Decelerating decent

[Jaybird180]

Yesterday, I was in a situation which I've only found perhaps twice before. I needed to decend to my target altitude and slow to my target speed. I have found some airplanes less willing to do this than others and wondered perhaps if there was a better technique I should use.

I typically accept whatever decent rate I'm getting while I slow the airplane. Sometimes it takes awhile, but I default to getting the speed under control first.

I spoke about this after my flight in the SR-22 awhile back with a Cirrus pilot and he suggested (insert unintelligible mumblings here).

Yes, I confess that better fore planning would have prevented me from being in the situation in the first place.

What technique do you use in this situation?

 

[Rex Kramer]

All things being equal, I'd go for altitude first then speed. Once you are level the speed will bleed of fairly quickly. If you are slowing down and going down at the same time, it will take forever to get either of them (or both) done. You can always ask the controller (I'm assuming a "cross XXX at xxx and xxxkts) which one they'd prefer to have. If you can't do it, tell them unable. Worst case is a vector.

 

[Teller1900]

If the altitude is most important (eg: crossing restriction, traffic conflict, etc) I pitch for Vmo/Mmo and drive it down, then slow down.

If the altitude is flexible (I was kept high entering the pattern, cleared for the visual from way high up, etc) I'll hold my altitude, sometimes to the point of being uncomfortably high, until I can slow to gear/flap speed. Once I get the gear out or a notch of flaps, I can come down MUCH steeper without really getting too much closer to the airport. It's much easier to stabilize an approach, in most situations, if your speed is under control and you just need to lose a bit of altitude.

En route I prefer high speed, idle descents. In the terminal area I prefer slower, configured, steeper descents. YMMV.

 

[wabower]

The "slam-dunk" approach procedures commonly used at busy reliever airports impute the capability for such maneuvers to the pilot. In my case it was common to get "maintain 170 'til the marker" in the B-200 which meant that I was going to execute the ILS starting at 2,200' and the assigned speed and be able to descend on the glideslope to ~900' while slowing to final approach speed of 100 kts. It can be done, but helps if you have refined and practiced the process in advance.

PS: And the technique required isn't published in any of the "stabilized approach" SOP's in the CRH.

 

[zaitcev]

I'm nowhere as sophisticated as the posters above, so I just slip a lot, sometimes horizontally. Works wonders in Mooneys BTW (although I only had a chance to try it once there). Most of the time I fly Arrow, and I remember having a problem getting speed under control in it initially. Eventually I had to adopt airliner-like glideslope and imagined it was the only way to do it. A month ago I went up with my CFI to do an activity of WINGS which included short field ops. He taught me to slow ahead of time (a tick under 1.3 Vso or 78..80 mph), and once there, Arrow can dive just like a Cherokee, using its constant speed prop as a brake. I was quite amazed to discover that it worked.

 

[Henning]

What do you do when you're hot and high in the pattern?

 

[Teller1900]

What do you do when you're hot and high in the pattern?

1. Pull power.
2. Raise pitch (level if need be).
3. Prop(s) forward (if not already).
4. Configure on-speed.
5. Dump the nose (if altitude permits).
6. All that fails - go around, try again. Four minutes in the pattern should be plenty to keep me from pushing a landing past the point of dumb ideas.

Slip MIGHT be in there at 3.5 if passengers aren't on board.

 

[DouglasBader]

It's difficult in many aircraft, big and small, to slow down and go down. Usually one then the other.

If getting to the destination sooner is the goal, then maintaining speed in the descent might be in order. The controller might not want that, or you might have been left high, close to the field. In that case, you're better off slowing before going down, if you can; reduce your forward progress toward the field as much as possible to give yourself more time to descend. You may need it.

