Apr 21, 2007

Required Rate of Climb?

All jet transport take-offs are planned to meet specific minimum climb constraints in case of an engine failure. Of course engine-out performance is more critical for a two-engined aircraft than for a three or four-engined version. When we lose an engine on a twin our remaining performance falls to less than half.

To meet a gradient of 200 feet climb per nautical mile, at a climb speed of 120 kts (i.e. 2 nm/minute) this translates to a 400 fpm rate of climb on a single engine. Climbing at 180 knots, we'd need a vertical speed of 600 feet per minute. So generally, our normal two-engine climb rate never gets much below 1,000 feet per minute.

With a lightly-loaded A319, which has a better power-to-weight ratio than the A320, I've seen the VSI hit almost 5,000 fpm. That's just plane fun! (pun intended - sorry).

These are very general numbers to convey the concept. For a more technical discussion of climb requirements, here's an excellent article:

Transport Canada > Civil Aviation > Civil Aviation Publications > TP 185 - Aviation Safety Letter

Obstacle Clearance During Takeoff
by Captain Robert Kostecka, Civil Aviation Safety Inspector, Foreign Inspection, International Aviation, Civil Aviation, Transport Canada

A failure to understand some of the important aspects of aircraft performance can have a tremendous impact on flight safety. It is not hard to imagine a situation where a lack of aircraft performance knowledge could have catastrophic consequences.

Let’s assume that you are the Captain of a Transport Category jet aircraft that is about to depart from Québec City on a flight to Europe. Tonight your aircraft will be very heavy. You are carrying a full load of passengers and are tankering extra fuel. The weather is 300 ft overcast, 1 mile in rain showers. As you taxi to position on Runway 06, you review the Québec Two Departure again: “Climb to ‘BV’ NDB then track 064° outbound...” maintain 4 000 ft.

You advance the thrust levers and the aircraft accelerates down the runway. Your First Officer calls “V1,” then“rotate” and you smoothly pitch the nose up. As the aircraft lifts into the night sky, your First Officer advises, “positive rate,” and you reply, “gear up.”

Just after you become airborne, the No. 2 engine fails. Instinctively, you apply rudder to control the yaw and adjust your pitch attitude. You fly the aircraft smoothly and precisely. Your many years of training appear to be paying off. It flies just like the simulator, you quietly think to yourself.

As per your company’s standard operating procedures (SOP), you engage the autopilot, select heading mode and call for the engine failure drill. You continue to follow the Québec Two Departure: “Climb to ‘BV’ NDB then track 064° outbound...” As your First Officer proceeds with the drill, the ground-proximity warning system suddenly barks:“Too low, terrain.” This can’t be right, you think, as your heart races. Your eyes dart to the vertical speed indicator. It indicates that you are in a steady climb. But the radar altimeter only shows 100 ft—and it is decreasing rapidly. You have no time left to understand what is happening.

How could this occur? Why would an aircraft that is being flown smoothly and precisely impact the ground? Aren’t Transport Category aircraft supposed to have sufficient climb performance—even with an engine failure? Isn’t obstacle clearance guaranteed if we fly the published instrument departure procedure? Most importantly, how can we ensure that an accident like this doesn’t actually happen? These are important questions. In answering them, we’ll review some of the important issues of aircraft performance.


It is vitally important for pilots and air operators to realize that the obstacle clearance provided by a published instrument departure procedure is based on all-engine aircraft performance. Following a published instrument departure procedure will not necessarily guarantee obstacle clearance following an engine failure.

To begin, we must understand the obstacle clearance requirements for published instrument departure procedures. These can be found in Transport Canada publication TP 308, Criteria for the Development of Instrument Procedures. TP 308 states that an obstacle clearance plane, with a slope of 152 ft/NM, is required. Aircraft must remain above the obstacle clearance plane and are expected to maintain a climb gradient of 200 ft/NM. In the event that an obstacle penetrates the normal obstruction clearance plane, a climb gradient greater than 200 ft/NM is specified. This is the case in Québec City on runway 30, where aircraft are expected to climb at least 290 ft/NM.

It is vitally important for pilots and air operators to realize that the obstacle clearance provided by published instrument departure procedures is based on all‑engine aircraft performance. In the event of an engine failure, the aircraft may not be able to achieve the required climb performance. Following a published instrument departure procedure will not necessarily guarantee obstacle clearance following an engine failure.

The aircraft’s climb performance with an engine inoperative may not meet the obstacle clearance requirements provided in published instrument departure procedures.
The aircraft’s climb performance with an engine inoperative may not meet the obstacle clearance requirements provided in published instrument departure procedures.

The regulations require airline operators to limit weight during takeoff so that the aircraft will clear all obstacles during takeoff—even with a failure of the most critical engine. Subsection 705.57(1) of the Canadian Aviation Regulations (CARs), Net Take-off Flight Path, specifies that, “No person shall conduct a take-off in an aeroplane if the weight of the aeroplane is greater than the weight specified in the aircraft flight manual as allowing a net take-off flight path that clears all obstacles by at least 35 ft vertically or at least 200 ft horizontally within the aerodrome boundaries, and by at least 300 ft horizontally outside those boundaries.” (The “net take-off flight path” is the aircraft’s actual or “gross flight take-off flight path”—that was determined through flight testing—decreased by a margin. For two-engine aircraft, the gradient is reduced by 0.8 percent. This margin is intended to account for less-than-perfect pilot technique and slight degradations in aircraft performance.)

