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Forward Slipping a 737-800

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Forward Slipping a 737-800

Old 25th Jul 2002, 12:07
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This may be akin to the alter-boy telling the priest how to conduct high mass

Forward Slip
In a no-wind situation, you’re flying along in your Spad and decide to land in some green pasture in Flanders. Noticing you’re a bit high, you stomp on the rudder bar while adding in an appropriate amount of opposite aileron to counter the yaw. The relative wind is now displaced from the centerline of the aircraft and a wing is lowered and forward of the other wing. This lowered wing is in the direction you’re traveling. With the wind displaced from the nose you have increased your wetted area, increasing your drag coefficient, and with the same power, increased your rate of descent – no flaps needed. The direction of travel is off-set from the nose of the airplane.

Side Slip
On a windy final approach you crab toward the crosswind, away from the runway. Moments before touchdown you may desire to align the longitudinal axis of your airplane with the centerline of the runway with minimal transverse movement. You apply a bit of rudder pressure while countering with opposite aileron. The longitudinal axis is aligned with the runway and thankfully is in the direction you’re traveling. You have succeeded in negating the effects of the cross-wind, but you’re not level, the up-wind gear should squeak on prior to the down-wind gear. The direction of travel is parallel with the nose of the airplane

Favorite Slip
When my girlfriend forgets to wear hers with a translucent skirt.

I have read various accounts of slipping heavy jets, B747 and DC8s, and understand they exhibit very docile qualities in the slip. It may be a fine maneuver for our freight brethren or when empty, but it’s not a normal maneuver for me with passengers on board, and will not do it when transporting such.

Last edited by '%MAC'; 25th Jul 2002 at 13:18.
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Old 25th Jul 2002, 12:56
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Is it not true that the Air Canada 767 that ran out of fuel in 1983 did a monster side-slip to get into Gimli?

Obviously with no engines he had only one chance to get it on - as a lowly PPL I'd guess that would be the only time you'd ever want to heave a 767 on it's side.

Even in a Tomahawk, the momentum caused by side-slipping can get a bit hairy of you're not, pardon the pun, 'on the ball'...
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Old 25th Jul 2002, 13:22
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%MAC,

I far prefer your third explanation and concur without any reservation ... however, as to the first and second ...

Could we revisit the very nice graphic at the site to which dvt directed our attention ..... which had, if I recall correctly, two pictures ... that on the left described as a "forward slip" .. and that on the right as a "side slip".

Might we make some minor alterations to the "side slip" image and do an interesting thing .... ?

(a) erase the reference to wind. Does anyone take issue with my suggestion that an aeroplane, other than via sophisticated electronic wizardry, knows naught about a steady wind ? If so, then the reference to wind is irrelevant.

(b) having removed the irrelevant reference to the wind, draw in an arrow which might represent the path, or track, which the now wind-less aeroplane might, perchance, take ....

(c) now, with scissors keenly sharp, cut out the resulting picture, apply some magic spray to render it somewhat transparent, and

(d) move the said somewhat transparent image to the left and superimpose it on the "forward slip" image

(e) rotate the superimposed image such that the arrow which you drew .. now is aligned with the runway

To my simple engineer's mind ..... the two appear to be quite similar .......

... which brings me back to my original question .....

... what is the difference between the conventional side slip ... which is well understood ... and this newfangled "forward slip" ?

I think that it is all waffle ....... any takers for a discussion on the pros and cons of my position ?

Last edited by john_tullamarine; 25th Jul 2002 at 13:28.
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Old 25th Jul 2002, 15:52
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I think that in some circles the term 'forward slip' is now applied to what used to be called 'side slip', ie the aircraft tracks a straight line over the ground, usually the approach centreline. In these circles, 'side slip' has become slipping laterally across the ground, starting to one side of the extended centreline but arriving over the runway on the centreline, assisted where possible by any crosswind.

From Handling the Big Jets , by Dai 'The Book' Davies, ARB Chief Test Pilot: "It is wrong in principle to allow a swept wing aeroplane to suffer significant angles of sideslip". Good enough for me, except, as mentioned earlier, when faced with a no-thrust approach.
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Old 25th Jul 2002, 16:04
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I don't claim any real-world experience beyond low hours solo glider flying, but I'm with John 100% on this one.
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Old 25th Jul 2002, 17:46
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"From Handling the Big Jets , by Dai 'The Book' Davies, ARB Chief Test Pilot: "It is wrong in principle to allow a swept wing aeroplane to suffer significant angles of sideslip". Good enough for me"

What do we PROFESSIONAL pilots call this principle..."The Because I Said So Principle." I want to hear it from the lips of a structures engineer, that "we didn't design the airframe to handle the stress of this manuever. I suspect this may be the case...at least over the long term.