Throw the gear out early; slow to the appropriate gear speed and toss it out; that makes for a great speed brake. You can always retract it later. In my current mount, we can get 4,000 fpm downhill with just the gear out at idle. In a light airplane, however, it's usually not a good idea to do idle descents if you can avoid it. I can put out speed brakes and really come down, too. However, if I'm left exceptionally high by ATC and need to descend, I'll request a turn in a hold or a 360 degree turn to get down, rather than trying to force the airplane down. If I can, I'll configure a little early, too; it lets me go slower which gives me more time over the same distance to make my adjustments.

There's the half-and-half method, in which you really do go down and slow down at the same time, but it's half-baked, involving not the best of both worlds; reducing speed somewhat and vertical speed someone, one tries to do both at once. It's usually to better to do one then the other, regardless of which you choose to do first.

Do yourself a favor; if you're in a piston airplane, don't let ATC goad you into a "170 to the marker," or "180 to a 4-mile final" kind of clearances. I don't. It's possible in turbine equipment with speed brakes and engines that can be pulled to idle without harm, and it's easier the heavier you are, too; the airplane slows more quickly just by raising the nose a bit, and configuring. Not so much in a T-210. Sometimes ATC thinks you can hold a high speed to the numbers and then make the first turn-off; it's a good way to make a landing a more expensive one. If ATC isn't paying for the airplane, don't let them dictate how you're going to get down and get landed. If you need something different, let them know as early as you can so that everyone can plan for the action; it works out best for all.

 

[Ted]

Comes down to knowing your airplane and practicing such things. I've shot an ILS in the 310 at 175 KIAS while slowing down to get the flaps, more flaps, and gear out. While doing so I've actually still been doing my progressive power reductions to be kind to the engines, and kept the needles centered. And since I'm a mere mortal and not Henning, that should be proof that you can do it, too.

What you're really looking for is a particular slope that you're trying to descend. In many aircraft, you will have an easier time getting the speed and altitude loss where you want them by slowing first, getting the drag out, and then pointing the nose down. Once you're above flap/gear speed it can be difficult to get below it in many aircraft.

Don't be afraid to do circles to descend, though. I had to do two 360 circles for a descent in the Navajo the other day while on a charter. If it were just me, I would've pointed the nose at the ground and made it in without those circles. The two passengers in back probably wouldn't have enjoyed that, though.

As Wayne said, you won't find it in any SOP. And you should only accept doing something like that if you're comfortable doing it. Never let ATC push you into something you aren't comfortable doing.

 

[Let'sgoflying!]

 

[poadeleted20]

If you've worked out the six configurations, then to slow in a descent, pull the power from cruise descent back to approach descent while holding pitch attitude (I.e., don't let the nose drop further).

 

[LearDriver]

If you are stage cooling and atc is late with a descend and speed restriction then just ask which they want first. The altitude or the speed. They will usually want the altitude first when en-route. As you get closer to the airport speed becomes more of an issue.

 

[peppy]

It depends on the plane. In a Bonanza the gear allows a lot of flexibility by acting as a speed brake. I love the Precise Flight speed brakes. In a Cirrus, pull power, pitch up to bleed speed, get flaps in and descend. Half vs. full flaps depends on how close you are to the airport. In general half flaps are best since that is best for the pattern with full flaps on base.

 

[bbchien]

Throw the gear out early; slow to the appropriate gear speed and toss it out; that makes for a great speed brake...

Uh. Doug. He flies an SR 22. His drag options do not include gear.

He has to just plan ahead.....

 

[Jaybird180]

C-172SP actually. 1st encounter was in a 22, then again in 182? But yes, all fixed gear. Could only control prop on the 182.

 

[timwinters]

A two turn spin would work nicely.

 

[DouglasBader]

Uh. Doug. He flies an SR 22. His drag options do not include gear.

Whatever. That's not the case, actually, but I think you get the point.

Use what's available, given the circumstances. Obviously if lowering the gear isn't an option, then don't do that. If it is available, then it becomes an option.

 

[poadeleted20]

C-172SP actually.

