Climb performance data poses challenge for pilots

 - February 2, 2007, 9:58 AM

Since the early days of powered flight, aircraft performance has been a mainstay of the pilot-training process. By the time pilots reach the left seat of a turbine-powered airplane they understand how their aircraft perform, or at least they believe they do.
Nat Iyengar, a training captain at Cleveland-based Flight Options, suggests, however, that this is not always the case. He says, “Aircraft performance, especially takeoff performance, is a horribly misunderstood subject.” Rich Boll, a training captain at Koch Business Holdings in Wichita, agrees: “There is probably more confusion in corporate aviation about performance than any other area of aircraft operations.”

Iyengar elaborated, “There are misconceptions that have been passed down through training over the years, and these misconceptions just snowball. We learn all about Part 25 performance and limitations in training. But then you get one instructor who gives you one idea of what to do [in the real world] and another who tells you something else.”

Boll added, “This system of explaining aircraft performance can confuse people if they don’t know how we got to this point.” Boll is referring to the differences between the methods of calculating aircraft performance, with one system for certification performance and another set of requirements designed around terminal instrument procedures (TERPs) used to build approach and departure procedures.

In the real world of aircraft takeoff performance, for example, these seemingly minute differences mean that tab data (the tabular data in the back of the operator’s manual that allow the pilot to develop a formula that reveals how much power is actually available and provides a framework for using that power most effectively under varying circumstances) for takeoff at Richmond has little relevance when that same aircraft departs Aspen for a nonstop flight back to the East Coast. In sharp contrast to Part 91 operators, Part 121 carriers have it much easier. Airlines conduct an obstacle and performance analysis and publish all the necessary data in a format that is simple to understand. If airline crews want to operate from an airport for which no data exists, the answer is simple: they don’t.

Since the operator of a Part 91 corporate aircraft is normally involved in all flights and may even be sitting in the back seat, the feds give business aviation much greater leeway regarding how they fly their aircraft. It is here that pilot techniques– both good and bad–enter into the operational equation.

Providing Performance Data

One problem with teaching performance issues is that much of the data necessary to advance a pilot’s education is scattered through a variety of publications. That means pilots must make a conscientious effort to locate it all. “Sometimes you must hand all the work to them on a plate before they’ll use it. But even then, many won’t. They think, ‘I go to school and I know how to fly this airplane,’” said Iyengar.

In addition, as AIN learned in reviewing the 2001 Gulfstream III accident in Aspen, the information that is available is not always in a format that is easy to understand. In that crash, a variety of opinions existed about how to determine aircraft category. Some were correct; some were not. In an attempt to at least explain some of the differences in performance calculations, the FAA recently published an updated 2004 edition of the Instrument Procedures Handbook (IPH) at http://av-info. faa.gov/terps/.

Again, the agency has not revised the variety of performance data standards, but has simply looked at new venues to explain what already exists. Some experts believe these standards have not and will not change because they were originally created for the airlines that still use them today. And no one wants to drop another shoe on Part 121 operators right now.

Takeoff Performance Requirements
The basis for takeoff performance confusion begins with certification. All Part 25 aircraft are required to meet a variety of criteria that prove the aircraft is capable of specific climb requirements if one engine should fail. Part 23 aircraft, by comparison, need only be able to maintain a positive climb rate under similar circumstances.

Part 25 data is derived primarily from actual flight testing, although computer modeling may provide some additional data about the aircraft’s flight envelope. This data is eventually transformed into charts that must be observed as true aircraft limitations relative to density altitude. But this data does not consider whether or not there are obstacles in the flight path of the aircraft on departure, hence the involvement of TERPs.

As an example, departure charts were renamed in early 2001. The standard instrument departure procedures (SIDs) are now called system enhancement departure procedures because they apply only to procedures developed to better organize ATC. The old IFR departure procedures are now called obstacle departures and are intended only to keep aircraft clear of terrain in unusual situations that require standards different from certification numbers.

Tab data was developed as a simplified method of determining takeoff numbers based upon a specific temperature, weight and density altitude, without the need to calculate every number precisely. Essentially, this works at low field elevations.