Differences.....

Side Slip is a precision manuever to land when you have drift. You kill the drift with rudder and fly the appropriate amount of aileron to a landing. Here the wing gets dipped into the wind and you touch down on one wheel first.

Forward Slip is a nonprecision manuever when you need drag. It is an intentionally uncoordinated turn but you don't turn. You slowly feed in rudder and use aileron to level the wings. You keep your forward vector. The ailerons offset the rudder for zero turning effect. The vertical fin and fuselage are now exposed to the relative wind creating lots of drag. And I suspect, lots of stress.....more than Side Slipping anyways.

Which brings me to the reason for my post. Where am I in this stress envelope, when I forward slip at final appraoch speeds? This is the question which I submitted to Boeing. If they answer me, I'll post.
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Old 25th Jul 2002, 21:45
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dvt

I appreciate that your question was about the structural implications but unless I was about to crash I cannot think of any occasion when I would choose to carry out such a manouevre.

For what it is worth I also happen to agree with JT, there appears to be no fundamental difference between so called 'forward slip' and 'side slip'.
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Old 25th Jul 2002, 22:01
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Sideslips and forward slips are very similar, but the previous explanation is correct.

The generally accepted definition is that the side slip aligns the longitudinal axis of the aircraft with the runway and utilizes bank angle to maintain track on the centerline (crosswind situation). The actual path through the air is to the side to compensate for the wind. Zero wind and you're not heading for the runway, but going to the side of it.

A forward slip involves applying rudder in one direction and enough opposite aileron to prevent a turn. In zero wind you will still be tracking the centerline of the runway. Usually you apply full rudder and enough aileron to compensate.

These are the standard definitions used by FAA, etc.

I think the main difference is that in the sideslip you are using enough aileron to control the drift and just enough rudder to keep the longitudinal axis aligned with the runway. In a forward slip you are using a whole lot (to the stop in a light airplane) of rudder and then aileron as required to prevent the turn. Consequently, you generally use a whole lot more rudder in the forward slip. I would agree with those that say that if you need to do this to lose altitude you've botched the approach and should just go around.
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Old 25th Jul 2002, 22:21
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With apologies for paraphrasing one of the the old wise Yank pilots on this forum some time ago who offered this gem on another not dissimilar subject..
"Boy, if the manufacturer don't recomend doin' something,dont go stikin' yer dick in there!! "
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Old 26th Jul 2002, 01:08
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With a pointer to the FAA (thank you, Prof2MDA) as the source of this terminological excess, I did some research (using my trusty US-sourced CD containing lots of FAA bits and pieces). A bit of study is always a good thing as one invariably learns something in the process.


The term, "forward slip" indeed is to be found within some areas of the FAA flight training and flight standards bookwork.


For the interest of others who might, like me, not be working within the US environment and not have come across the term before, may I cite some references which you might care to examine -

(a) AC 61-89D, Appendix 1, Lesson #37

(b) FAA-H-8083-3, Chapter 7, from which comes the graphic referred to by dvt (at Fig 7.6). Interestingly, this text makes it clear that the two are the same, except for the (groundbased runway frame of reference) observed track.

(c) Order 8400.10, Volume 5

(d) FAA-S-8081-5C, -6A, -14

(e) Dictionary of Aeronautical Terms


If I may quote from the last document (even if I might take the view that the definition is somewhat inadequate) -


"slip (aircraft flight maneuver).

A maneuver in which the aircraft moves through the air sideways, rather than straight ahead. Slipping produces a large amount of drag so the airplane can descend at a steep angle without gaining excessive speed. An airplane is slipped by crossing its controls, using right rudder and left aileron.

A forward slip is one in which the aircraft turns its side into the wind, but continues in forward flight.

A slide slip is one in which the aircraft continues to point straight ahead, but slips to the side."



Do I have to eat of the weekly umbles pie as befits my lowly status for not knowing more about this matter ? .... I guess so.

Do I still think that the two are the same thing waffling as distinct entities ? .... absolutely.

Is either view more important than the other in the overall scheme of things ? .... probably not.