Airlines comply with this regulation by considering the obstacles in the take-off path and verifying that their aircraft will clear all obstacles by the required margin. In addition to obstacles, this analysis considers all of the factors that could affect the takeoff: the characteristics of each individual runway—including the slope, pressurealtitude, ambient temperature and wind component. This information is used to produce special charts that are known as Airport Analysis Charts. (Some air operators refer to their Airport Analysis Charts as WAT Charts.)

Airport Analysis Charts specify the maximum allowable weights for takeoff under various conditions. This data is based on the aircraft following a specified engine-out path during the takeoff. The airline may choose to follow the published instrument departure procedure or they may choose a straight-out path, along the extended runway centreline, as their standard engine-out flight path.

In some cases, because of high terrain or other obstacles, following the published instrument departure procedure or a straight out path will not provide the required obstacle clearance following an engine failure. In these cases, “special” engine-out departure procedures—that allow obstacles to be avoided laterally—are provided. These special procedures include a turn (or a series of turns), as well as the specific headings or tracks that must be flown in order to avoid obstacles.

In our fictional engine failure during takeoff that we discussed earlier, the aircraft ran into the high terrain that is northeast of the ‘BV’ NDB. This could have been prevented if the proper engine-out path—on which the Airport Analysis Chart was based—had been followed. This special engine-out procedure required the aircraft to turn right at the ‘BV’ NDB, so that the obstacles could be avoided. (Instead we followed the published instrument departure procedure.)

It is important to understand which procedure has been used to establish the engine-out departure path. If an engine failure occurs, flight crews must know whether they should follow the published instrument departure procedure, fly straight-out on the runway heading, or follow a “special” engine-inoperative procedure.

Weight must be limited so that the net take-off flight path will clear all obstacles by at least 35 ft vertically (CAR 705.57). The“net take-off flight path” is the aircraft’s actual or “gross flight take-off flight path”—that was determined through flight testing—decreased by a margin that is intended to account for less-than-perfect pilot technique and slight degradations in aircraft performance.
Weight must be limited so that the net take-off flight path will clear all obstacles by at least 35 ft vertically (CAR 705.57). The“net take-off flight path” is the aircraft’s actual or “gross flight take-off flight path”—that was determined through flight testing—decreased by a margin that is intended to account for less-than-perfect pilot technique and slight degradations in aircraft performance.

Increasing the altitude for level acceleration and flap retraction (extending the second segment of climb) is another method that is used to ensure obstacle clearance. Pilots must know if the engine-out procedure requires this technique. In addition, if a special engine-out procedure has a turn (or a series of turns), pilots should know whether they should delay flap retraction until after completion of the turn. (This is because of the effect of acceleration on turn radius.)

In an emergency, pilots are authorized to deviate from published instrument departure procedures in order to ensure obstacle clearance with an inoperative engine. (An emergency should be declared as soon as practicable, so that air traffic control is alerted and can take appropriate action.) These special engine-out procedures allow airlines to carry profitable payloads, and still comply with the engine-inoperative obstacle clearance requirements of CAR 705.57, Net Take-off Flight Path.

When obstacles such as high terrain are a factor, it is important to have a way out should an engine fail. Properly-designed engine-inoperative take-off procedures will ensure that the aircraft is able to achieve a safe altitude. These procedures should terminate with the aircraft at minimum radar vectoring altitude, minimum sector safe altitude or 100-mile safe altitude. The obstacle clearance requirements for takeoff described in CAR 705.57, Net Take-off Flight Path, must be complied with until the en route obstacle clearance criteria of CAR 705.58, Enroute Limitations with One Engine Inoperative, can be met. Net take-off obstacle clearance requirements do not always end at 1 500 ft above ground level (AGL) or at an arbitrary distance from the runway.

A diversion to an alternate airport due to poor weather or a medical emergency can pose unique challenges. In addition to having correct take-off data for airports that are normally used by the airline, it is recommended that arrangements be made for obtaining take-off data in the event of an unscheduled diversion. Pilots and dispatchers should know how to obtain accurate take-off data—which properly assesses obstacles—when an aircraft has had to make an unscheduled landing at an unfamiliar airport.

Good airmanship requires us to expect the unexpected. To fly safely, we must anticipate what can go wrong—and develop a plan. The engine-out departure paths, on which the Airport Analysis Charts are based, provide a plan that allows airlines to take off at heavy weights, while still ensuring obstacle clearance in the event of an engine failure.

References:

TP 308, Criteria for the Development of Instrument Procedures
CAR 705.57, Net Take-off Flight Path
CAR 705.58, Enroute Limitations with One Engine Inoperative
TP 12772, Aeroplane Performance


Prior to joining Transport Canada, Captain Kostecka worked as a pilot and instructor for several Canadian airlines. He has flown over 12000 hr and holds a Class 1 Flight Instructor Rating as well as type ratings on the A320, A330, A340, B757, B767, CRJ, DHC-8 and B-25.

Copyright and Credits

The Aviation Safety Letter is published quarterly by Transport Canada, Civil Aviation. It is distributed to all holders of a valid Canadian pilot licence or permit, and to all holders of a valid Canadian aircraft maintenance engineer (AME) licence.The contents do not necessarily reflect official policy and, unless stated, should not be construed as regulations or directives. ...

Reprints of original Aviation Safety Letter material are encouraged, but credit must be given to Transport Canada's Aviation Safety Letter. Please forward one copy of the reprinted article to the Editor.

2 comments:

Flyin Dutchman said...

Amazing article thanks for posting it !

I don't have to worry about single engine climb performance it's all I've got :)

As for twin engine time being so sought after by employers I can maybe see why. In an engine failure scenario for us we trim for glide, cross your arms and point your toes cause your going in deep ! :)

Cheers

Chris said...

Very good article! I learnt a lot from it.