Dumping height; why not increase speed

[whatunion]

it has been suggested on here that one way of loosing height with an engine out would be to approach the stall or fly below best l/d speed.

would anybody consider increasing speed to increase rate of descent above best l/d ratio speed. perhaps even right upto flap limiting speed with full flap?

this argument of course assumes you have height 'in hand' to correct judgement errors.

could this method be safer than decreasing speed to correct error or technique?

[Gertrude the Wombat]

right upto flap limiting speed with full flap I've had one instructor telling me you can fly a 152 faster than the flap limiting speed with the flaps down, all you can't do is raise or lower flap above this speed.

This did not coincide with my recollection of the POH, so I checked, and could not convince myself that the POH allowed this. Anybody else heard of this theory?

[whatunion]

flap limiting speed is a never exceed speed unles described differently. he is correct in saying that the limiting speed may be brought about by the speed the motor can cope with. however my recollections of flying the very early c150s were that the limits were the same and it had manual flaps so the limit may have been used for aerodynamic consideration on the flap surface

so would you consider doing just that to inc rate of descent to get into a field?

[Gertrude the Wombat]

would you consider doing just that to inc rate of descent to get into a field No, I would not exceed the flap limiting speed on an instructor's say-so without seeing it in the book with my own eyes. It's just a little academic in the 152s I've flown anyway, as it feels like you can point them almost straight down, and lose height quite rapidly, and they don't actually speed up that much anyway (with full flap).

[Algirdas]

Whatunion, the problem I can see with increasing the speed to dump height is that it gives you a problem approaching the flare - you would need to allow extra horizontal distance at a very low height, in order to bleed off the excess speed - so you would need to aim to level out before the thresh-hold - and it would be easy to misjudge this either way - running out of height early and touching down before the 'runway', or vice versa, floating along merrily until running out of runway altogether! Not a problem I guess if you have lots of runway length and a short LDR, but if we are talking engine out, then that scenario is hardly probable - more like you would finish up in the hedge at the beginning of the selected farmer's asset, or in the hedge at the end of it ......... :{

[whatunion]

thanks
i was thinking more on the approach than over the fence

[FlyingForFun]

Whatunion, am I right in thinking that you're talking about being on the final approach to your field, and being a little too high? If that's the case, I don't believe that increasing speed would work.

If you think of the approach in terms of energy management, the situation you are describing would be a situation where you have too much energy. Increasing your speed will change that energy from height to speed.... but it won't reduce your total energy (at least, not significantly). You'll still have to get rid of the energy even after you've converted it from height to speed.

Compare this with sideslipping, which actually removes energy from the aircraft - a far more effective technique, since removing energy is exactly what you want to do. In fact, the idea of stalling, as suggested in the other thread, would probably be more successful in removing energy than your suggestion of increasing speed - assuming, of course, that you did manage to control the stall and land without spinning and crashing.


That's all true for the final approach, which, as I said, is what I think we are talking about. But if we are talking about being at altitude, then descending at high speed (in a steep descending turn) is exactly what I was taught to do for my CPL in the case of an engine fire, when the engine fire checklist failed to put the fire out. In this case, the steep descent serves two purposes. Firstly, it gets you close to the ground quicker - hopefully enabling you to land before the fire can take hold sufficiently to burn through something structural. Secondly, increasing the airspeed will aid in putting out the fire.

The technique I was taught for the CPL was to descend with full flap and gear down, at Vfe, with 45 degrees AOB, until the fire goes out. Then, wings level, gear up, flaps up, and commence a normal PFL.

However, on my Instructor skills test, my examiner suggested another method, which was to descend with no flaps, at Vne. The difference is that descending with full flap will give you a better rate of descent, and is preferable if you don't think the fire will go out. However, descending at Vne will obviously give a much higher airspeed, and therefore an increased probability of putting the fire out. So unless the fire has taken hold sufficiently that you don't think it can be put out by the airspeed, descending without flap at Vne may be preferable.

Your thoughts?

FFF
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[Genghis the Engineer]

Gertrude - if you'd care to send that instructor round to my place for an explanation of aircraft controls stress analysis, followed by a good slapping, please feel free. The flap limiting speed is there because above that the flap attachments may be overstressed, the wing itself may be overstressed by the torsion due to the excessive flap loads, and also the whole aircraft can be overstressed by a severe gust if flying above Vf when it hits.

Of course, there are a few talented individuals who have such a depth of understanding of aircraft structures, AND flying that they can carefully judge this and nibble the safety factors. However, so far as I am aware, neither of them fly a Cessna if they can possibly avoid it.