So far as structural implications might be concerned, I would be less concerned with moderate slip angles than with rapid and varying rudder inputs. There is, also, the possibility of asymmetric stall and unintended spin if the manoeuvre is done at lowish speed and in a ham-fisted sort of way. As always, we in the field do not have access to the manufacturer's design records ... therefore, if something is not prescribed in the manufacturer's documentation then the manufacturer ought to be consulted prior to embarking on one's own little flight test card.

In sim training, I have seen some prodigiously extreme slip angles employed successfully for the purpose of recovering the aircraft from an all-engines failed scenario.

In real life line operations, though, the intentional use of slip (other than that which necessarily is used during the final stages of a crosswind approach and landing) is both unpleasant and unnerving for the SLF and probably has no routine place in large aircraft operations.


Having said all that ... as a very young and comparatively inexperienced pilot many years ago on glider tug operations using the lovely SuperCub .... nothing since gives me the routine delight as that which I derived from an extreme sideslip down to the flare with the tow rope just clearing the boundary fence by inches .... and occasionally leaving the last few inches on the said boundary fence. Probably not very sensible flying and I don't think that I would have any interest in doing so any more .. but it was all rather good fun at the time ...
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Old 26th Jul 2002, 02:08
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leadslead-

If the manual doesn't specifically prohibit it, then it's OK to do it? I'm finding it personally difficult to buy into that line of reasoning. The manual would have to be many thousands of pages thick, and even then, I think something would be missed. Much safer (from the manufacturer's legal liability standpoint) would be to say what you "can" do, and anything else would be on your head.

Aviation Week, in the aftermath of AA587, has done a number of excellent, eye opening articles on rudder usage. Some of which bring out statements from the structural design engineers saying words to the effect of, "..we never expected the pilots would do that...".

dvt-

If Davies says it's not a good idea, that's good enough for me (and untold thousands of other professional pilots). If you can't learn from him (and others like him), who are you going to learn from?

For what it's worth, my company prohibits slips (to lose altitude) across our fleets. I was part of that decision, and I'm comfortable with it.
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Old 26th Jul 2002, 02:43
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"For what it's worth, my company prohibits slips (to lose altitude) across our fleets. I was part of that decision, and I'm comfortable with it."

And this decision was based on what?.........The "Because I said so principle." I'm curious...what's your company's policy on hand flying, raw data approaches, appropriate levels of automation, and the like? I've seen many once fine aviators who've forgotten the BASICS of flying. Can you believe a commerical airline pilot with over 25,000hrs of instrument flying can have NO functional crosscheck. They can't trim hands off in a turn or for level flight. They can't hand fly and think, they're task saturated. Guess what? Their out there. And I find they work for companies that set policies like yours. I'd be careful of setting company policies that stiffle pilot skills.

Every pilot needs a bag of tricks. And he needs to inventory that bag from time to time. They don't need micro-managing desk jockeys who take stuff out of their pilot's bag of tricks. BTW, some of the worst flying pilots I've ever seen have been Check Airmen/Instructors. I know what's in my bag? If you're gonna take something out of my bag, you better have a damn good reason. Some better than "Because I said so".
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Old 26th Jul 2002, 03:37
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"All things and all people in life have to sink or swim on their own merits, not their reputation; that just as a wise man can say a foolish thing, a fool can say something wise." Vincent Bugliosi
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Old 26th Jul 2002, 03:49
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I believe the dicision was based on sound reasoning, with safety being primary. Believe it or not, there are a "few" pilots who aren't quite as good as they think they are. It only takes ONE bad mistake to create an accident. Too many times in history, has there been a pilot who has said at the end, "I wish I hadn't done that". That's too late to admit you made a mistake.

Our policy is to leave the number of raw data and/or hand flown approaches up to the individual pilots. You don't have to be concerned with the abilities of our pilot force. We are one of the few carriers whose Ops Specs allow hand flown CAT II approaches. Our POI must have the same confidence in them as I do.

I'll be the first to admit that my flying skills are not what they used to be. But over the years, I've become a lot more knowledgeable and well rounded, and not tempted to do some of the things I did when I was full of **** and vinegar. (Including getting baited into an argument with you on these points).

