forward slips on a jet safe ??

[shanx]

Is it structurally safe to perform a forward slip (full rudder deflection) on a jet like the B737, 777 etc in the event of any emergency ?
(perhaps a rapid emergency descent or depressurization, or like in the case of Air Canada Flight 143 aka the Gimli Glider)

Im asking this because I've read somewhere that the rudder in a heavy jet are not the strongest of the flight control surfaces and might not be able to withstand the pressures felt during full rudder deflection by the pilot while attempting a forward slip to lose altitude.

[Pugilistic Animus]

At the practical slip limit the engines may flame out,...fuel tanks may unport,...and high side loads imposed upon the engines,...read what BelArgUsa wrote about ED's good stuff:ok:


edit: to add I've already had a little talking to from J_T about sideslips,...but in a purely aerodynamic sense it does not hurt the airframe,..but may do other bad things as stated above
PA

[PEI_3721]

P A Re “may do other bad things”.
Not necessarily so. These aspects are tested during type certification and if encountered, any serious problems would probably define a side slip limit short of the aerodynamic limit. Of course there can always be exceptions and some aircraft may have AFM limiting or warning statements for the systems affected so that the full ‘aero limit’ can be used. Also see aircrafts’ limit re Va – max speed for full scale control deflection.

There should not be any structural problem up to the limiting sideslip angle (possibly defined by physical rudder limit / airspeed schedule). As with other structural limits there is a margin of safety, but historically, the rudder requirements did not fully cover reversed or high dynamic forces; thus most aircraft manufacturers publish warnings / advice on these subjects.

AIC P 85 2008. (http://www.nats-uk.ead-it.com/aip/current/aic/pink/EG_Circ_2008_P_085_en.pdf)
IFALPA Safety Bullletins 2002. (http://www.ifalpa.org/publications/safety-bulletins/2002.html)

P.S. Engine limit more likely to be encountered in a crosswind take off, if so, then the aircraft may have to conduct a rolling take off, with full power only being applied at some predefined airspeed.

[Pugilistic Animus]

PEI_3721,

The most important concern regarding sideslip is fuel tank unporting and engine upsets[at extreme sideslip autoignition may help,...but the air entering,... may be so low energy that it wont matter,...of course as you said it depends on the spsecific AFM limits,...as far as aerodynamic issues these are most related to instrument indications [if uncorrected] etc...not strength

but attaining the practical slip limit on a jet to me seems...:\

from a prior post relating to 'control reversal'


there is no more hazardous a maneuver than sudden control reversals in an airplane ---the certifcation requirements do not even consider that--as resonance may develop and quickly result in a catastrophic failure even on a Pitts special or an extra or an f-22 or a cub or a 737---and even at or below Va ---



PA

[Mad (Flt) Scientist]

There should not be any structural problem up to the limiting sideslip angle (possibly defined by physical rudder limit / airspeed schedule). As with other structural limits there is a margin of safety, ....

I wouldn't be prepared to bet on that, especially if the manoeuvre is being enacted in response to a sudden depressurization and/or a need to initiate a quick emergency descent. Something already went badly wrong with your aircraft; are you that confident that the same event - whatever it was - didn't disrupt the nice tidy conditions assumed for the structural design calcs? Never mind that there is always the issue that for certification there is always the option - certainly for considering handling limits - to define various aircraft responses as "deterrent" - and high lateral accel may be one of those. So the OEM and authority may have satisfied themslves that "no one in their right mind" will do exactly what is now being attempted.

Even Va I'd take with a pinch of salt - yes, there are margins in the structural calcs in terms of UL/LL factors - but those are also there to account for things like an imperfectly flown manoeuvre compared to the 'perfect' design case, or errors in the calcs, or repairs to the structure, or fatigue weakening the structure, or .... Just because there's a margin in a cert requirement, certainly does not mean it's all going to be there to rely on for any given event.

You'd have to have a bloody good reason to be aggressively sideslipping a swept wing transport.