In a 172 (SP or otherwise), if you are stable in a cruise descent at cruise speed and 500 fpm, and then reduce power about 300 RPM (which should leave you right about the approach descent power) while pitching to keep the descent rate the same, and then trim for the reduced speed, you'll end up right around approach speed at the same descent rate. And I know you have those power settings and speeds on a card on the panel of your plane, because I put them there before you owned it.

 

[airguy]

The whole game here is energy management. You can slow down and then go down, or go down and then slow down - both routes take a certain amount of time to bleed a certain amount of energy. Aircraft bleed energy (drag) at a higher rate when they are travelling at higher airspeed - so technically you'll be accomplishing both faster by dropping at max airspeed (with all available drag options in use) to target altitude, then slowing to target airspeed. Keeping the airspeed high will maximize your energy bleed rate.

This of course assumes a TIME constraint for accomplishing both. If you have a DISTANCE constraint for accomplishing both that is tighter than the equivalent TIME constraint, you would be better off slowing first and then dropping. While less efficient at bleeding energy, you'll cover less distance in the process.

 

[bbchien]

When ATC holds you uncomfortably high uncomfortably close, slash airspeed to that compatible with what drag devices you have. Unlike Mr. Bade, you dont have spoilers, drag fences, nor gear at your disposal, and the 172 is basically a kit, rather than the desired "power off brick" with which ATC is epectant.

This is "staying ahead" of the airplane. Then when you can finally get descent authorized, out come the flaps, and the throttle, having been eased back, doesn't get "suddenly closed".

You can't throw the gear out on a 172 SP.

It is about energy management, and having sufficienty little energy so as not to bend the aircraft, and anticipating the "pickle" of being too high and too fast.

 

[wabower]

A mixed blessing of older airplanes like my 1960 Cessna 180 is the low flap extension speed. It's a bit of a nuisance in some ways but forces the pilot to plan ahead.

When ATC holds you uncomfortably high uncomfortably close, slash airspeed to that compatible with what drag devices you have. Unlike Mr. Bade, you dont have spoilers, drag fences, nor gear at your disposal, and the 172 is basically a kit, rather than the desired "power off brick" with which ATC is epectant.

This is "staying ahead" of the airplane. Then when you can finally get descent authorized, out come the flaps, and the throttle, having been eased back, doesn't get "suddenly closed".

You can't throw the gear out on a 172 SP.

It is about energy management, and having sufficienty little energy so as not to bend the aircraft, and anticipating the "pickle" of being too high and too fast.

 

[DouglasBader]

Eons ago, we did descents in 182A's, after dropping jumpers, with about 15" of manifold pressure and steep turns to descent. Dump the lift and let the airplane come down. Not always practical, but it worked keeping the engines warm and getting down fast. I often beat the jumpers to the ground. We'd drop at 15,000 or 16,000 and descend away from the drop zone. Back then we often used full slips at slow speeds in steep turns to come down.

I did that once with a load of jumpers on board after the jump got cancelled, without really stopping to think about it. When we stopped and opened the door, one of the jumpers crawled away from the airplane and vomited repeatedly.

 

[j1b3h0]

It takes a 737-800 12nm in level flight with idle thrust to slow from 340kts to 210kts indicated - without the use of speedbrakes. So, airline pilots plan on being at a certain geographic place at a certain altitude and airspeed. For us 737 guys, 35mn from the airport at 10,000ft and 250kts is pretty good advice, IFR crossing restrictions notwithstanding. If ATC asks for the impossible, tell them you're unable. Doesn't matter if it's an untenable crossing restriction, a turn to the ILS final from too high and too close to the marker or 'hurry and make that runway exit' from the landing roll. If you can do it, sure. Otherwise "unable".

 

[Henning]

The whole game here is energy management. You can slow down and then go down, or go down and then slow down - both routes take a certain amount of time to bleed a certain amount of energy. Aircraft bleed energy (drag) at a higher rate when they are travelling at higher airspeed - so technically you'll be accomplishing both faster by dropping at max airspeed (with all available drag options in use) to target altitude, then slowing to target airspeed. Keeping the airspeed high will maximize your energy bleed rate.