Tab data charts also quickly identify the important “V” speeds necessary to evaluate takeoff numbers. The V1 decision speed is where some of the confusion often begins.
Many pilots view this as the speed at which they decide whether or not to continue the takeoff. However, pilots must have decided whether to complete the takeoff by the time the airplane reaches V1. If the pilot waits until V1 to make the decision to abort, the aircraft will actually be committed to the takeoff at conditions that almost guarantee a trip off the end of the runway. As a safety precaution, many departments actually call V1 five knots early to give the pilots time to act before reaching the critical spot in the takeoff run.

The logic is simple. If V1 is 100 knots, the groundspeed is approximately 170 feet per second. The time required for the pilot to recognize and react to a problem significant enough to warrant an abort, when added to the time and distance required for the aircraft to react to control inputs, can easily overextend the ground run. And most important, during that time between passing V1 and the time needed to begin slowing down, the aircraft is actually still accelerating, eating up even more valuable concrete.

An “80 knots” call is commonplace as the time for pilots to cross check all critical flight instruments. This point should also trigger the transition from a speed arena where an aborted takeoff is possible and still quite safe, to a time where a rejected takeoff becomes an extremely serious maneuver that can trigger other unexpected consequences if handled incorrectly. Most takeoff briefings include a discussion of aborts above 80 knots and before V1. It is wise to review them to ensure both crewmembers completely understand the circumstances that will call for that high-speed abort. Normally, the only reason for an abort in this regime
is an engine failure, a directional control problem or something that might prevent flight.

Vr is the speed at which rotation to the proper liftoff attitude should commence. In some aircraft, rotation will be synonymous with aircraft liftoff. V2, also known as takeoff safety speed, is the initial climb speed of the aircraft in the takeoff configuration. It is not necessarily the speed for best rate or best angle of climb. What it does mean is that holding V2 for a predetermined time will ensure a climb pretty much as detailed in the aircraft performance manual. While many pilots of turbine aircraft choose to climb at a higher speed, this technique trades precious altitude for speed at a time when it could yet be needed should one engine quit.

Another takeoff consideration is flap settings. A standard flap setting under most conditions is designed to offer the shortest possible takeoff ground run. Seldom, except when they are forced by aircraft weight, or in high-altitude operations, do crews consider the possibilities of a reduced flap setting on other kinds of takeoff.

Certainly there are tradeoffs. While the downside is that a reduced flap setting increases takeoff ground run, it will significantly improve climb performance should an engine fail after takeoff.

The takeoff path is divided into segments and, according to the FAA, is measured from a reference point 35 feet above the runway after liftoff. Once they leave 1,500 feet agl, however, pilots are on their own. During certification an aircraft’s gross climb rate for these segments is based upon a minimum of 2.4 percent, or 2.4 feet vertically for every 100 feet of horizontal travel.

Net climb rate, based upon a 1.6-percent gradient, is simply the gross figure with a fudge factor for two-engine aircraft. Those original aircraft flight manual numbers were gathered by a test pilot under sterile conditions with new engines, so pilots should add a small margin for safety in case of an emergency.

The second segment is normally the most limiting factor of takeoff and begins where the first segment ends, around 400 feet agl, unless there are obstacles.
Second-segment climb rates are actually lower than most pilots believe and are based upon an average of 200 feet per nautical mile. This rate offers a scant 48 feet of clearance from any obstacle if the pilot maintains the required rate, hence the need for some extra performance margin options. The third segment begins when the aircraft levels off and flaps and slats are retracted and ends when the aircraft climbs through 1,500 feet agl.

Determining Takeoff Criteria
Looking at the numbers, determining safe takeoff criteria is not easy. Aircraft manufacturers’ performance charts give climb gradients in percentages. TERPs procedures, such as obstacle departures and SIDs, list performance figures in feet per nautical mile, requiring more arithmetic for safety.

Pilots talk about aircraft performance in feet per minute. What cockpit instrument measures net climb gradient? The answer, quite simply, is none. That means the crew must calculate a target rate to keep in the back of their head to be certain they will clear the granite. Work the math backwards to calculate the net climb gradient and look at the aircraft performance charts for confirmation. But without a target vertical guidance rate, obstacle departures are a bit like flying an aircraft in actual IFR conditions near areas of thunderstorms and simply hoping not to hit one.

Gulfstream’s vice president of flight operations, Randy Gaston, agreed that lack of cockpit information is a problem. “No pilot wants to go flipping through printed charts for climb information. Nothing I know gives feedback to the pilot on how the climb is progressing. It would be a small effort to do this, but I don’t know anyone who does. When they go true single engine, most pilots are amazed at how much performance they’ve lost. Your eye closes the loop very quickly and you focus on your speed. But we should have something that takes all the mental gymnastics out of this for the pilot.”

