Forward Slipping a 737-800
 

Sometimes you just get absolutely "crap-vectors" from ATC. Sometimes you find yourself high and steep on final, and need to lose altitude. As new pilots, we're introduced to the Forward Slip manuever. It's a basic skill we seldom call upon flying large aircraft. Recently, I've begun to wonder about the applicability of this manuever in larger aircraft, with the recent Airbus crash in NY.

I've forward slipped the 737-800 a couple of times in the past. At final approach airspeeds it seems to handle quite nicely, and is very effective when "S" turns and "360's" aren't an option. I know of no specific prohibition on this maneuver from Boeing. But recently, I wonder if I might be putting and an undue twisting moment on the fuselage. Does anybody know what Boeing's position is on this manuever?

I'd be more concerned with the passengers' positions than Boeing's...

How many passengers would be comfortable in a slip that was significant enough to increase the rate of descent moreso than full speedbrakes, idle thrust, and max flaps for the current speed?

Granted, it feels a little odd when you intially enter a forward slip. It's about the same sensation as kicking out the crab angle on short final in a X-wind landing. There is a high rate of descent associated with a forward slip but that's the point of doing the manuever. You can look in just about any basic pilot training manual and see that its a taught manuever.

Anyways, I've flown some airplanes that strictly prohibit the forward slip manuever in certain flap configurations. Boeing makes no mention of the maneuver at all. I was hoping this site might have an aeroengineer or other specialty that would have more insight into this.

Q: in a jet, does a slip lead to compressor stalls?

Id be very dubious about applying slip if the standard operating manual doesnt mention it.... Its a bloody good trick to get your sailplane on the deck ASAP, however, PAX generally get pi**ed off when they have to lean sideways to drink their G and T.
Reasons behind flap limitation to aircraft sideslip occur as a result of decrease in control surface authority (horiz. Tailplane mainly) and even stall of the rudder. Not Healthy. Buffet Generated by high alpha manoevers on flexible tailplanes can significantly decrease the fatigue lives of their constituent components, as per tornado GR1, B707 F18, F16 etc etc...
As regards buffet entering engine intakes, its probably studied in the design process, however, having a background investigating Resonance induced blade failures and its complexities, it would definately be worth asking the chaps at boeing.... I cant imagine them looking at it in too great detail, pilots are supposed to stick to straight and level as much as possible to keep the boffins happy.Could always try it in the sim, or just ask your fleet chief instructor...
From an engineering standpoint, its probably best to avoid, as you are entering flight envelopes that are cleared for flight, but not as standard procedure and thus can have no idea as to the long term effects.
Hope this was helpful

I don't think inducing high RODs close to the ground is very healthy. Keep to your SOPs as far as you can and I'm pretty sure that no sensible airline would include forward slips as an approved method to regain your approach profile. I'm sure the regulator would have something to say about it too...

If things are that desparate why not go-around?

When asked about this a couple of years ago, UK CAA test pilots said that slipping up to max rudder does form part of the certification flight testing. That doesn't necessarily mean that it's a 'good thing'.

I agree with Mr. Mouse.

A rudder is designed to cope with the stress of a deflection, but it is not designed to cope with that stress continiously.

I was taught thta it is absolutely not done to sideslip larger a/c. If s-turns and flaps 40 don't do the trick then you're in go-around area anyway.

I wouldn't try S-turns in a congested area.

How strange that this topic should suddenly appear!!

I was sitting at the holding point at Orange H.Q. a couple of days ago waiting for an aircraft to land. (We presumed this because ATC had merely told us to hold position when we reported ready for departure).

Gazing up the final approach path we spotted an aircraft in the distance that appeared to be manoeuvring randomly (or so it seemed) and getting closer. We therefore surmised that we were being held because of an unidentified intruder.

As it got closer and much to our surprise, it became evident that it was one of Stelios’ finest, vigorously performing a series of “S” turns and side-slips in order to lose height. The determination to “get in” was obviously paramount and quite alarming.

Correct me if I’m wrong, but the only reason that I can think of to perform such prolonged and extreme manoeuvres would be in the “loss of all engines” case.

Unless there are pertinent details that I’m not party to, this was the worst case of commercial flying I’ve seen in more than two decades.

As M. Mouse quite rightly suggested, the PROFESSIONAL thing to do is GO AROUND.

Hmm....Seems this topic is skewing off from the intent of its original post. I'm interested in the opinions of aircraft structual engineers and the like. The fact is...the Forward Slip IS a valid flight manuever.