The most important concern regarding sideslip is fuel tank unporting and engine upsets[at extreme sideslip autoignition may help,...but the air entering,... may be so low energy that it wont matter,...of course as you said it depends on the spsecific AFM limits,...as far as aerodynamic issues these are most related to instrument indications [if uncorrected] etc...not strength

but attaining the practical slip limit on a jet to me seems...:\

I'd agree with that last sentence, but not the bit in bold. Sideslip is turned into an effective increase in angle of attack on the leeward wing, in terms of margin to the stall. Sideslipping a swept wing aircraft to its limits is not something you should be doing lightly. There could be some extremely nasty aerodynamic characteristics waiting to bite you.

And with respect to control reversal - you don't even need repeat oscillations. The Part 25 manoevres don't even apply a single full throw reversal. If you stamp on a pedal, wait until full sideslip, then stamp to come back - as opposed to just centering the pedals, which is the cert manoeuvre - well, the fin and rudder might stay attached. But no-one's promising you that.

[Pugilistic Animus]

Thanks, MFS for the clarification

that post of yours closes the book on 'jet side slipping' with me ---now I really know

[john_tullamarine]

... but, for SuperCubs and the like, the only problem is not enough rudder to really be able to flaunt a sideslip ...

[misd-agin]

Read transport jet certification standards. They do test to a full side slip and rapidly release the rudder. It's the opposite rudder (doublet) immediately after that to a sideslip in the other direction that steps outside of the 'certified' envelope.

Please do not take that as an endorsement of doing sideslips. Just explaining the certification process.

Another place to read on this subject is -

NTSB - American Airlines Flight 587 (http://www.ntsb.gov/events/2001/AA587/default.htm)

[Mad (Flt) Scientist]

Read transport jet certification standards. They do test to a full side slip and rapidly release the rudder. It's the opposite rudder (doublet) immediately after that to a sideslip in the other direction that steps outside of the 'certified' envelope.

Please do not take that as an endorsement of doing sideslips. Just explaining the certification process.

But bear in mind that a "full sideslip" for cert may be less than what you could get if you applied the full rudder at a given speed. Especially above Va, where there's no need to be able to apply full rudder.

[SNS3Guppy]

The hazards of slips in a swept wing airplane have been touched upon, but there are issues beyond control to be considered. A control reversal has been mentioned, but in the sense of a pilot rapidly reversing control inputs. This can be dangerous, but an aerodynamic reversal may also occur.

I've slipped one turbojet airplane, and technically I didn't do the slip. It was an individual with me who did the slip, whom I quickly corrected...and he performed a full slip without any announcement regarding his intent, when he found himself high on approach. That was in a barely-swept wing (a Lear 35, which is also a very strong airframe). In a highly swept wing, a slip offers far more disadvantages than advantages...which is part of the reason we have high-drag or lift-killing devices on many airplanes, rather than simply performing slips.

I've done full slips in large four-engine airplanes on many occasions while performing low altitude maneuvering as part of aggressive firefighting activities. However, I stopped doing it when I learned about the after effects. The use of the slips was commonplace among most performing that type of work at the time. However, as most of us were also mechanics/engineers and were involved in performing the inspections on the airplanes as well as flying them, we had opportunities to see far more of the airplane than most pilots. During heavy maintenance checks, I began to see damage to attaching brackets, surfaces, and bolts, primarily on the vertical stabilizers.

Most pilots would be rather shocked, I imagine, to find out just how little holds the vertical stab in place on many aircraft. Then again, many were shocked to find that the vertical stab left the aircraft during a pilot-control-reversal in American Airlines 587, too. In our case, some of the aircraft had very little, and very small bolts attaching the vertical stab...and not so many as one might expect...and we were finding them cracked.

The side load on the vertical stab is significant, as is the torsional or twisting moment when large rudder deflections are applied. As airspeed increases, this load increases in the sideslip. Unlike a wing with a spar that's intended to take a load and a bending moment and has wing root support through the spar, attach points, and additional member, the vertical stab was simply designed to help align the aircraft with the slipstream. It's a stabilizing surface, not a lifting surface,and was never intended to be exposed to the types of loads that repeatedly slipping the airplane can impose.

