mark

For all round parachutes, including those with pulled-down apexes, the air that goes in the bottom has to come out somewhere. It can go through permeable fabric; it can vent through the apex vent or other vents; or it can spill out from underneath the skirt as the parachute oscillates. For a typical round parachute (no pulled-down apex), as you enlarge the vents, you reduce the amount of air spilling out from under the skirt. The same amount of air goes from inside to outside, so there's the same amount of drag, but the canopy is more stable. You are right, though: at some point as you enlarge the vent(s), the amount of air going through the vent(s) exceeds the amount of air that would have spilled from underneath the skirt. Then the canopy has less drag and descends faster. I think you'd be surprised at how big you can make the vents before there's an increase in descent rate. Modified 28' rags and T-10s frequently had the equivalent of 2 entire gores (triangular section between radial seams, from skirt to apex) removed. The vent in a vented pilot chute is at the apex, but not at the top. Because the apex is pulled down more or less even with the skirt, the top of an inflated pilot chute is really a ring about half the diameter of the fully open pilot chute. Whether a pulled-down-apex apex-vented pilot chute has more or less drag depends on how far the apex is pulled down and how big the vent is, which is to say, how much the vent contributes to stability. Mark

I disagree. I don't think a student who has trouble with canopy control is helped by making him pay for and jump with a freefall lifeguard. Mark

Might be you. Might be absurd. Kinda depends. I'd support a nuisance fee that encourages a clingy student to take more responsibility for her jump. There's not enough info in your post to tell if that's the case with you. If I were a DZO, I'd be willing to accept the maintenance and repairs for student radios for students, and I'd accept that occasionally radios are lost or damaged in normal use. I'd try to build those costs into what I charged for student jumps. Gear rental for graduates doesn't normally included radio, though. In that case, I'd see the fee as a kind of insurance premium. Is there a charge when graduates use student altimeters, helmets, jumpsuits, goggles, or gloves? If so, then a charge for using a radio isn't out of line. Depends on how rentals work there. If a graduate student uses a radio, are there enough radios left for other students? Where I work, when an up jumper borrows a student altimeter off the rack, I don't worry about it much. But if a student needs that altimeter to make her jump, I'm not shy about getting it back from the experienced jumper. If I knew the altimeter were rented, I'd need to find a different one for my student. Is it like that there? If I were a DZO and I had enough equipment for other students, I'd be inclined to let an instructor use a radio for a Level 8/Cat F-G-H student, figuring I'd make it back in the rest of the equipment rental and lift ticket -- but I'd like to see that student off radio as soon as possible. And I'd be concerned that that hadn't happened yet. Mark

Well, we teach Up-Down-Go, but usually in the door or on the step it's more like Wiggle-Go. Sometimes just Go. Sometimes something else inventive and challenging. And it's always only the student or his legstrap moving, not the instructors. Mark

I'm going to agree, then disagree. In terms of reducing oscillation, I don't think it matters where you put the vents. The pressure is the same everywhere inside the canopy, so allowing air to escape through vents halfway between skirt and apex will be as effective in reducing oscillation as vents near the skirt or vents closer to the apex. The virtue of vents near the skirt is the vector of the escaping air. The closer to horizontal we can make the escaping air flow, the more effective the vents are as drive windows or turn windows. And then I'm going to agree again. If the apex vent were large enough to allow the same airflow as vents near the skirt, the descent rate would be greater, again due to the direction of the vented air. Mark

I think you meant to say that there was less (lift-)induced drag with higher aspect ratios. If higher aspect ratios require more lines, their parasite drag increases. As with many things aerodynamic, there's a trade-off. Mark

That depends on how high you set the par. If it's under the par, it's definitely at least a birdie. Mark

Here's a quick method that gets a similar answer: Average the wind speed to the nearest 10 knots. Go upwind a quarter-mile for every 10 knots. (Subtract a quarter-mile for forward throw.) Because of the limitations of winds-aloft forecasts, I don't think it's worth making a more accurate estimate than that. The winds aloft forecasts are derived from radar tracks of weather balloons released twice in each 24-hour period at selected locations. The nearest one to any particular dropzone may be 50 to 100 miles away, and the winds do not change magically at 8, noon, and 4. The forecast is most accurate in between fronts, least accurate when there is a frontal passage occuring between the forecast station and the dz, and least useful when the winds aloft are opposite the surface winds. Mark