I'm not interested in the opinions of pilots who keep their bank limiter set to 10 degrees in the traffic pattern and who don't know the difference between a Forward Slip and a Side Slip. Who consider a standard rate turns, "S" turns and "360's" to be the realm of aerobatics. Who "**** their pants" with the loss of their FDs. You guys can "kiss my @$$". BTW, Eli Vator, that wasn't me.

So....If you have any knowledge from a structures or areodynamics standpoint, "like UAM's post". I'd like to hear from you. Otherwise, move on!

Given recent events (A300 tail failure), I would be dubious about doing such a manueovure. What happens if you are in a slip with right rudder, and the right engine stalls / fails ?

OMG! Your throttles are at idle when you Forward Slip! For the LOG, if you have no knowledge of aircraft structures, please don't waste my time.

You know what. See ya! ....I'll email my concerns to Boeing.

Excuse my ignorance I know what a 'side slip' is but what is a forward slip?

Dear DVT,

My my such language... I don't consider S-turns aerobatic, but I do consider S-turns on final approach to cause an unstable approach, and I do consider forward slipping to cause the pax to be uncomfortable, so better to go around, or to ask for a longer vector to final, then to start steering around the localizer or to get your airplane down.

As I said, on the structural level you are asking for: A rudder is made to cope with the stress in an Engine-Out-situation and for decrabbing and the like, not to cope with continuous or repetitive stress in various approaches where your own true-airman-I-use-30-degrees-of-bank-descent planning wasn't right on the spot.

I do think the opinion of less brave pilots who stick to SOPs could be very valuable, and though sidewards slipping or forward slipping is possible in a 737 it might not be ideal or worse even it might cause structural damage when done too often.

Almost all posts in this thread include the question "is it wise to perform such a manouvre near the ground?" or "why not perform a go around and try again?"; before attacking all these professionals as non-airmen, or before reducing your attention to Wild E Coyote-pilots who don't give a damn about SOPs, you could consider the experience that is behind these questions.

On top of that: Shouting 'OMG' (o my god?) if somebody talks about an engine failure during such a slip manouvre: yes the throttles are idle if you are to lose altitude, still, especially in the approach, with the throttles at idle you have about 35% N1, which is lost during an Engine Failure, and even for that amount of thrustloss you need to correct with the rudder. Might seem to be a minor detail, but not less important.

P77

PS By the way, I lost my flight director once and didn't "**** my pants", but landed uneventful.

All,
To quote Joe Sutter, late of Boeing some years ago, when I this asked the question, the answer was: “if the manual doesn’t prohibit it, the can do it”.

It was prohibited on the 707, because at that time SFAR 422B Certification did nt include it, any wiping engines of a pylon in an unintentional slip was demonstrated on more than one occasion.

All the 747's, up to the 744 slip quite nicely, and the quite moderate actual angle of bank and out of balance forces than can produce quite a useful increase in RoD, if you get it right, won’t spill anybody’s G&T.

All the rules you were taught in basic training still apply, regardless of the size of the aircraft.

As for the suggestions that a missed approach is the “professional” answer, firstly I hope nobody is suggesting continuing this or any similar maneuver within the “stable approach” envelope, usually 1000 agl and down.

Secondly ATC planning only ever accommodates a missed approach a second best option, ATC can suffer from getinitis as badly as a pilot.

Tootle pip !!

I, like M. Mouse, had no idea what a "forward slip" is ... and, having looked at the page referred to by dvt, I still don't ..... could someone please explain to me what the difference in the two examples is ? I can see absolutely no difference between the two .... or am I just too dumb to see a subtlety in the detail ? At the risk of having to eat my words .. as I have to do from time to time ... is this set to become one of those wonderful myths of aviation ?

As a general practice, I would be very wary of adopting the attitude that, if the book doesn't prohibit it, then it is OK to do something .... the absence of prohibition in respect of a specific matter may only indicate that it didn't come up in the certification process .... far better, I suspect, to follow the guidance material without too much potential experimental flight testing in line operation ....

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.

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'...

%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 ?

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.

I don't claim any real-world experience beyond low hours solo glider flying, but I'm with John 100% on this one.

"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.

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'.

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.

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!! ":D

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 ...

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.

"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".

"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

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.

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.’

dvt

For the benefit of my curiosity when would you envisage needing this particular 'trick'?

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.

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?

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.

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.