Flaps

[TST Tom]

Not sure if this the correct place to post this, but I was wondering whether flaps can be operated in the absence of engine power?

What's prompted this query is a discussion I was having in the pub (where else?). My friend was of the opinion that if an airliner ditches in the sea the occupants are pretty much fish food. I was arguing that a controlled ditching should be survivable. My basic research on the interweb has revealed this list (Water landing - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Ditching)) of commercial passenger aircraft ditchings, and it's noticable that the worst incidents involved the absence of engine power.

Without flaps the impact speed would be considerably higher than normal and perhaps helps explain the aircraft breaking up - with consequent loss of life? :sad:

Not a cheerful subject, but I'd like to know the answer.

[SNS3Guppy]

Tom,

It's not quite so simple. Without flaps, the stall speed is higher, but in order to fly a stable approach with flaps down and avoid an excessive descent rate, then considerable power is required. (Which is why greater flap settings have greater noise impact on approach).

The speed that does the most damage to the human body and to the aircraft on impact isn't forward speed; it's vertical speed. The human body in a seated position can take considerable forward impact, but very little vertical impact, as this is transferred directly to the spine.

Obviously the slower one is moving forward, the better, but also the lower the vertical speed,the better.

With respect to the use of flaps, the specific answer will depend on the specific airplane. In our 747, we have two options for extending trailing edge flaps. One is hydraulically, which requires engine power. Without any engine power this also means no engine powered hydraulic pumps. Without engine powered hydraulic pumps, this only leaves air driven pumps, which require pneumatic bleed air to run...which is also provided by the engines. The only possible means of operating those pumps would be the auxilliary power unit, which in most cases is restricted to ground use only...can't be run in flight. There aren't too many reasons that all four engines would be unavailable save for fuel loss, or possibly a volcanic encounter. In the case of the former, there's no fuel to run the APU, because it comes from the same main fuel tank that feeds the No. 2 engine. In the case of the latter, there's a possibility that the APU could be used.

In lieu of the normal hydraulic flap extention, the backup system uses electrical motors. However, these are going to require something more than the use of the aircraft battery...which would mean the APU...same problem again.

Also in the airplane, all flight controls are hydraulic, with no manual reversion. This means that without hydraulics, there is NO aircraft control, period. To account for this, in the case of a four engine flame-out, we can obtain enough hydraulic pressure from windmilling engines to operate the flight controls, but NOT gear and flaps...those need to be extended alternately...and without a means of doing that, they'll remain up (as they will anyway for a ditching...you don't want gear down when ditching). In order to maintain enough airflow through the engines to windmill them for hydraulic control, however, there's a minimum airspeed: 160 knots...or maneuvering speed for the existing flap setting...whichever is greater.

With flaps and no fuel we could get down to a lower speed...but without adequate means to control the aircraft...so there's little value in attempting that, as well as few options for achieving it even if we could fly slower.

--After re-reading the post, I neglected to mention the leading edge devices. In addition to the trailing edge flaps, there are leading edge flaps or devices which are extended pneumatically normally. In normal operation this uses the same bleed air that runs the air driven hydraulic pumps described above...the bleed air won't be available with the engines inoperative for either hydraulics or to run the leading edge devices (LED's). In lieu of the bleed air operation, the LED's can run on electric motors as well...but in the absence of the four engine generators which normally provide power to the airplane, one is left with the possibility of an APU and it's dual generators (if one has that capability), or alternately the battery...and the battery is needed for standby power for instrumentation on the captain's side...it's all we have left...not going to use that on the LED or trailing edge flap motors.

Our ditching emergency checklist, incidentally, covers a ditching with power...but doesn't address a power off ditching. In a case such as that, one already has multiple emergencies in progress, each with their own procedures and checklists; the ditching is just one more. With power, the checklist calls for a flap 30 (full flap) contact with the water.

The end results of contact of course are very difficult to predict. Water conditions and the timing of the aircraft with swells, vertical descent rate, forward speed, configuration, aircraft loading, and numerous other factors can combine to determine the ultimate outcome. It's not unreasonable to plan for a successful egress and survival, however. Yes, the worst case can certainly happen, but prior to that happening we're going to do everything in our power to create the best situation possible out of an emergency. Prepare for the worst, strive for the best and deal with what actually happens.

