Paro International Airport, in the kingdom of Bhutan, is high in the Himalayan Mountains. At 7,300 ft (2.23 km) above sea level, with a runway 6,500 ft (1.99 km) long, surrounded by deep valleys and 18,000-ft (5.48-km) peaks, Paro is one of the world’s most difficult airports for takeoffs and landings.

In February 2003, a Boeing 737-700 successfully completed 11 test flights at Paro International Airport. The series included two technical demonstration flights and eight customer relations flights with Druk Air Royal Bhutan Airlines, the national airline of Bhutan. Druk Air, which operates two 72-passenger BAe 146-100 jets from Paro to six cities in five countries, is considering upgrading its fleet and extending its routes. The rigorous test flights proved that the 737-700 is capable of meeting all performance and procedural requirements for safe operations at Paro and other airports in high-elevation, high-terrain environments.

The 737-700 performed flight maneuvers as predicted and met or exceeded performance expectations for simulated one-engine-inoperative maneuvers, which were accomplished by reducing thrust on one engine to idle power. The expected performance levels proved conservative when compared with the demonstrated performance of the 737-700.

Test flight data were verified by flight data recorder (FDR) information, indicating that predicted airplane performance is representative of actual airplane performance as recorded by the FDR.

The test flights verified procedures for takeoff and landing operations at Paro. The 737-700 demonstrated engine-out takeoff procedures, which is required for Paro operations, engine-out missed approach and go-around procedures, and Druk Air procedures for landing on both directions of the runway at Paro.

This article discusses

1. Technical demonstration test flight airplane.
   
2. Technical demonstration test flights description.
   
3. Technical demonstration test flight analysis.

1. TECHNICAL DEMONSTRATION TEST FLIGHT AIRPLANE

The demonstration airplane was a 737-700 Boeing Business Jet (BBJ) configured with blended winglets and a business jet interior (fig. 1 and table 1). The 737-700 BBJ used for the demonstration flights is aerodynamically equivalent to the commercial variant of the 737-700 being offered to Druk Air.

2. TECHNICAL DEMONSTRATION TEST FLIGHTS DESCRIPTION

On February 6, 2003, two technical demonstration test flights were accomplished from runways 33 and 15 at Paro International Airport. Boeing pilots Captain Buzz Nelson and Captain Van Chaney flew the 737-700 accompanied by the Druk Air chief pilot on the first flight and a senior first officer on the second flight.

To prove the capability of the 737-700 at Paro, the technical demonstration flights had to show that the airplane could take off following a simulated single engine failure at the most critical point during the takeoff ground roll (V1) and safely return to the airport on one engine.

Terrain in the valleys surrounding Paro limits takeoff performance. Flight operations into and out of Paro only occur when the visibility in the valley is clear. This visibility is required to allow an airplane to turn around safely within the steep valley walls and reach the minimum safe altitude to depart the valley or return to the airport in the event of an engine failure.

Runway 33 Technical Demonstration Test Flight
The first technical demonstration test flight was performed from runway 33 (fig. 2). After takeoff, a right bank was initiated for a heading change of approximately 30 deg to avoid terrain that extends from the west valley wall. This maneuver was followed by a left bank to position the airplane along the east wall of the west fork of the river. The climb continued close to the east wall until the turnback initiation point. A teardrop turnback was initiated just after passing abeam the Chhukha village. Here the terrain falls away off the right wing where a stream empties into the river. The turnback was flown with a 30-deg bank while maintaining speed throughout the turn.

After completing the teardrop maneuver, the pilots performed a flaps 15 (engine-out landing flap) missed approach to runway 15. This was followed by a go-around and a teardrop turnback south of the runway using the Druk Air runway 15 turnback procedure. The condition was completed successfully with a normal flaps 40 landing using the Druk Air straight-in landing procedure.

The takeoff weight for runway 33 is limited by the turning radius required to perform a 30-deg bank turnback. The available turning radius is based on the valley width at the net height achievable while maintaining not less than 492 ft (150 m) of lateral separation to the terrain and all obstacles on either side of the intended track. The limit weight calculations were based on the valley width at the net height for turnback initiation, assuming that the airplane was positioned within 492 ft (150 m) of the valley wall.

The takeoff gross weight for the technical demonstration was calculated based on the airplane empty weight and the weight of the crew, passengers, and fuel on board (table 2). Table 3 lists the airport conditions and airplane configuration and takeoff speeds.

Engine failure was simulated by throttling back the left engine to idle at 125 kias, the V1 speed for takeoff.

The second technical demonstration test flight was performed from runway 15 (fig. 3).

After liftoff, a right bank was performed for a 10-deg heading change, followed by a left bank for a 60-deg heading change. The left bank took the airplane across the valley toward the east wall and avoided a hill that extends from the west wall of the valley.

