jibboo, You came to the right place, I wrote that gibberish, or at least was principal author, but by the time the tech pub writers finished....... The basic idea of Power Assurance is to be sure the engine will let you meet published performance. The trend method allows some daily wandering of the margin, called scatter, which is assumed to be merely statistical and not actually indicating an engine problem. The rationale for each:
1) means = The engine meets the min specification power.
2) note: The "daily trend margin" means any individual daily power margin check. Means = No daily check can be very much out of whack - this spells trouble, maybe the engine has a real problem, and the result greater than 4% from the average is beyond scatter.
3) Means = if the margin at low power is less than the margin at high power, if you really had an OEI event, the engine might not give you 30 min and 2 1/2 min power. This covers an engine that has a "negative slope" which means it has less margin at high power than at low power.
While NickLappos provided the appropriate technical rationale to address your issue, I don’t think he answered your (specific) question which was “to what do you compare today's rolling average with to get the answer for the 3rd criteria?”
Using your example that “the last 30 min power check had a margin of +4” .......when you subtract today’s rolling average (average of the most recent 20 checks) from the rolling average baseline (average of the first 20 checks), the result cannot exceed 4.
Yes, good catch, I really didn't answer his question! The current (rolling average) margin cannot have degraded from the original (rolling 's average) margin by more that the original 30 min power margin.
(Original RAM) - (Todays RAM) < 30 min margin
One more thought. We do this low power check to predict the behavior at high power, without exercising the engine to high power every day. Rules 2 and 3 are potective of the margin at high power without making you check the engine there. Note that in the maintenance manual, after maintenance, you re-do the 30 min check, and start a new rolling average. In other words, you assure power with an actual chack at the required condition, then start tracking it at lower power to maintain confidence in that original high power check.
Thank you for your responses! It is clearing up a little now (excellent description of why the 3 criteria are used).
I thought I'd add that of course don't mean to criticize the RFM... just needed some clarification! This thread is such a powerful tool. I hope it continues!
Having said that... I do have more questions:
------------------------------------------------------- 1.) Are you familiar with Customer Service Notice No. 76-88A?
(it outlines the how to manage/analyze power assurance and trend data)
Nick and Heli Tester said:
(Original RAM) - (Todays RAM) < 30 min margin is the formula to get that 3rd criteria
The CSN says you compare
(Base Line) - (Todays RAM) < 30 min margin
"Base Line" = the 5th check after the 30min power check...
2.) To what should I compare (Todays RAM)? Base Line or Original RAM?
"Note that in the maintenance manual, after maintenance, you re-do the 30 min check, and start a new rolling average."
3.) If you do a 30 min power check at the 50hr interval (no maintenance done necessarily), do you restart the RAM?
(we only restart the RAM if a component on the engine has been changed ie. Fuel Control, N2 governor, compressor etc)
With how much we are flying, on the 20th day (probably close to 50hrs later) it's usually time to do a another 30 min check... if we restarted the RAM every time the 30 min was done, we'd barely get the "original RAM" before it is time to start another "original RAM"! 4.)Is this the reason for that "base line" possibly in the CSN?
I understand your issue. Regrettably I don’t have an S-76A RFM and I have not seen Sikorsky CSN 76-88A, so I’m working from memory.
You stated that the CSN defines "Base Line" = the 5th check after the 30-min power check. So I assume that means the baseline (against which the current 20-point RAM is compared) is the RAM of the first five 135 kt power check points following the original 30-min power check (and not the original 20-point RAM as I previously suggested). As I recall, you don’t have to establish a new baseline every 50 hours just because you do another 30-min power check. Re-baselining is only required after a maintenance action. Therefore, I would modify the equation in Nick’s last post to read:
(5-point baseline) - (Todays 20-point RAM) < most recent 30 min margin
So the 20-point RAM could be calculated continuously for say 500 hours through 10 30-min power checks as long as there is no maintenance action.
Hopefully Nick will read this and confirm. I think the only other place you can turn for guidance is the Sikorsky Aircraft Help Desk.
