Busting Crossing Restriction / Calculate RoD???

[jordanal 70]

OK, JD and his new crossing restriction of "Descend to an Altitude in 30 miles or less" still gives me problems every once in a while and FSDO is starting to get on my nerves, LOL... I usually get no errors on a flight anymore but if I do, it's because I didn't calculate the descent rate properly. Over the years of simming I've just been pluging something in the FMC off-the-cuff, like -2100ft/min which obviously has become a bad habbit.What is the formula (using my hand-held calculator in flight) to properly figure out the proper rate of descent in ft/min, given a constant speed (in knots), and a limited amount of miles? Thanks for algebra lesson, I always did stink at math. ;)AL

[ronzie 21]

If you are using a GPS, you can estimate your ground speed on the higher side of the decimal. I also don't worry about the roughly 10% difference between a nautical mile and statute mile . . . hopefully ATC is talking in nm anyway.So if your GS is 250 knots which is 250 nm/hour, then dividing by sixty gives you just over four nm/minute. Within 30 nm div by five nm per minute gives you (rounded to the least) 6 minutes to do your descent from current to desired. So now lets say to drop 12,000 feet in seven minutes is 12,000 div by 6 or 2000 feet/min. for your minimum descent rate .I frequently roughly estimate my ground speed in nm/minute to estimate waypoint arrival times in minutes. Without a GPS you can watch DME assuming you are on a route in line with a VOR/DME or VORTAC and not within slant range error. Then look at your DME now and one minute later. The difference is your GS speed in nm/minute.Now, maybe I'm all wet about this, but so far it has worked for me.Is this sense or nonsense?

[Guest Scott Campbell]

The most basic rules are:1) start the descent by cutting power and slowing down2) when you are under FL240, use IAS not mach,3) when you are below FL200, try to be at 310 knots or less,4) do the distance calculation from where you are to 30 miles ahead of you. You can do this either by looking at the DME of the VOR if there is one, using GPS to see distance to the next waypoint and do the calculation, or looking at the RC AdvDisp display and noting the distance to your waypoint from where you are and subtract 30 (same as using GPS),5) slow to 250 knots some 10 nm before the crossing/descent point. That gives you time to get the weather.The main thing to do is to think ahead.Let's say you're cruising at FL350 at 0.84 Mach. You get the descend to FL240 instruction. That's the ToD point. You are doing something like 420-470 knots GS, depending on wind, so you're moving fast. IAS will read something like 280 knots. You know you're going to get a descent and a PD and a crossing point instruction.If you're using the Autothrottle, set speed to Mach 0.8 or 0.78 (if you're flying a 737-600 or older, the specs vary, but assume Mach 0.7). Set the altitude to 24000. Set VS to somewhere around -1800 to -2100 fpm, depending on how fast you want to descend, but keep the speed to your setting. You can also control speed somewhat with spoliers. The rule with the big iron is you can descend fast, or you can slow down fast, but not both. So try a medium of both. Slow and descend.By the time you reach FL240 you probably have already been given the PD to anywhere from 19,000 to 15,000. Head down at the same settings. When going below FL240, switch the Autothrottle to IAS and set to 300-310 knots. Or note the IAS and keep it to between 300 and 310.You should try to maintain 300-310 knots by adjusting your descent rate. Use spoilers where necessary but sparingly.You should now be at the PD altitude before getting the crodsing point altitude. When you get it, you know you are 30 miles from it. The calculation should be fairly simple. Your altitude (17,000' say) to 10,000, say. So you need to drop 7,000' in 30 miles, or 233 feet per mile. Let's say 7,000' in 20 miles to be there 10 miles beforehand - so that's 350' per mile. How fast as you doing? Hopefully between 300 and 310 knots, which is now close enough to the ground to be close enough to GS (without wind). So rate=310, distance=20, time=X: rate*time=distance: time=distance/rate: time=20/310: time=3.87 minutes. You have 3.87 minutes to descend 7000 feet. 7000/3.87=~1800 fpm. You need to descend at 1800 fpm. QED. Of course your descent rate will decrease as you slow to 250 knots. I can calculate a good rate of descent while slowing down (usually by looking at the arrow on the FSNav map ;-)).You can do that simple calculation anywhere along the line to know how fast to descend at what speed at what time.But the main rules are: Think ahead and slow to descend.Do I have a clue what I'm talking about? No, but I don't bust any altitude instructions...well, hardly ever. ;-)Hope that helps.

[ronzie 21]

Just a quickie to an emphasis to what Scott said. Speed limits exist below various flight level threshholds. You have to slow down anyway which gives you more time to descend.I have seen posts from "real pilots" that when required, they increase heavy descent rates up to 3500 fpm at the higher levels. With pressurized cabins, it should not be too discomforting for the passengers, but there are some g-forces that may provide a little discomfort.There is a balance between fuel use efficiency demanded by operations that determine a compromise between flight comfort and minimal fuel use. The faster you descend with the max airspeed allowed, probably the less fuel is used. Then again, if level out too short of the crossing point, you may have to use more fuel in level flight then if you descended at a slower rate.The airline ops manuals have this all figured out for max efficiency but the pilot has to insure operating within the ATC tolerances as issued.

[CactusDx 0]

If you want a real "quick and dirty" no brainer low math method a good general rule of thumb is that it will take you 3 miles to descend 1000 feet. Scenario: You're at FL350 and you need to make a published crossing restriction at FL240. 35,000 - 24,000 = 11,000 feet. Drop the zero's and multiply times 3: 11 x 3 = 33 miles. That will give you a DME fix, give or take a mile or two, as a point where you need to start down to make your crossing. Using this method will generally give you a descent rate of around 2000 fpm, however don't get in the mind-set that a descent rate of greater than 2000 fpm is "wrong". Depending on traffic or other considerations ATC may make requests that will yield descent rates of 3000-4000+ fpm.Enjoy!Michael CollierDispatcher/SOCAmerica West Airlines