That brings up the question then, can one simply slip it once and be safe? Setting aside the aerodynamic effects of slips in a swept wing configuration, one must ask just how strong is that vertical stab? Even if the vertical stab is strong, what about the attach bolts? What about the brackets? Do you really want to find out? How many times has it been slipped before, and under how much stress have those components been placed in the past? Have the bolts ever been inadvertantly overtorqued, and the fittings and bolts themselves ever been exposed to over-tension? Who's to say?

We were finding cracked fittings and bolts and had no idea at what point the failures had occurred. Just barely, or some time in the previous 100, 200, or 300 hours? We didn't know. What we did know was that the evidence clearly indicated that full slips in the airplanes was contraindicated; a bad idea.

We did full slips because maneuvering close to terrain (down canyons, in mountains, etc) was a job function, and slips were preferrable to flaps. A slip can be ended instantly, restoring performance, whereas changing aircraft configuration with flaps or gear takes time...time which one might not have inside a canyon or variable, steep terrain. It worked very well, too,and was very effective in accomplishing the mission we had before us. After I began to be exposed to the after-effects, however, I elected to find other ways to accomplish the same mission.

Some airplanes are subject to other types of control reversals than simply pilot-applied changes in the direction of control input. In the C-130, for example, a control reversal or more correctly a loss of authority could occur with "fin stall." Over-deflection in a slip could lead to loss of rudder authority, and ultimately result in the aircraft yawing farther into the slip. This is not the case on all aircraft, and even on the same aircraft it's not the same in all scenarios. A combination of angle of attack and sideslip, such as in a steep descent, can cause combinations of airflow disruption, separation, or loss ("blanking the tail"), and this can in turn cause the rudder (and elevators in some cases) to lose effectiveness, or even cause the vertical stabilizer itself to lose effectiveness. Flaps can also play a role in such a situation.

Many years ago I worked in the shop for a company that flew 30 or so Cessna 207's. A far cry from a large, swept-wing turbojet airplane, these were simple, strong airplanes that were primarily used for tours, for back-country charter, a lot of dirt-runway and rough field work, and all in the mountains. When flying the airplanes, the use of slips was very common. However, over a period of time, I began to find the vertical stab attach brackets broken...on all the airplanes. Eventually a fix was found and most 200 series airplanes have improved steel attach brackets now. Over the course of a single year, I found nearly every attach bracket broken on those airplanes, until they were upgraded and pilots were encouraged to stop slipping so much.

I mention that because even a small, light airplane which is commonly slipped can face structural problems, and there's nowhere near the load on a small airplane as there is on a large airplane. However, as the airplane gets bigger the proportion of the attach bolts and brackets doesn't increase accordingly. Big airplanes have bigger brackets, bigger fittings, bigger bolts...but not so much as you might think...and certainly not in proportion to the increased area and loads that can be developed in uncoordinated flight.

Add to that the uneven airflow into engines, the potentially drastic change in airflow over swept wings, and controllability issues, there's little to be gained. When one slips a swept wing airplane, one isn't increasing drag so much or losing lift; one's transferring it from one wing to the other. If one applies spoilers or speed brakes, on the other hand, one is increasing drag, and this is far more effective.

Another consideration for performing a slip at a time such as an emergency descent is control authority and deflection. Performing a slip at high altitude from cruise speeds may not even be possible. In the B-742 for example, our controls are locked out with only the inboard ailerons available, and rudder travel is limited; we can't put much control authority into a slip and can't create much of a slip. There's a lot of wing out there to twist and flex, and the outboard ailerons are locked out for this very reason. The whole airplane flexes to a much greater degree than many people realize, anyway, and bending it around even more isn't in anybody's best interest.

I submit that slips are best left to supercubs, but I submit that slips aren't so good for supercubs, either. I've been flying ag airplanes and fire airplanes along with other bigger equipment for many years now, and I won't slip an Air Tractor or a Dromader into a fire or over a powerline, either. I used to...but having seen what I've seen from a mechanic's perspective, those days are past.