HeliTester, I have to admit I haven't thought this much about it in about 31 years (the procedure was developed about 1980). I believe your last explanation is correct, I don't have a copy of the CSB but your explanation is consistent. Certainly, the intent of the 3rd rule is to be sure the engine has not degraded enough so that 30 min and 2 1/2 min power cannot be obtained. That is why we get suspicious if the engine's average daily margin slides down by the amount of the margin shown at 30 min power, we suspect the 30 min margin will fail. The confusion as to what is the "baseline" is perhaps a little academic, because an engine seldom slides down in power if it is kept clean and not eroded, so the difference between the RAM based on the first 5 points and the RAM based on the first 20 points should be very small. This is a good thread to help me clear the cob webs, thanks!
Trying to understand the collective / yaw couple in the S76 mixing unit and I have emerged confused. With the anti-clockwise direction of the main rotor the fuselage would, with increased torque, want to turn right with increase in collective pitch, and vice versa. So why in the mixing unit will collective increase with right pedal input? My small brain seems to only understand that with an increase in collective, the nose will swing right, therefore requiring left pedal input to compensate. I apologise for my limited brain power and lack of understanding on this mechanical matter, and an explanation to drag me from the depths of ignorance would be very much appreciated.
My small brain seems to only understand that with an increase in collective, the nose will swing right, therefore requiring left pedal input to compensate.
Had fun when I tried to groundtaxi a tight turn to the right; the collective kept coming up. Apparently there was insufficient tq effect at flat collective pitch to yaw the nose to the right.
What it in general does, is ensure you don't need to shove in a bootful of left pedal when lifting the collective, nor shove in much right pedal when lowering the collective, to maintain balanced flight.
Under normal circumstances, up collective adds left pedal (increases TR pitch) automatically, and the pedals don't move at all. This mixing is done in the mechanical controls, via a mixing link. Down collective makes the TR go the other way. It does just what you would do, so that a liftoff to a hover takes virtually no yaw input. All Sikorsky helos after the S-58 (CH-34) have had this mixing.. IF the Tr is against a stop, so that no more TR pitch is available, then this automatic yaw input "bounces back" and is perceived in the cockpit as you describe: If the TR is against the right stop (full right pedal) and the pilot pushes harder on the pedal to the right, he will simply back drive the collective and drive it up. Under this circumstance, no more TR pitch is being added, the TR is at its physical stops. The pilot is simply finding new combinations of collective/pedals that equal full TR pitch. It is somewhat rare, but certainly possible, to find this spot while taxiing, and also if one is in a right autorotative turn in descent. The same thing happens with vastly up collective and left pedal, but the combination is very hard to find in normal operations. In that case, near the left pedal stop with near full up collective, adding left pedal drives the collective pitch downward. If you ae still confused, I can draw a simple cartoon to show how the fixed TR setting creates a relationship between the yaw and collective. BTW, if the TR control system should jam and freeze in place, this pedal/collective mixing will be seen in any flight regime. In a rare failure, due to a mode now fixed, a PHI pilot flew home and landed with stuck pedals and what he thought was stuck collective (but was not, the collective was free to achieve full range if he let the pedals move back and forth.)
Seems hard to believe but I couldn't find any thread dedicated to the venerable S-76, I'd be happy to get this one merged with a previously existing thread that I would have missed, Mods go ahead!
I will start by posting this press release from Sikorsky, a farewell to the C++ :
S-76D™ Helicopter Continues Reliability and Safety of its Predecessor as S-76C++™ Production Program Closes
June 23, 2011
PARIS, France - The S-76C++ helicopter is officially “sold out.” Sikorsky Aircraft Corp., a subsidiary of United Technologies Corp. (NYSE: UTX), made the announcement at the Paris Air Show today. With a 30-year legacy of safety and reliability as its foundation, the S-76D™ helicopter is the next generation and continues to progress toward first customer deliveries in 2012.