Everything discussed to this point basically relates to Part 25 certification data. TERPs uses different standards. Another problem trying to comply with TERPs is that the required climb gradient does not account for changes in aircraft climb performance during the climb itself. And TERPs applies only to the U.S. and a few other countries.

Outside the limited TERPs arena, ICAO’s PANOPS is the determining document. Perhaps the new FAA IPH offers some guidance. All the IPH says, however, is, “The rate of climb table (see graphic on page 38) used in conjunction with the aircraft performance specifications in your aircraft flight manual will help you determine your ability to comply with climb gradients.”

Interestingly, a departure procedure designed under TERPs requires a 3.3-percent climb gradient under most conditions, which means that pilots need to convert everything into feet per nautical mile and then to feet per minute to ensure obstacle clearance. But there are also close-in obstacles on departure deemed “low, close-in obstacles.” These are simply noted on a departure chart with symbols like a tree or a small hill. No increase in the climb gradient is added, so the pilot had best be able to see and avoid these on takeoff if an engine stops turning and burning.

Lou Poth, an instructor in the G550 program at FlightSafety International’s Savannah Learning Center, said, “This is odd that the pilot must go elsewhere for the actual climb gradient data [converting feet per nautical mile to feet per minute]. Honeywell knows the lateral and vertical guidance you need. But how pilots interpret the regulations [to clear the obstacles] is up to them, I guess.”

Determining Takeoff Data for Special Circumstances

Think we’re being too cautious? If you haven’t been to Eagle, Colo., lately, look at the departure procedure and think about that big hill off the west end of the runway as you enter the clouds. Gathering the numbers for high-altitude departures is definite work and it’s made more difficult by the variety of standards that manufacturers and the FAA use.

Boll said, “Day-to-day flight operations don’t worry me too much. But when they’re operating from high-altitude airports, or in the mountains, pilots need to think about what their aircraft can and cannot do if an engine fails on takeoff. If that engine does quit, what exactly will they do?”

Here, a general takeoff briefing simply will not suffice. A common takeoff procedure out of Aspen is to maintain VFR and head down the valley for Rifle if an engine quits. “Think about this,” said Bolls. “You’ve lost an engine and you’re at V2 with a 15-degree deck angle. Navigating around those rocks to Rifle might get pretty exciting, especially if you’ve never tried it before.”

Iyengar asks, “Why don’t the manufacturers give us data the way it is developed for large aircraft? I don’t think they want to in case someone cuts it too close and hits a hill.” However, he’s also learned that pilots are much more receptive to very specific climb performance training when they understand the pitfalls behind the data that currently exists. “Once they understand the logic about how these procedures were designed and the real problems they might face, they start to see the big picture,” he said.

Boll said, “It is important to truly understand the rules and know the differences between commercial Part 135 operations (clear the obstacles) and Part 91 (you’re on your own). Although I haven’t heard about anyone losing an engine out of Aspen or Eagle recently, that’s probably because turbine engines are very reliable. But it is going to happen and someone will hit one of those hills.”   

COMPOSITE HELIPAD

French company Thomann-Hanry is offering a new, lightweight helipad made of composite material. Dubbed Helistop, it is said to be easily adaptable to existing buildings that were not necessarily designed to withstand the weight of conventional, concrete helipads. The concept was developed under Eureka, a European research-funding program, with Spanish and Italian composite-expert firms. Helistop sells for approximately $600,000 for a helipad that can accommodate a Eurocopter EC 135.

“Our composite helipad weighs about 10 pounds per square foot,” Rémy Platel, Thomann-Hanry’s manager, told AIN. Under European standards, approximately 5,900 square feet are necessary for a helipad for an EC 135 light twin-turbine rotorcraft. The total 28 tons compares to “about 300 tons” for a conventional helipad, according to Platel.

According to the company, the composite material offers “a better resistance to fire and corrosion” than aluminum, another material used to construct helipads. In addition, repairing a composite helipad is simpler than repairing one constructed out of concrete; since the helipad is made of a number of boxes, local damage can be repaired by replacing only one or two of them.

Four Helistops have been sold so far, to customers in the Middle East. The manufacturer is expecting a number of sales in the emerging market
of French hospitals’ replacement of obsolete helipads