Complete power failures do occur, albeit very, very, very rarely. In a number of cases, a successful landing has been made on a runway following a complete power failure. In absence of this outcome, pay close attention to the briefings you're given and follow them...especially important items like making sure not to inflate your vest until you're outside the aircraft and in the water. Remember that you're not alone; a lot of other anxious people are in the same boat, so to speak, and you all have the same goal. A water emergency can quickly turn to every man for himself, so don't allow that to overcome you. It's in your best interest to remain as calm as possible, and offer as much support to your fellow passengers as possible...their interests are your own. Rest assured, the crew has your interest in mind long before the flight leaves the ground, and well after it's come to a rest.

[smala01]

Hats off to you sir, a very well written reply.

Smala01

[BladePilot]

SNS3Guppy
Another masterful and informative reply, brill:ok:.

You've mentioned achieving a degree of hydraulic pressure through windmilling engines, what about adding a couple of lines to explain the RAT fitted to some commercial aircraft?

[SNS3Guppy]

Bladepilot,

Some aircraft have a RAT, or Ram Air Turbine, where others don't. In the case of the airplane I'm presently flying, we have no RAT.

Airplanes are electrically divided into buses, or power sources which serve groups of components. These buses have various names, such as a Main AC bus, or an Generator Bus, generally referring to the type of operation they do (Radio Bus) or the critical nature of the components they serve (Essential Bus). When electricity is limited, such as in a case where all the aircraft generators have failed or engines have failed (and again, I stress that this is a very, very unusual situation), electrical buses generally shed loads, or drop off line, and quit powering their components, in order to preserve power for the most important items (like flight instruments, radios, navigation equipment, etc).

This equipment is then whittled down to a bare minimum, either by the flight crew or automatically, and is served by the aircraft battery. The battery won't last long. In the 747, it's all we've got. We even have a switch that takes our third inertial navigation unit (INS) out of the loop by stealing it's battery power, and routing it to other battery power to give us just a little bit more time. We don't have the Ram Air Turbine because unlike many airplanes, we have the benefit of four engines to lose before something like that becomes necesary.

A RAT is really nothing more than a propeller on a generator or a pump. What is attached to the RAT depends on the airplane and what it's intended use is. When I flew ag airplanes, we used a ram air powered hydraulic pump to move and pressurize chemical, to spray crops. On other airplanes, even light general aviation airplanes over the years, a small generator attached to a little wooden propeller is used; the slipstream drives it and that produces electricity. Externally mounted electrical generators are a rare sight on light airplanes these days...generators are nearly always engine-driven. Older light airplanes also used "venturi tubes" to produce suction from the slipstream, which was used to power the flight instruments by drawing air through gyroscope wheels in the instrument cases...these have also been nearly universally replaced today with engine driven pumps of one kind or another.

On larger airplanes, especially airplanes in which electricity is the critical system, the RAT is necessary because the airplane would soon run out of battery. The 747 is a hydraulic airplane...everything else can fairly well quit and go away, but it won't fly without hydraulics. Hydraulics are still important to other airplanes such as the various airbus models, but these are even more dependent on electricity; that's their critical system. The battery for electrically dependent airplanes is really more of a means to tide the airplane over until the RAT can begin providing power to the essential systems.

RATs are found on airliners and on a number of military aircraft. Typically the RAT provides electricity, and the electricity is used to power pumps or other needed equipment, or electric motors where required. Some aircraft also have the capability of operating their APU in flight. The APU is nothing more than a small jet engine. The Garrett 731 turbofan engine that's found on some learjets and other business aircraft actually started life as a component in an APU many moons ago. It's been improved and modified and changed and is much more powerful and efficient now..but the APU exists basically as a wind machine, with accessories.

Just like the main aircraft engines produce bleed air for pressurization, anti-ice, moving flight controls, etc, the APU produces bleed air to put into the same manifold to do the same thing. It's also got generators attached which produce electricity to power aircraft systems, and in some cases, one or more hydraulic pumps to provide limited capability with aircraft systems. Some of these APU's can be run in flight, some can't (we have a few that can, but they're very restricted in terms of how fast we can go, how high, etc). On airplanes in which the APU can be run in flight, it provides more capability than the RAT. In situations such as fuel exhaustion, then the APU has no fuel source, and it's the RAT or nothing.

A few aircraft also use special auxilliary emergency devices known as EPU's or emergency power units. Some military aircraft use these, powered by a fuel called hydrazine, as short term power supplies until the problem can be solved, or the pilot can eject. (We also don't have those in the 747, and you may be pleased to know we can't eject, either).

It's worth mentioning that while different aircraft employ different means of powering systems in an emergency, the goal is to always avoid being in that situation. All aircraft on long overwater routes, for example, are operated with "Equal Time Points" in mind...points along the route at which it will take the same amount of time to reach a given set of alternate airports. If one has a problem before reaching the ETP, one turns back to the closest planned alternate...and the fuel planning is always done to take this into account. If one is past the ETP along the route, one knows it's faster to go to the alterante designated farther ahead.