On a wider issue, losing height is all about losing energy (whilst obviously maintaining control of the aircraft). Sideslipping, by presenting a larger area to the flow (the side of the aircraft) does that quite well (just stay below Va with more than 1/3 control deflections, and no rapid control reversals out of the sideslip - for similar structural reasons).

Some aircraft however have a very high profile drag (a C150 or C172 with 40° flaps selected for one example, most traditional microlights for another, a glider with airbrakes out for a third). In those, increasing speed will push form drag up through the roof, which means that you are dumping a great deal of energy. On the other hand, some other types, such as a PA28 for example, have relatively low profile drag, and no drag flaps or airbrakes - in those all you are doing is turning potential energy into kinetic, which will then need to be dissipated in ground braking, or as you go through the hedge at the far end.

G

[Tinstaafl]

FFF, what Whatunion says is correct. Yes, there is a 'simple' transfer of Potential Energy into Kinetic Energy however the efficiency of this process is reduced if the a/c is faster or slower than Vg. Descending at any speed faster (or slower) than Vg will result in a greater energy loss - seen as a reduced glide range.

Why else is maintaining the correct Vg (for weight ie the most efficient AoA) emphasized as a necesessity for best glide range?

[TonyR]

G the E,

I too have seen the results of pilots in Cessnas putting the flap on at to high airspeed.

I know of one just a few weeks ago, when a pilot was left with asymmetric flap on a 206. he managed to raise the other one as he was passing through 90 degrees in roll.

As I said in another thread about the Rallye, "electric flaps can be damaged without the pilot being aware"

I do use high speed to decend in both the TB20 and the C340 (clean) keeping the pots hot, from a good distance out, (it also helps when mixing with Jet traffic)

With an engine out (in a single) I think Vg is what I'd aim for.

Energy management is the key to flying any aircraft, from a Cub to a 747.

Tony

[Snigs]

FFF

If the fire is an engine fire and is outside the cockpit, then the best thing to do is descend, 45 deg AOB and at just below Vne, this will give you a high vertical and forward speed and hence the best chance of blowing the fire out (and getting down quickly)!

If the fire is in the cockpit (i.e. an electrical fire) you don't want to fan the flames, so you descend at just below Vfe with full flaps, this will give you a high ROD but a low forward speed so you don't give the fire more oxygen to burn.

As for the other subject in discussion, S-turns or side-slip would be my preferred way of dumping height in order to hit my aiming point, IMHO fewer variables to misjudge.

(Edited to say this)

As a stress/design engineer I can unequivocally say that Vfe is a structural limit (not a motor limit) and you do run the risk of breaking bits of the aircraft if you put flaps down at higher speeds.

[FlyingForFun]

Snigs, your explaination of the different methods of emergency descents makes perfect sense. Pity it's never been explained to me like that before - thanks!

FFF
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[Gertrude the Wombat]

I can unequivocally say that Vfe is a structural limit (not a motor limit) Pity I can't remember which instructor said that to me, isn't it, otherwise I could point them at this thread.

[englishal]

perhaps even right upto flap limiting speed with full flap?
An emergency decent in many twins is described as gear down, full flap, 45° spiral descent right up to flap limiting speed. You can come down bloody quick as well ;)

EA

[Potter1]

Went flying in Palm Spring a couple of years ago, got checked out on a 172. Approaching the airfield we asked to expedite the approach, the instructor suggested a cruise descent at 115kts and 10deg of flap, worked well and the wings didn’t fall off. :O

Although I wouldn’t do it to my own ac!

P...

[englishal]

What a lot of students are not taught properly is that the white arc is not always the absolute limiting factor for flaps. It can be the FULL FLAP OPERATING RANGE. Read the POH though, and you may see its safe to use 10° of flap at higher than the top of white arc.

EA

[Kingy]

Just referring back to the original point... assuming there is no compelling reason to get down quickly e.g. fire/bits falling off/someone injured. I would suggest it is a lot safer to trim at the best glide speed and simply fly the distance required to get down at that descent profile. Sure, pick a field - it it's close and you are high, simply extend the circuit a little, keep the field in sight and use a standard constant aspect method in order to judge your turning points. Once you are sure of making the field, dump some energy using flaps or better still by slipping. This is standard gliding practice and has to be better than making a 'dirty dive' at the selected landing area.

Kingy

PS this is, of course an 'armchair' view, and I did not follow my own advice when I did it for real last year....! :}

[bookworm]

Genghis

Are you saying that a situation where Vfo < Vfe does not exist or just that the limits are equal on a small Cessna?