[clivewatson]

[quote]especially if the manoeuvre is being enacted in response to a sudden depressurization and/or a need to initiate a quick emergency descent.[quote]

Mad Flight Scientist, the purpose of any side slip is not to give maximum rate of descent, it is merely used to increase ROD while limiting airspeed, and in a variety of types it aids forward view. With the greatest of respect therefore, I suggest that your scenario of an emergency descent is an unlikely use of the manouvre.

John Tullimore - my recollections of all Cub variants are clearly different from yours, although I would readily agree that the Tiger, Stampe and Pitts do it much better! The secret is to get the speed down before one starts slipping.

Never had the need to try it in a jet - but then I never got too high and fast!! :ooh::uhoh::suspect:

[Pugilistic Animus]

Mad Flight Scientist, the purpose of any side slip is not to give maximum rate of descent, it is merely used to increase ROD while limiting airspeed, and in a variety of types it aids forward view. With the greatest of respect therefore, I suggest that your scenario of an emergency descent is an unlikely use of the manouvre.

Clive Watson, MFS certifies transport aircraft he knows all about that stuff:

Guppy, that post was eye opening,... I think that post would be helpful for the CFI's I work with,...and perhaps only ONE ED and slip to land. one visit to the practical slip limit is enough for every student

You mechanics must sometimes think pilots are a little thick:E

PA

[clivewatson]

Mad Flight Scientist, the purpose of any side slip is not to give maximum rate of descent, it is merely used to increase ROD while limiting airspeed, and in a variety of types it aids forward view. With the greatest of respect therefore, I suggest that your scenario of an emergency descent is an unlikely use of the manouvre.

Pugulistic: With even GREATER respect to you, and the GREATEST of respect to Mad Flt Scientist (WHO I KNOW, KNOWS HIS STUFF), I suggest you, MAY have missed the point I was trying to make......that being that the side slip manouvre would be of little use in the scenario of an emergency descent.

He (and you) should know that side slip manouvre would rank among one of the most USELESS manouvres to consider if the MOST rapid rate of descent was needed. This was the ONLY point I was trying to make - nothing more - nothing less, and certainly nothing that requires complex thinking.

If he (MFS) thought that rapid rate of descent was derived from side slipping, then he was WRONG, but I don't think he wad suggesting that!

[SNS3Guppy]

You mechanics must sometimes think pilots are a little thick


It seems to work both ways. Mechanics think of pilots as very limited in their capacity, and pilots tend to see mechanics as the lower class. Sadly, where I fall into both categories, I fill a special lower-tier role beneath either party in which I can walk but not chew gum at the same time, can recognize a twelve-point socket two out of three times, but not on the same day when taxed with flying a localizer.

[glad rag]

Brilliant and thought provoking post SNS3Guppy.

Most pilots would be rather shocked, I imagine, to find out just how little holds the vertical stab in place on many aircraft

Too bloody right!! It is a real eye opener when you see how that part of the airframe is designed and built!! :suspect::ooh::ooh::ooh: Goes to show how important the secondary structures/skin around the fin base are.....

sparty

[411A]

Hmmm, forward slips.
Aside from the rather large rates of descent that can be obtained, these maneuvers are not especially helpful when performed close to the runway...say in the last four miles or so, whilst on final, as an example, for excess height reduction.
We had one very senior Captain who used to train (and perform) these forward slips for excess height reduction, on a somewhat regular basis.
And in a 300+ passenger airplane.
Management got wind of this, and contacted the manufacturer, who promptly dispatched a company test pilot out to our location, with actual data obtained from certification testing.
Result?
NO pilot ever again forward slipped the airplane.

[Pugilistic Animus]

Clive Watson, MFS certifies transport aircraft he knows all about that stuff:

J_T I meant certificated,... I'm Certified:}:}:},...btw You OS and GF had me spilling my coffee :} in the 'Obstacle Thread' the best:ok:

I'm just not sure if folks realize the resources available here and the level of expertise available for free,...apparently you do

With most greatest respect,
PA