By CHARLES W. LENT JR. Directorate of Evaluation and Standardization, Fort Rucker, Ala. June 1, 2011
Since 2005, a tactic, technique and procedure has been slowly gaining acceptance in the UH-60 community that may not be the correct response in all decreasing rotor situations. The mission requirements in Afghanistan have forced H-60 aircrews to perform missions at the limits of aircraft engine performance. Most Army aviators have not experienced these environmental conditions, which require an understanding of engine gas generator speed and fuel flow limiting.
Although the operator’s manual includes information on turbine gas temperature limiting, there is little information on fuel flow and NG limiting. Because TGT is the only method of engine limiting mentioned, pilots may believe that bypassing the TGT limiting function of the Electronic Control Unit/Digital Electronic Control Unit will always offer additional power. It is critical for aviators to understand the conditions that cause the engine limiting before placing an engine in lockout.
The General Electric T700-series engine limits maximum torque available in one of three ways: TGT, NG or fuel flow. Typically, H-60 pilots have been trained to rely on TGT as the best indicator of aircraft power. Until recently, most H-60 pilots flew missions in environments in which TGT was generally the engine-limiting factor. When limited by TGT, bypassing the ECU/DECU limiting function would allow the pilot to increase torque by 2 to 4 percent beyond the dual-engine limiter. When operating in cold environments (below 0 F), the T700-series engine may reach an NG or fuel flow limit before a TGT limit. Below minus 20 C, the engine will always be NG limited and TGT will not reach the dual-engine limiter value.
Here is the danger; pilots who rely only on TGT and fail to consider NG or fuel flow limitations when determining the additional power beyond the maximum torque available may be in for a nasty surprise. That additional power may not be there, a situation that could delay a successful recovery or escape plan. The current charts in the operator’s manual, tabular data and the integrated performance aircraft configuration software do not specify whether the maximum torque available figure is TGT, NG or fuel flow limited. However, all give an accurate maximum torque available value regardless of the limiting factor.
Power Limited Approaches and the Value of Escape Routes
Rotary-wing aircraft supporting Operation Enduring Freedom are often required to take off and land at high gross weights in power-limited situations. Anytime a pilot determines he is in a power-limited situation, it becomes even more imperative to have an executable escape plan for the entire takeoff or landing sequence. A limited power situation is not a go/no-go event since conditions such as wind, turbulence, pilot control input and power required for the deceleration for landing aren’t precisely predictable and aren’t factored into torque values. Variables may change during the takeoff or landing, causing pilots to exceed the planned and calculated power limit. It is critical while conducting landings during TGT, NG or fuel flow limited power situations that an escape must be executed whenever a rotor droop occurs or anytime power is in question.
Limited power margins should be an indicator to the pilot in command as to whether to attempt the maneuver. As the margin between power available and power required becomes smaller, the quality and necessity of an executable escape plan should be the determining factor in deciding to conduct an approach. Issues such as power to overcome wind, turbulence, downdrafts and deceleration must be factored into the maneuver. Climb/descent power available must be determined before beginning the maneuver and the ability to execute an escape at any point is critical. Where power requirements may be marginal and cannot be accurately calculated, it may be necessary to verify power available by applying power at the same conditions as the landing zone before the approach.
When conducting limited power approaches, Task 1011 of the aircrew training manual states: “Determining aircraft performance using tabular data, requires that aircrews update performance data when there is an intent to take off or land when operating within 3,000 pounds MAX ALLOWABLE GWT OGE, and when there is an increase of 1,000 feet pressure altitude, and/or 5 C from the planned PPC.” Currently, the only method of calculating the data to meet this standard is the tabular data located in the operator’s checklist or by using the charts in the operator’s manual. During the next revision of the ATM, Task 1011 will be updated to include the use of IPAC software to derive values.
Landing Zone Sequence a Proven Procedure
The Directorate of Evaluation and Standardization, in coordination with U.S. Army Forces Command and 21st Cavalry Brigade, have been involved in training units before deployment to Afghanistan in these limited power situations. The High Altitude Mountain Environmental Training package includes mountain flying considerations, power management, multi-aircraft and night vision goggle operations. It also includes an LZ sequence that is used for all approaches, simulating marginal power and includes terrain analysis. Originally adopted from the High Altitude Aviation Training Site program of instruction taught at Eagle, Colo., it is an invaluable and proven technique for determining margin available versus power required, a vital consideration when conducting limited power operations. Although trained in mountainous conditions, the techniques can apply to takeoffs and landings in any limited power environment. The next revision of the H-60 ATM will include the following procedure:
LANDING ZONE SEQUENCE
-Note temperature at LZ.
-Note pressure altitude of LZ on altimeter setting of 29.92.
-Size, slope, surface, long-axis, obstacles.
3. POWER REQUIREMENTS
-Tab data/IPAC Max OGE wt _______
-A/C wt (zero fuel wt + fuel) _______
-Difference (+/-) _______
-Percent torque (TQ) (+/-) _______
-Max TQ (Verbalize) _______
-Hover TQ (Verbalize) _______
-Assessment of the direction and velocity of the wind by cockpit indicators, visual indicators, GPS, last known forecast wind, or flight maneuvers.
-Analysis of terrain, trees, buildings and their effects upon wind creating updrafts, downdrafts, headwinds, tailwinds, crosswinds and demarcation lines from a large scale down to the touchdown point.
5. ROUTE IN/OUT/ESCAPE
-Wind should dictate route in, out and escape.
-In calm wind, use the route that affords the best escape.
6. LOW RECONNAISSANCE
-Verify wind by using cockpit indicators.
-Ground track versus heading.
-Airspeed versus true airspeed (convert IAS to TAS to make this step accurate).
-A/S versus TQ versus VSI (vertical speed indicator).
- Verify touchdown point and suitability.
-Predicted TQ for approach, hover and takeoff. This is an adjustment of the hover TQ, considering level surface and zero wind.
-Expended TQ is the highest amount of TQ used during any part of the maneuvering, approach and takeoff.
-Actual TQ is the amount of TQ to hover.
-If there is a difference between TQ values, discuss why.
In summary, the GE T700 engine limits maximum torque available in one of three ways: TGT, NG or fuel flow limiting. Pilots must have an understanding of the conditions that cause each type of limiting and should rely on the maximum torque available figure derived from the IPAC software, operator’s manual or tabular data when determining maximum power available. Pilots should not focus on TGT as the sole indicator of engine power below 0 C when operating with a T700 engine. Nor should they make the false assumption that placing an engine in ECU/DECU lockout will offer additional power in all environmental conditions. The torque increase of 2 to 4 percent gained when the T700 series engine limits by TGT and is placed to lockout must be secondary to having an accurate knowledge of power margin available and an executable escape plan during limited power approaches.