That would be (2^10) - 1, assuming each digit has two possible positions and you start at 0. If each digit has more possible positions, then you could count higher. A finger could be fully flexed, partially extended, or fully extended to enable you to count to (3^10) - 1. You are limited by how small the extension can be and still be detectable. If you could detect 10 increments for each extension, you could count to (10^10) - 1. You could count even higher if hand position had some value: one palm (or both) toward you or away from you would increase the count by a factor of 4, or by 9 if an intermediate position had value. Also even higher if hand altitude could be assigned a value, depending on whether the hand was held up toward the ceiling, at waist level, or toward the floor. What do I win? Mark

32 is correct if you assume each of the options is independent, and each option can have only two values. Some options may have more than two values. Paint color, for example. Or paint finish: matte, eggshell, semi-gloss, gloss, satin. Some options may be dependent on others. For example, one style of cabinet door may be available only in white laminate, while another may be available in oak, maple, or cherry. There's not enough given information for a complete answer. Mark

I've tried no-sew fingertraps and gone back to previous methods. Whatever method you use, the running end which you'd like to stay inside frequently winds up poking out. Some jumpers can live with that, but most want it trimmed off. With the no-sew method, there's a limit to how much you can trim off before the the locking part is affected, and I don't want to find out exactly what that limit is. Mark

I vaguely recall that Cypres was designed with this capability, but not produced. There are some tantalizing Google hints (dead links here and here), one of which includes Bill Von's name. Perhaps he can remember the gist of that rec.skydiving exchange 10 years ago -- or perhaps AirtecKai or Helmut can chime in here. Mark

You are not alone in having that impression, but there is no FAR requiring it. Doesn't matter how many doors, how many engines, how big the cabin, how far from door to pilot, or intentions of the occupants. If there were such an FAR, every Cub pilot in the world would be in violation. Some time just for fun, you and a friend should take up a 152 and experiment with using the doors as a substitute for the rudder. Mark

US cloud clearance and visibility requirements are given for altitudes above sea level, so you'll need to make an adjustment for your non-sea level dz. Visibility is in statute miles, not nautical. The cloud clearance and visibility requirements apply during freefall and under canopy. If you violate the requirements, your pilot could be in big trouble with the FAA. Sometimes it looks as though we've punched clouds, but it's usually just parallax, a kind of optical illusion. We should never brag about punching clouds, since we would never do it deliberately and it only gives ammunition to the McNasties and hostile airport boards. Below 10,000 MSL: Skydivers must be at least 1000 feet above clouds below them, at least 500 feet below clouds above them, and at least 2000 feet horizontally from clouds. The flight visibility must be at least 3 miles. Above 10,000 MSL: Skydivers must be at least 1000 feet above clouds below them, at least 1000 feet below clouds above them, and at least 1 mile horizontally from clouds. The flight visibility must be at least 5 miles. These requirements are not for the benefit of skydivers. They are for the safety of IFR traffic legally allowed to fly through clouds. IFR aircraft are required to see and avoid when they are in visual conditions, and the cloud clearance and visibility requirements are intended to allow them the time to break out of a cloud and have some reaction time for collision avoidance. Mark

It was a typical hinged-at-the-top inflight door. I don't think we would have exceeded the door-open speed. The real problem would have been climb performance. Mark

It's in the ISP (SIM Section 4), Cat D. Mark

Wait! There's more! He also told me that since it was such a hot day, I could open the door at 1000 feet, so we could be cooler on the rest of the climb to 11K! Mark

I'll bet Jan will be making a correction to that page soon. That "operation procedure" is for the Sentinel-Sentry (or Sentinel/Altimaster?), not the Sentinel Mk 2000. The Mk 2000 does not have a zero needle, black quadrant, or windows (except for the red/green calibration lights. The Mk 2000's rate-of-descent sensor means it doesn't have to be turned off for normal (1970's - 1980's) canopy flight. In fact, there is no on-off switch, so if you want to turn disarm it for, say, an emergency descent in the aircraft, you have to physically unplug the control unit from the firing cartridge. Mark

THERE IS NO FAR requiring the pilot to wear a parachute or have one available during jump operations. Doesn't matter how big or small the aircraft is, how far the pilot is from the door, whether there is a bulkhead between cockpit and cabin. Sometimes the STC for a jump door will specify that the pilot must wear a parachute, but the STCs mostly deal with how the door is made, installed, and operated. Some STCs are silent with regard to whether the pilot should wear a parachute, and that means it's pilot/operator choice. Pilots (and other occupants) must wear parachutes during aerobatic maneuvers. Aerobatic maneuvers are usually defined as flight attitudes not necessary for normal flight. Mike Mullins' normal descent to the surface has been litigated and found to be normal for jump operations. If a parachute is available to the pilot for emergency use, it must be in date. ------------- What's legal and what's safe are not necessarily the same thing. I am not prohibited from wearing a parachute when flying jumpers, so I do. The straps are snug, and I'm ready to go skydiving if I need to. I have had a 182 pilot tell me he'd buckle the harness straps if he needed to, but otherwise he would rather be comfortable, just use his parachute for a back cushion. I have yet to see an Otter pilot wear a parachute. The few King Air pilots I know do not wear a parachute either. This in spite of a number of tail strikes (mostly King Air), a wing strike (Deland), and a wheel/tail strike (? I don't recall the details, Birdman/Perris). In a fatal accident several years ago in Australia, the Caravan pilot survived only because he was already wearing a parachute when an accidental deployment in the door resulted in loss of aircraft control. Mark