This of course assumes a TIME constraint for accomplishing both. If you have a DISTANCE constraint for accomplishing both that is tighter than the equivalent TIME constraint, you would be better off slowing first and then dropping. While less efficient at bleeding energy, you'll cover less distance in the process.

This is not necessarily true, the plane will only 'bleed energy off' if the drag is greater than the input. At maximum speed this is incorrect, actually if you are stable at maximum speed you aren't bleeding any energy off, you are at an equilibrium and you are carrying the Vne's worth of kinetic energy.

Any time you are on either side of L/D max (best glide) you are increasing drag. If you reduce speed below L/D max your increase in drag is much more effective at bleeding energy than any increase in drag from increase in speed (kinetic energy). The aerodynamics of most planes make's it impossible to reduce kinetic energy with the nose down without employing some high drag device or control input.

 

[poadeleted20]

Folks, this ain't a big, slick, jet bombing in out of the flight levels at speeds approaching the Mach numbers -- it's a draggy old 172 going something south (probably well south) of 140 knots indicated. Everything the OP needs to do this is in post #11.

 

[jesse]

Eons ago, we did descents in 182A's, after dropping jumpers, with about 15" of manifold pressure and steep turns to descent. Dump the lift and let the airplane come down. Not always practical, but it worked keeping the engines warm and getting down fast. I often beat the jumpers to the ground. We'd drop at 15,000 or 16,000 and descend away from the drop zone. Back then we often used full slips at slow speeds in steep turns to come down.

That is how I do it, in the year 2012, in 1960 182C models. Not much has changed

 

[airguy]

This is not necessarily true, the plane will only 'bleed energy off' if the drag is greater than the input. At maximum speed this is incorrect, actually if you are stable at maximum speed you aren't bleeding any energy off, you are at an equilibrium and you are carrying the Vne's worth of kinetic energy.

Sorry buddy - basic physics. You're dead wrong.

At maximum speed (or any speed above zero for that matter) you have drag that is bleeding energy - unless you've managed to find a perfectly frictionless aircraft, which we would all be interested in seeing. If you are stable at maximum speed, that means you are putting in as much kinetic energy as you are bleeding off (thrust equals drag), but during a closed-throttle descent that incoming energy comes at the expense of potential energy (altitude). The higher the drag rate (drag devices, slip, closed throttle, flaps - anything to dirty up the airframe), the faster you are dumping kinetic energy into frictional heating and turbulence of the airstream you are flying through and the more potential energy is required to be traded into kinetic energy to maintain a specific velocity. For dumping any given quantity of potential AND kinetic energy, the shortest TIME for doing so will always be accomplished by use of a maximum possible drag, maximum airspeed descent to new target altitude followed by deceleration to new target airspeed.

Unless you are planning on slamming into the ground and wrinkling metal in the process, there is absolutely no way to get rid of potential energy in a conventional aircraft without first converting it into kinetic energy, and then dissipating that resulting kinetic energy into the world around you via some type of drag device.

You wanna pick a fight about physics and explicit terminology, I'll be your huckleberry.

 

[Henning]

Sorry buddy - basic physics. You're dead wrong.

At maximum speed (or any speed above zero for that matter) you have drag that is bleeding energy - unless you've managed to find a perfectly frictionless aircraft, which we would all be interested in seeing. If you are stable at maximum speed, that means you are putting in as much kinetic energy as you are bleeding off (thrust equals drag), but during a closed-throttle descent that incoming energy comes at the expense of potential energy (altitude). The higher the drag rate (drag devices, slip, closed throttle, flaps - anything to dirty up the airframe), the faster you are dumping kinetic energy into frictional heating and turbulence of the airstream you are flying through and the more potential energy is required to be traded into kinetic energy to maintain a specific velocity. For dumping any given quantity of potential AND kinetic energy, the shortest TIME for doing so will always be accomplished by use of a maximum possible drag, maximum airspeed descent to new target altitude followed by deceleration to new target airspeed.

