Engineering Assumption versus Operational Reality: Repeated Lift Operation.
Bogdan Gajewski
Corrective Action Engineer
Transport Canada
Introduction
The unique flight capabilities of a helicopter allows for a multitude of uses. During flight the helicopter accumulates damage known as fatigue or wear. However, the way in which the rotorcraft is utilized will dictate the amount of damage accumulated. A rotorcraft used for transportation accumulates damage differently than that operating in a utility role.
The most demanding helicopter role is Repeated Lift Operations. Any helicopter, no matter its size could be affected by constant utilization in a repeated lift role. During certification certain mission profile is assumed. When operating outside to this assumed profile component lives and wear may be adversely affected resulting in a risk to safety. This paper is intended to support the use of actual loading conditions rather than assumed operation to establish component replacement schedules and support the use of condition health monitoring.
1. Helicopter usage
Helicopter usage is typically considered in terms of these roles:
- Transportation
- Industrial
1.1. Transportation
Helicopters that provide basic transportation of people.
Examples: Oil rig workers, medi-vac, sightseeing, air-taxi, airlines, pleasure and corporate flights.
1.2. Industrial
Helicopters that perform work. Examples: Forest industry, seismic, construction, firefighting.
Both of these roles are uniquely different and will sustain different fatigue damage.
Of interest is the use of actual physical data to dictate component replacement and maintenance. Specifically, this paper is concerned with repeated lift operations.
Transportation Safety Board of Canada (TSB) evaluation of the repeated lift operations like heli-logging and firefighting lists “torque event” (a collective application) as a significant event that contributes to a helicopter’s fatigue.
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Presented at AHS International Helicopter Safety Symposium, Montreal, Canada, September 26-29, 2005.
It means that not only how much has been lifted, it is also important how often the torque event has occurred. As per TSB findings, the number of torque events per one cycle during heli-logging operations was four per cycle. It translates into 120 torque events per hour during intensive logging.
Helicopters involved in heli-logging and firefighting are exposed to a significant number of torque events. These events are repeated themselves and, therefore, there is a need to define such repeated occurrences.
Fig. 2. Torque and hook weight over time in heli-logging (source - TSB)
Fig. 3. Torque and hook weigh over time anticipated during design.
Fig. 4. Torque application in heli-logging (thin line) vice versa torque application (thick line) anticipated during design - presented on same time line.
Number of torque application for helicopters in transportation:
On average: 2 torque events per cycle.
(Pleasure flight: 4 x 2 = 8 torque event per hour)
Number of torque application for helicopters in RLO:
Up to 4 torque events per cycle in heli-logging.
Minimum: 20 x 2 = 40 torque events per hour
Maximum: 30 x 4 = 120 torque events per hour
The data indicating that the Certification Spectrum Conditions are different than those in real applications are extracted from the following report:
“Continued Evaluation and Spectrum Development of a Health and usage Monitoring System - DOT/FAA/AR-04/6 May 2004”.
This report describes the results of a research project to evaluate structural usage monitoring and damage tolerance methodology using data collected on a Bell Model 412 helicopter equipped with a health and usage monitoring system (HUMS). The helicopter was operated by Petroleum Helicopters Inc. The three missions were monitored and compared to certification data.
We are interested in two missions:
The first one, which was described as a short haul mission, was considered a severe usage mission because it involved many short maneuvering flights to provide pickup and delivery services at the Summer Olympics in Atlanta.
Second one, which was a Gulf Coast mission, was considered a mild usage mission because it primarily involved long cruise flights.
The data (provided at the end of the text) shows that helicopters being flown in short haul missions are exceeding some Certification Spectrum Conditions a few times and, in one case just over 18 times.
We have no data from heli-logging operations, but one can suspect that there will be some exceedences of greater values.
2. Component fatigue - Cycle vs. flight hour.
Some airframe and engine manufacturers have developed unique formulae to calculate the number of cycles related to one flight hour of operation, or has developed new retirement criteria for parts affected by industrial applications. However, these formulae are all different, some require more complex accounting and mathematical calculation. Also, there is little formal exchange between operators to share operational experience.
2.1 Examples of Repeated Heavy Lift (RHL) definitions:
Sikorsky Alert Service Bulletin ASB 61B35-67A, Dated 11 October 2002.
Applicability: All Commercial S-61 Models used in repetitive external lift (REL) operations.
Definition:
A repetitive external lift is defined as accomplishment of lift, enroute travel, approach to hover, release of load or land, and then depart. This sequence, when repeated several times within one flight hour period, is referred to as repetitive external lifts per hour.
Sikorsky Service Bulletin 61B General 2N Rev.14 - 11 October 2002
Repetitive External Lift (REL):
Lift, enroute travel, approach to hover, release of load or land, and then depart. This sequence when repeated more than six (6) times within one flight hour period is referred to as repetitive external lifts.
An external lift operation is defined as up to 6 lift-carry-drop cycles per hour without landing or external lift operations with up to 6 takeoff and landings per hour.
Sikorsky Alert Service Bulletin 61B35-68B Rev. B - 6 July 2000.