The indicator window on the control unit is marked with a red zone below 1000 feet (no accurate adjustment possible), and with tick marks every 500 feet from 1000 to 4000. Spacing of the tick marks combined with parallax make it difficult for a user to accurately set the firing altitude any closer than a couple hundred feet. Unless you are jumping in the factory test chamber, the accuracy they report is not useful information. The accuracy standard for the repack chamber test is + 300 feet. If the firing altitude is set for 1000 feet, the unit may fire as low as 700 feet and still be within acceptable calibration. The warning sticker on the side of the control unit says you must have an open canopy at least 1500 feet above the unit pre-set altitude. If you set the indicated firing altitude at 1000 feet then the possible firing range is from 700 feet to 2500 feet. I hope that your local FXC test chamber is better than the $5000 chamber FXC sells. Their chamber includes an aircraft Vertical Speed Indicator, for which there are no FAA calibration standards. With an electronic AAD, you can have some confidence that estimated vertical speed is derived mathematically from changing pressure (indicated altitude) measurements rather than from an uncalibrateable (is that a word?) instrument. Some riggers use a plastic bag instead of a test chamber. They wrap the bag around the control unit, then squash the bag. The rapid increase in pressure simulates a rapid descent. Unfortunately, the pressure inside the bag before it's squashed is surface pressure, so all the test does is show the unit will fire at impact. Installation of a pin-puller type AAD requires some attention to detail. Installing the puller eye too close to the ripcord pin means that the puller may not retract the pin, either because there is not much energy left in the spring at the end of it's stroke, or because there's still slack in the puller cable at the end of the stroke. Installing too far away means a chance of accidental deployment without an actual pull. Since most rigs are set up only for Cypres/Vigil, installation of an FXC 12000 requires extra expense for firing spring unit pocket, hose/cable channels, ripcord and puller cable clamp mount, and control unit mount. Putting a puller eye around a ripcord pin lifts the pin away from the grommet and flap, making the pin more susceptible to an accidental pull from rubbing the cover flap on part of the aircraft in the course of normal movement in the aircraft. Finally, packing with a large FXC box in the reserve container requires some adjustments to regular packing technique to even out the bulk. For example, with manufacturers' permission you can pack the lines into just the right half of the line-stow pocket on the freebag. What does your rigger think about that? Buy a Cypres or Vigil. Mark

FXC makes two varieties of AAD for the sport market. The FXC 12000 is a mechanical (no batteries) ripcord puller, available used for not much more than the cost of shipping, but with high maintenance costs: about $125 every two years for a factory visit, plus the cost of chamber-testing at each repack (nearly $40 at one west coast facility). That makes it more expensive to use than a Cypres. You must have an open canopy by at least 2500 feet or you risk a two-out situation. FXC also makes the Astra electronic AAD, also-ran competition for Cypres. it's better than a 12000. Sorry, I'm having trouble saying more about it. This classified ad mentions an "FXC Sentinel" AAD. There's no such thing, and it probably refers to an SSE Sentinel. You might want a Sentinel for your skydiving museum, but parts and service are hard to some by. IIRC, the firing cartridge has a 2-year shelf life from the date of manufacture, so it's almost impossible to find a supplier who turns over stock fast enough to get you a fresh one. Plus you need to find a rigger familiar with it so you can get your rig repacked -- and there aren't many of us left. Mark

Could you cite the FAR, please? Mark

Why rot in the earth when you can rot above it? Visit the Body Farm, now or later! Mark

Okay, I watched the RWS video pilotdave pointed to, and I see that a baglock does not generate as much drag as I thought it did. I was wrong. My apologies, tr027 & ruschmc. Mark

I'll confess I don't understand the physics of this, and I'm prepared to be wrong. Could you describe the event in more detail, please? For example, what kind of pilot chute, and was there any problem with the pilot chute? Were the lines fully extended and just the locking stows failed to unlock, or were the lines tangled in some way? Thanks! Mark