Unless you are planning on slamming into the ground and wrinkling metal in the process, there is absolutely no way to get rid of potential energy in a conventional aircraft without first converting it into kinetic energy, and then dissipating that resulting kinetic energy into the world around you via some type of drag device.

You wanna pick a fight about physics and explicit terminology, I'll be your huckleberry.

That is where I see the difference, altitude that is not being converted into kinetic energy is not yet energy, it is only potential; you aren't "burning it off with drag", you are leaving it on the table unrealized.

 

[Jaybird180]

What! Henning this is not the thread about selling your 310.

 

[airguy]

That is where I see the difference, altitude that is not being converted into kinetic energy is not yet energy, it is only potential; you aren't "burning it off with drag", you are leaving it on the table unrealized.

So potential energy is not energy in your world?

Better rewrite all those physics books boys...

The OP on this thread is asking about a decelerating DESCENT - or did you not catch that? Descent = loss of altitude = loss of potential energy. Now you're trying to turn it into an altitude-maintaining level deceleration, that's not the situation he asked about.

 

[j1b3h0]

Folks, this ain't a big, slick, jet bombing in out of the flight levels at speeds approaching the Mach numbers -- it's a draggy old 172 going something south (probably well south) of 140 knots indicated. Everything the OP needs to do this is in post #11.

You're right, Ron. The only similarity between the method of the 172 pilot and the jet pilot is that they both must plan for the descent...that' what I was trying to say (but didn't say very well).

 

[Jaybird180]

You're right, Ron. The only similarity between the method of the 172 pilot and the jet pilot is that they both must plan for the descent...that' what I was trying to say (but didn't say very well).

And what was said in the original post. However the question was posed for the situation where poor planning setup the situation in the first place.

 

[Threefingeredjack]

Decelerating descents are a piece of cake, can't figure out what all the hoo hoo is about. You just roll off a little throttle, ease the cyclic back a touch and pull up on the collective a smidge, you can decelerate to a stop if you want......... There, see you're all smiling and the heat is gone. OBTW...I'm going with post #11 too.

 

[Fearless Tower]

Folks, this ain't a big, slick, jet bombing in out of the flight levels at speeds approaching the Mach numbers -- it's a draggy old 172 going something south (probably well south) of 140 knots indicated. Everything the OP needs to do this is in post #11.

And here I was going to recommend reverse thrust on the inboard engines, but I guess that only works in the DC-8....

 

[Henning]

So potential energy is not energy in your world?

Better rewrite all those physics books boys...

The OP on this thread is asking about a decelerating DESCENT - or did you not catch that? Descent = loss of altitude = loss of potential energy. Now you're trying to turn it into an altitude-maintaining level deceleration, that's not the situation he asked about.

No, POTENTIAL energy is not yet energy, you don't "burn it off".

 

[Threefingeredjack]

And here I was going to recommend reverse thrust on the inboard engines, but I guess that only works in the DC-8....

I do not have any DC-8 time, but I'm guessing if you reversed the other two as well you get an accelerating descent???

 

[j1b3h0]

And what was said in the original post. However the question was posed for the situation where poor planning setup the situation in the first place.

Jay- Regardless of type aircraft, one can't do the impossible, so, tell ATC you can give them either altitude or airspeed, but not both. They will tell you altitude or airspeed first or they could turn you in a direction which you'll have the room to do both. Much sillyness has resulted from extraordinary efforts of pilots trying to comply with ATC...much of it from their poor planning. Whether it's a new tailwheel pilot asked to take the first available exit from the runway, an instrument pilot in IMC cleared for the approach from well above the glideslope, just outside the marker, or a crossing speed/altitude resriction the laws of physics won't allow, the correct reply from the pilot is UNABLE! The standard Nancy Reagan answer: Just say no.