Wednesday, May 10, 2006

Losing One

The captain that I flew with last trip, Steve, is a rather unlucky fellow when it comes to keeping engines running. He has experienced six engine failures at this airline, and several others before working here (including a catastrophic crankshaft failure in a single-engine C206). His most recent engine failure happened on his last trip, on takeoff out of Redding. They had just lifted off and retracted the gear when there was a bang and the left engine shut down. It was a nice VFR day so they just kept the airplane in the pattern, ran the appropriate checklists, and landed. The engine will be sent back to Pratt & Whitney for a postmortem, but preliminary analysis suggests a problem with the accessory gearbox, which drives the fuel metering unit.

Engine failures are relatively rare, but they are the failure that pilots train for most often, since a poorly handled engine failure can easily lead to a fatal accident. In a single-engine airplane, engine failure is deadly serious. Losing the only engine just after takeoff, over mountains, or at night all rank among pilots' worst nightmares. An alarmingly high number of accidents involve stalling and spinning in after the engine quits, even over terrain suitable for an emergency landing. (Edit: mechanical engine failures are rare. Fuel starvation is appalingly common.)

You would think that engine failure would be more survivable in a twin engine aircraft, but the available data suggests this isn't so. An old joke among pilots is that the second engine is there to fly you to the scene of the accident. When the failure occurs shortly after takeoff, you have two problems: low airspeed and low altitude. At low airspeeds, it's tough to maintain control of a twin that has full power on one side and lots of drag from a windmilling prop on the other. The airplane will yaw and roll violently, and the pilot must immediately apply aggressive control movements to maintain control. A large number of engine failure accidents in twins are simply due to failure to maintain control. I suspect that the last time many light twin pilots practiced engine-out maneuvers was during their training for the multi rating.

Then you have the low altitude in an aircraft that may or may not want to climb. There is no requirement that light twins be able to climb on a single engine, and some of the lower powered airplanes have dismal single engine climb rates, particularly at high density altitudes or heavy weights. Even capable airplanes like the Piper Navajo or Beech Baron are quite vulnerable if the engine failure occurs in the time between liftoff and Vyse, or single-engine best rate of climb speed. There are cases where even the best pilot may not be able to save the day.

In an airliner, engine failure is still a big deal, but you have a lot more going for you. For starters, you will always have adequate single-engine climb performance. For each takeoff, we look at performance data that is specific to each runway and takes temperature, wind, and aircraft configuration into account. It will give us a maximum weight for takeoff that assures adequate performance in the "worse case scenario" - that is, an engine failure right at V1 (see below). So, assuming the performance data is accurate, we know that the airplane will climb so long as we follow the proper engine-out procedures. For this, we have the benefit of realistic simulation training, and the fact that the non-flying pilot can take care of checklists, radio calls, etc, while the flying pilot concentrates on maintaining control of the airplane.

Airliners don't have the same V speeds as light aircraft, such as Vx, Vxse, Vy, or Vyse. Instead we have V1, Vr, V2, and Vse. These are calculated for each takeoff and are based on considerations such as aircraft weight, configuration, and runway/weather conditions. They are defined as follows:
  • V1 is "Decision speed." If the engine failure occurs prior to V1, the captain will abort the takeoff. If the engine failure occurs at or after V1, you keep going. V1 is calculated for each takeoff to ensure balanced field length. This means that for an engine failure at V1, it will take the same distance to continue the takeoff as it will to abort. So by aborting prior to V1 or continuing after, you're choosing the option that'll take less distance.
  • Vr is rotation speed, same as in a light plane.
  • V2 is takeoff safety speed. In case of engine failure, you climb at this speed until acceleration height. It's analogous to Vxse (single engine best angle of climb) in a light twin.
  • Vse is single engine climb speed. You would maintain this speed between acceleration height until level off. It's like Vyse (single engine best rate of climb) in a light twin.
As an example, a Megawhacker taking off from a dry runway into non-icing conditions at 59,000 lbs with the flaps set to 15 degrees would have the following bug speeds: V1 114, Vr 114, V2 115, Vse 150.

So... you're hurtling down the runway in a Megawhacker, and just as you reach V1 , the master warning goes ding ding ding and the airplane starts yawing massively. What do you do?

Obviously, the first priority is to keep the airplane under control. This will take considerable force on the rudder and ailerons. You want to keep the nose pointed right down the runway. Then, as you reach Vr, you rotate to 10 degrees pitch, which should give you a speed close to V2. By this time, the offending engine should have auto-feathered its propeller so you don't have so much drag on that side, and the remaining engine will "uptrim" to give you 10% more torque. Once the airplane is positively airborne, the pilot not flying retracts the landing gear, and the pilot flying adjusts pitch to maintain V2. You don't run any checklists yet, you just concentrate on keeping the airplane under control and climbing to acceleration height (usually 1000' above airport elevation).

Each runway we use has a "turn procedure" assigned, to be used in the event of an engine failure. Many of these are rather simple and do not require a turn below acceleration height. Where obstacles present problems to slowly climbing aircraft, however, a complex turn procedure may be established. Most have textual descriptions, although a few are complex enough to require their own Jeppesen chart. Here's the turn procedure for runway 10L at Portland: "Climb via PDX radial 085 until reaching PDX DME 7.8, or IVDG DME 7.6, then turn right heading 280. Acceleration height 1030'." In this case, you'd make the turn to intercept the PDX 085 radial at 50 feet (!).

Upon reaching acceleration height, you level out, retract the flaps at V2 + 10 kts, and accelerate to Vse before resuming the climb at that airspeed. Only then does the pilot flying call for the engine failure checklist. This emergency checklist leads to several others ("engine failure cleanup items" and "single engine approach and landing"), which you'll do in between talking to ATC and your dispatcher (and possibly maintenance control), coordinating the arrival with your flight attendants, and perhaps making a PA to the passengers. Then it's time to land, smile at the passengers as they deplane, and go drink some beer.


Traytable said...

Then it's time to land, smile at the passengers as they deplane, and go drink some beer.

That's the part that I usually get to see! (Plus the instruction, 'go smile at the passengers'... ;)

Greta post, I'm sure a lot of non-flyers will appreciate the explanations...

traytable said...

errr... I meant 'great', obviously...

Anonymous said...

Yeah, even warm cheap beer! Super post, thanks!

wil said...

You might be interested in this incident

Too low & slow to make the runway after engine failure and never gained enough airspeed to complete the go-around.