“With aircraft number 822, the last S-76C++ helicopter closes out that production program and we are looking forward to introducing the newest version in the series, the S-76D, when we deliver the first aircraft to the launch customer next year,” said Tim Fox, Senior Program Manager for the S-76® helicopter program. “The helicopter market is beginning to show optimism again. The S-76 C++ aircraft weathered that storm and the S-76D helicopter will carry forward a legacy of durability and safety to a new generation of customers.” Final assembly of the S-76D helicopter began in December 2010 at Sikorsky Global Helicopters’ manufacturing facility in Coatesville, Pa. Aero Vodochody in the Czech Republic, which has produced more than 300 S-76 airframes for Sikorsky, is manufacturing the S-76D fuselages. Sikorsky Global Helicopters will install Pratt & Whitney Canada PW210S engines, blades, and gearboxes during final assembly, followed by acceptance flight tests and customer completions. Since its inception in 1979, the S-76 helicopter program has introduced six variations including the S-76C++, introducing improvements during that time that included more powerful engines and an inlet barrier filter to provide improved engine protection and smoother airflow.
“The S-76 helicopter continues to be the premier executive aircraft around the world. The fleet has logged 5.8 million flight hours, and Sikorsky is committed to remaining at the forefront of aircraft safety as the S-76D steps up to write the next chapter in the history of this successful program,” said Ed Beyer, Vice President of Sikorsky Global Helicopters. In November 2010, the S-76D helicopter program entered into certification flight status at the Sikorsky Development Flight Center in West Palm Beach, Fla. This is the final phase of the test program that provides critical data for FAA sea-level certification, expected by Q4 2011. Two prototype aircraft are in flight status, with a third preparing to enter the flight test program. All three prototypes were manufactured at the Sikorsky Global Helicopters facility. The new generation S-76D helicopter will perform an array of civil missions including executive transport, offshore oil, emergency medical services, and a multi-mission role. Among the S-76D helicopter’s features are all-composite, flaw-tolerant main rotor blades; an advanced THALES integrated avionics system and autopilot; optimized rotor system for quiet operation; active vibration control; powerful Pratt & Whitney Canada PW210S engines; and an optional Rotor Ice Protection System (RIPS) for all-weather capability. Sikorsky Aircraft Corp., based in Stratford, Conn., USA, is a world leader in helicopter design, manufacture, and service. United Technologies Corp., based in Hartford, Conn., USA, provides a broad range of high technology products and support services to the aerospace and building systems industries.
Hi Guys, I am thinking on incorporating an s76b in our fleet. Can anyone help me with some questions, please? I have checked prices for s76b on the market and they are around 1.6m$ isn't it cheap? Where is the catch? Are the operating cost high? Most of them are on power assurance programs for the transmissions, are they problematic? From your experience would you say is a problematic heli maintenance wise? For a low altitude flying(max 2000' & 25ºC average) what would be the payload with two pilots and 2hours fuel? Would you recommend the s76c+ instead? I hope I am not abusing here, but I am a little bit lost 'cos I do not know anything about this aircraft. Thanks in advance
I have checked prices for s76b on the market and they are around 1.6m$ isn't it cheap? Where is the catch?
No catch. With the C,C+,C++ and D models available the B is now an old model so prices reflect that. The B is still a very good aircraft with good range and payload due to the PT6s
Are the operating cost high?
No more than any other S76. Being older you will have more expense as components will be near the end of there life.
Most of them are on power assurance programs for the transmissions, are they problematic?
Most are on the PAP programme as an insurance. Like any manufacturer spares have a long lead time and priority is given to customers on PAP. The S76 gearbox is a proven design and very robust. PAP also helps with resale, I wouldn't buy a S76 without PAP
From your experience would you say is a problematic heli maintenance wise?
No more than any medium helicopter
For a low altitude flying(max 2000' & 25ºC average) what would be the payload with two pilots and 2hours fuel?
It depends on the spec. I would say about 1600lbs payload.
Would you recommend the s76c+ instead?
All a question of money
If you need anymore PM me. I know of a S76B for sale that a good offer would be accepted.
Sultan, If the 76D has a 4 year delay, and that means to you some major problems with the design, then what does a 20 year delay in the 609 signify - a completely bankrupt design concept? I suspect that both cases more rightly reflect the variability of continued funding in a self-funded R&D world. Nothing to do, in either case, with the engineering successes of the development team. Why, when your own track record is so abysmal, do you throw rocks at another team? Rocky