the take off point
At Decision Point of 100 feet call decision and rotate from this point you will have sufficient height and power single engine to go around should the other one fail.
Really restricts take off weight so does'nt tend to get used very often offshore. Probably only be used to come out of a confined area, hospital helipad, after dropping casualty brought in from a rig.
Last edited by jbrereton; 20th March 2005 at 18:19.