A right bank was then held for an approximate 95-deg heading change, which directed the airplane from the east side of the valley back toward the west side and the Silung Nang village. The airplane flew over Silung Nang and the ridge behind it, which required an altitude of 9,100 ft (2.77 km). After the airplane cleared the ridge, a turnback was initiated with a 30-deg bank while maintaining the designated V2 speed.

After completing the turnback maneuver, the pilots performed a flaps 15 (engine-out landing flap) missed approach to runway 33, followed by a go-around and a teardrop turnback north of the runway using the runway 33 turnback procedure. The condition was completed successfully with a normal flaps 15 landing using the Druk Air straight-in landing procedure on runway 15.

The turnback procedure limit for runway 15 originally was determined to be the turning radius required to perform the 30-deg bank turnback. This limit was based on the valley width at the net height achieved while maintaining a minimum 492-ft (150-m) splay outside the intended track.

However, before the technical demonstration flights, Boeing and Druk Air pilots flew practice flights. After these flights, the pilots determined that the critical requirement was clearing the ridge beyond the village of Silung Nang, which requires a net height of 9,100 ft (2.77 km) at the turn initiation point. The limit weight calculations were based on the requirement to achieve this height on the net flight path. The turn radius was not limiting at this condition, assuming a 30-deg bank.

Table 2 shows the airplane takeoff gross weight for the runway 15 technical demonstration flight. Table 4 lists the airport conditions and airplane configuration and takeoff speeds.

Engine failure was simulated by throttling back the right engine to idle at 124 kias, the V1 speed for takeoff.

3. TECHNICAL DEMONSTRATION TEST FLIGHT ANALYSIS

FDR Analysis
FDR information was downloaded from the airplane after the technical demonstration test flights. The FDR flight paths were compared with profiles of predicted performance to verify the capability to match actual flight profiles.

Figures 4 and 5 show the ground tracks and altitude profiles for the demonstration test flights from runways 33 and 15, respectively.

The calculated flight paths with one engine pulled back to idle thrust closely match the demonstrated flight paths. The calculated altitudes at the turnback initiation points for both flights were within 50 ft (15 m) on the conservative side of the predicted altitudes.

Turn Procedure Optimization
For the technical demonstration test flight from runway 15, takeoff weight was limited by the Druk Air procedural requirement to fly over a ridge after flying south over Silung Nang. To clear the ridge, a net altitude of 9,100 ft (2.77 km) is necessary at the turn initiation point.

Flying parallel to the valley wall near Silung Nang instead of crossing the ridge removes the requirement to reach 9,100 ft (2.77 km) and allows a greater takeoff weight. The performance then becomes limited by the width of the valley. Through careful selection of the turn initiation point, additional takeoff weight is possible.

The available turn radius as a function of altitude was determined by computing the maximum turn radius available in the valley at each altitude line on a digitized topography map. Maximum takeoff weight was calculated by plotting the available turn radius and the turn radius required as a function of gross takeoff weight.

Figure 6 is an example takeoff weight calculation. The bottom plot shows the airplane altitude at the turn initiation point. The middle plot shows the corresponding airspeeds. The top plot shows the turn diameter required for a 30-deg bank, which is solely a function of true airspeed and bank angle, and the turn diameter available at the corresponding net height.

Boeing continues to investigate product and operational improvements as part of its commitment to ensure that current and potential customers can maximize payload capability during safe takeoff and landing operations in high-elevation, high-terrain environments.

Editor’s note: Druk Air Royal Bhutan Airlines only operates BAe 146-100 jets at this time; Boeing is not maximizing the payload capability of the BAe jets.

• IMPORTANCE OF SPEED DURING TAKEOFF TURNBACK PROCEDURES
• TAKEOFF CAPABILITY REFINEMENT
• INCREASE TAKEOFF GROSS WEIGHT USING CFM56-7B26/B2 THRUST RATING AND ALTERNATE CENTER OF GRAVITY

VAN CHANEY
FLIGHT TEST PILOT
FLIGHT OPERATIONS PRODUCTION
BOEING COMMERCIAL AIRPLANES

MAGALY CRUZ
PERFORMANCE, AERODYNAMICS ENGINEER
FLIGHT OPERATIONS ENGINEERING
BOEING COMMERCIAL AIRPLANES

BUZZ NELSON
CHIEF PROJECT PILOT
FLIGHT OPERATIONS
BOEING COMMERCIAL AIRPLANES

ALLEN ROHNER
REGIONAL DIRECTOR
MARKETING
BOEING COMMERCIAL AIRPLANES

MIGUEL SANTOS
DIRECTOR
INTERNATIONAL SALES
BOEING COMMERCIAL AIRPLANES

JAMES WILSON
PERFORMANCE, AERODYNAMICS ENGINEER
SALES SUPPORT AERODYNAMICS
BOEING COMMERCIAL AIRPLANES