We plot ETP's for depressurization situations in which we must descend in an emergency. We do this because at lower altitudes, we have a much higher fuel burn, and we need to make sure that if we have a depressurization, we can still go down, fly to our destination, and have reserve fuel available in the event that weather intervenes or there's delay. We also calculate an ETP for a situation in which we've lost two of four engines, can't maintain altitude, and will be operating more slowly to the destination...it takes longer to get there, and consequently the ETP is located in a different place, and requires a different fuel burn, than one based on a depressurization with all engines. Additionally, we also calculate one which is an emergency ETP such as we might use for a medical problem...when we need to divert quickly somewhere. This one is planned at a lower altitude with a higher fuel burn, but with all engines and our best forward speed.

By doing this kind of planning, we take into account unusual situations that might consume more fuel. We always fly with reserves in addition to the required fuel to get to the destination, and to all of the alternates enroute. The goal is to prevent a situation where a RAT or other device might be necessary out over the water or in some remote place...we always leave alternatives and a "way out."

[JEM60]

Mr. GUPPY's posts are ALWAYS worth reading, unlike some...............

[DeRodeKat]

Our ditching emergency checklist, incidentally, covers a ditching with power...but doesn't address a power off ditching

If you have power, why ditching? What kind of emergency would make you ditch an aircraft that can fly? Fire on board when crossing an ocean? If you do not have power off ditching procedure, do you have power off landing procedure? Do you practise it on a sim?

[nebpor]

Guppy, thank you for that clear explanation :ok:

[SNS3Guppy]

DeRodeKat,

That's a good question...why would someone land off-field (away from an airport) without power? There are a number of reasons, really. The biggest and most obvious would be a situation in which one can't maintain altitude...say, for example, one has lost one or more engines (depending on the aircraft type and the number of engines one has to lose in the first place,of course)...one may not be able to hold altitude and may gradually descend. This is called a drift down...and typically one has an altitude that can be maintained on the remaining engines...called a "drift-down" altitude. But suppose the airplane has only a small percentage of it's total power available...a four engine airplane with only one engine operating, for example...depending on it's weight, it may not be able to maintain altitude for long.

In the past several four engine failures have occured when airplanes encountered volcanic ash. Volcanic ash can cause an enormous amount of damage to an airplane, especially the engines. One or more engines may not be able to restart or relight. This may mean a considerable loss of performance, loss of systems, etc.

Another situation may be an onboard emergency such as a fire, in which there is no option to remain aloft.

There are very few situations in which one would willingly land the airplane off field, or ditch...it's not a good choice if any other possibility exists. If one is faced between putting the airplane down in rough terrain, or making an approach to the water, however, the water may be the best, and even the only choice.

Do we practice forced landings in the simulator? No, not really. In fact, in the simulator the actual landing isn't that important, and most of the time when flying approaches to a runway, we don't land...we have some kind of emergency in the sim and concentrate on handling it. The stress is on cockpit and crew coordinationa and management, addressing emergencies while working together as a team, etc. There's really no good way to replicate a forced landing, in a large airplane. There are too many variables, too many possibilities, and the most important part that one can prepare for is getting the airplane stable and doing the airwork. If the airplane must be put down, it's coming down one way or another...the big thing is making sure it's under controlled circumstances. One can't account for all the variances in terrain and water...but one can train and prepare for a stable approach and keeping the aircraft under control at all times.

[Pontius Navigator]

Ditching used to be a significant possibility in early days of long over water flight. Aircraft of the 50s had insufficient fuel to make true long haul flights and would often stop off or land at coastal airfields like Prestwich and Shannon in Europe before refuelling and continuing to the continent.

These aircraft could not fly in the clear air mass conditions of modern trans Atlantic flight and were down in the weather. They had few long range navigation aids and would rely on professional navigators to take star or sun shots and calculate the winds. They could often experience significant navigation errors and arrive short on fuel. Things improved in the late 50s with the advent of the turboprop that could fly above much of the weather and of jets that could fly well above.

As suggested, the possibility of a decision to ditch now will usually be one caused by an event other than shortage of fuel or navigation error. That said, the Heathrow crash could, in different circumstances, have led to a ditching had the event occurred on a long polar flight down the Norwegian Sea.

[DeRodeKat]

Guppy and P Navigator thanks for the professional and comprehensive replies. Really enjoyable reading :ok:.