>>If you're gonna take something out of my bag, you better have a damn good reason. Some better than "Because I said so".<<

Well, I guess we'd have a problem there. You'd always be welcome to come in and discuss our differences, but in the end I'm gonna win that battle.
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Old 26th Jul 2002, 04:31
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It is one thing to have stated in the ops manual that forward slips in our fleet are prohibited, and another thing to give the reason why. I am very skeptical of companies that keep their decision making process hidden. I have flown for enough carriers to know that not all procedures were borne of enlightened knowledgeable professionals. When I was in fourth grade, the comment ‘because I said so’ held some credence; now such a statement is met with an acerbic demeaning comment. It is much more effective to state the reasons behind the policy, people then are more likely to be compliant. A statement such as ‘forward slips in our fleet are prohibited as it has been documented in Boeing study 249-7981 that such maneuvers lead to premature failure of the main fin spar.’
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Old 26th Jul 2002, 05:18
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Every pilot needs a bag of tricks.
dvt

For the benefit of my curiosity when would you envisage needing this particular 'trick'?
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Old 26th Jul 2002, 13:13
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dvt
Are you a clone of 411A, you certainly act like one? No, you can't be; while he too is often highly opinionated, at least he isn't rude and arrogant.
"What do we PROFESSIONAL pilots ...." - who are you excluding from this elite group, Dai Davies or me? You'd be wrong on both counts. If you want to hear it from an engineer: "...modern types of aircraft do not take very kindly to sideslipping. (Continues with reasons)" From Mechanics of Flight, A C Kermode CBE, MA, CEng, FRAeS.

You appear to want to concentrate on the airframe stress aspects and want to hear it from the lips of an engineer. You might well hear it from the lips of an engineer that a commercial aircraft is of course stressed for a 1g barrel roll (and it's possibly not prohibited in the 737 manual either), but should you do one?

Although you might be lucky, I doubt that you will hear from Boeing. I understand that for legal reasons Boeing will now only communicate with an individual or organization that has bought, or is actively seeking to buy, one of its products, and then will only comment on that specific product.

Last edited by Hew Jampton; 26th Jul 2002 at 14:16.
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Old 26th Jul 2002, 16:50
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dvt - You have a very poor attitude.

1. Just because you created a post, you do not own it. Nor do you have the right to tell people not to reply to the post with whatever spin or retort they wish. Just because it doesn't serve the purpose that you wanted, the fact that it already has 3 pages of replies, means that the rest of us are interested. Sorry, old bean - this board is a discussion forum, not a mechanism for you to get specific answers to your questions.

2. You said "You know what. See ya! ....I'll email my concerns to Boeing.". Well.... erm.... why didn't you? Why the three subsequent replies??! What did Boeing say?

3. Accept others advice/experience. You don't know it all. There seems to be a pattern here. Everyone else is saying "erm... can't see why you'd need to... a professional should go around... not in my SOP.... specifically not authorised in my airline". Yet there's you, who seems to *think* you know better... ask yourself a question!

4. Suggest you take the matter up with your chief pilot. But be prepared for a dressing down.


I sincerely hope none of my family are ever pax in your care...

I bet you're the type who doesn't believe in CRM training, aren't you?
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Old 26th Jul 2002, 18:33
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A telling difference between forward and side slips: Side slip by definition merely counteracts the crosswind component to allow a straight line of flight, but the useful forward slip is usually a much steeper banking turn counteracted by a much greater rudder deflection.

There is NO question that a forward slip will get you down faster than S-turns for an emergency. If turbulence is not a factor, controlability and loads might be within normal limits, but gust loads or a heavy foot will quite possibly put you far out of the surface loading limits for a big tail. Maybe also for pylon wobble on big wing-mounted fans. If the tail is one of the fiberglass type, the aircraft - in good conscience & with what we know now - may not be reuseable afterward.
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Old 26th Jul 2002, 18:56
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dvt, the purpose of a thread is that it changes direction slightly with each new post (provided it's still relevent). I've yet to see an irrelevent post on this thread. A new (and old) 737 driver would probably benefit from the advice and cautions of other fellow professionals.

I try to hand fly / do raw data approaches on a regular basis WHERE APPROPRIATE, plus I have the opportunity to do visual / circling approaches. I agree hand flying is important, but I think these days CRM, Teamwork, and making sound desisions are almost more important. Very few accidents are caused by pilots not having the necessary flying skills, MANY, MANY are caused by bad management, breaking rules, and get-in-itis.

I know ATC can mess you up, but if you can't manage your approach profile to such an extent that you have to side-slip then I think you're in the realms of a rushed approach. Have you ever thought what the other seat would make of you slipping on finals? I think it would make him rather uncomfortable.
You should be able to recognise early enough that you use the gear, or ask for further vectoring, or you do an early re-position. If you're high enough so sideslip, there are other, better, flight manual options available, if you're too low, then GA.

I agree that its an interesting question about whether there is any reason why you can't slip, and I would like to know the answer, but I think it's your attitude that 'only cissies don't slip' which has alarmed so many.
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