An external lift operation is defined as:
The accomplishment of external lift, enroute travel, approach to hover, release of load or land and then depart.
If the average number of external lift operations exceeds six (6) per hour during any 250 flight hour period of external lift operation during the main gear box time between overhaul (TBO) interval this is defined as repetitive external lift (REL) operation.
GE Aircraft Engines Service Bulletin CT58 72-162 Rev. 9 - 6 October 1998.
Repeated Heavy Lift (RHL) operations are those during which a lift-carry-drop cycle is repeated more than 10 times per hour or during which there are more than 10 takeoff and landing cycles per hour or during which there are more than 10 occurrences per hour of a combination of these cycles.
Utility operation is defined as 2 to 10 inclusive lift-carry-drop cycles per hour or 2 to 10 inclusive takeoff and landing cycles per hour or any combination of these cycles.
A non-RHL operation is defined as an average of
1 (one) complete takeoff and shutdown cycle with
1 (one) additional landing without shutdown and takeoff per hour or 1 (one) lift-carry-drop cycle per hour or any combination of these cycles per hour.
Bell Helicopter:
BHTI has introduced the Retirement Index Number (RIN) as an inspection interval and the life retirement limit.
Definitions:
* Torque event = a take off or a lift (internal or external)
* Cycle (in repeated heavy lifting operations) = Lift + ferry + drop off + ferry (ready for another lift)
A heli-logging cycle has TWO torque events. (One - lifting the log, and second - while climbing back for another)
As per Bell torque event definition, a Bell 212 helicopter in RHL will accumulate four RIN per one take-off (more powerful model), while model 205 will accumulate two RIN per one take-off and model 204 will accumulate one RIN per take-off.
FAA AD 2003-24-01 MDD 369 Helicopters. Based on Helicopter Technology Company, LLC Mandatory Service Bulletin, Notice No. 2100-3R2 dated 20 December 2002.
Record a torque event (TE) for each transition to a hover or landing from forward flight with airspeed of 30 or more knots or any external lift operation. An external lift operation is defined as the pickup and drop-off of an external load. (An external lift operation with a return flight at airspeed of 30 or more knots back to the pick-up location would be recorded as two TEs).
In summary, there is a need for common cycle definitions with a simple accounting system.
2.2. Maintenance for specific industrial applications (Firefighting, Logging)
There is a need to clarify the need for the mission-oriented maintenance based on a more relevant cycle time verses flight time. It is likely that helicopters used in RLO could be dispatched to transport mode or, some parts are being exchanged between helicopters of different mission profiles. Some manufacturers will assist in adjusting maintenance requirements. Of particular interest is on-condition component maintenance. Also, interchangeability of parts is a concern.
3.The basic issues regarding RLO:
1. Component fatigue - The question of usage defined in cycles vs. flight hours.
2. The lack of clear and consistent definitions regarding industrial operations.
3. The nature of the cyclic operations should be
reflected in maintenance.
No. / Certification Spectrum Condition / Certifi-cation(%) / Short Haul Atlanta (%) / Gulf Coast
(%) / Change: Atlanta/
Certifi-cation
1 / IGE 90o Right Hover Turn / 0.0700 / 0.9421 / 0.4330 / 13.4
2 / IGE 90o Left Hover Turn / 0.0700 / 1.2715 / 0.3809 / 18.1
3 / Normal takeoff and Acceleration to Climb Airspeed / 1.500 / 6.2583 / 0.1323 / 4.17
4 / 0.4 Vh Level Flight ant 324 rpm / 0.200 / 2.5246 / 0 / 12.7
5 / 0.8 Vh Level Flight ant 324 rpm / 3.000 / 25.7597 / 2.6945 / 8.5
6 / 0.9 Vh Level Flight ant 324 rpm / 4.000 / 13.6669 / 8.9187 / 3.4
7 / 0.6 Vh Right Turn / 1.000 / 4.7646 / 1.2422 / 4.7
8 / 0.9 Vh Right Turn / 1.000 / 4.0439 / 0.2726 / 4.0
9 / 0.6 Vh Left Turn / 1.000 / 2.8248 / 0.4894 / 2.8
10 / 0.9 Vh Left Turn / 1.000 / 4.3725 / 0.3962 / 4.3
11 / 0.8 Vh Level Flight at 314 rpm / 12.000 / 6.4399 / 0.6736 / 0.51
12 / 0.9 Vh Level Flight at 314 rpm / 16.000 / 3.4167 / 8.9187 / 0.21
13 / 1.0 Vh Level Flight at 314 rpm / 30.400 / 1.2926 / 12.641 / 0.042
Table 1.
Data from report “Continued Evaluation and Spectrum Development of a Health
and usage Monitoring System - DOT/FAA/AR-04/6 May 2004”
Conclusions:
* There is a need to enhance traceability and history of the critical components.
*There is a need for common cycle definitions with simple accounting methods.
*Critical component replacement should be based on operational cycles verses flight hours.
*Maintenance should be established based
on cycles verses flight hours.