Quick Questions from Math Class
Question:
My 12 year old son is in 7th grade, learning about slope (y = mx + b). A question in his math book was, "How long will it take a plane to descend from 30,000 feet at 1500 ft/min?" I told him that the question could not be answered because (1) you don’t know how high the plane is, and (2) you don’t know where the ground is (MSL).
The answer is still 20 mins even in pilot world. If you have 30,000 ft on the altimeter and you nail a 1500 ft/min descent rate on the altimeter and the VSI, then the altimeter will read zero exactly 20 minutes later (or never, if the ground intervenes). If you’re going to bring in non-linearities in the altimeter setting, then you’ll have to use those same ones in the descent rate. But aren’t we clever to have made a 7th grade problem really complex:-) Peter Cassidy Pilot and Pedant
Response:
My 12 year old son is in 7th grade, learning about slope (y = mx + b). A question in his math book was, "How long will it take a plane to descend from 30,000 feet at 1500 ft/min?" I told him that the question could not be answered because (1) you don’t know how high the plane is, and (2) you don’t know where the ground is (MSL). (1) is easy and obvious, but for (2) I don’t have a copy of the FARs handy, so I need some cites… If you are above 18,000 MSL, you use 29.92" on your altimeter according to what reg? Thanks Sandy
A perfect example of why teachers should be paid more… JP
Response:
- A question in his math book was, "How long will it take a plane to – descend from 30,000 feet at 1500 ft/min?" I told him that the – question could not be answered because (1) you don’t know how high – the plane is, and (2) you don’t know where the ground is (MSL). While you are correct, do you REALLY what to piss off the math teacher with pressures, FARS, and MSL? Remember most people think the ground is zero and 30,000 would be 30,000 feet above the ground. And if you son is anything like mine, they would NEVER quote FARs to the teacher. After all you are the PARENT…and everybody knows parents are the STUPIDEST creatures on earth 
— ——- Virginia Beach, Virginia —– (804) 471 6776
Response:
- Hide quoted text — Show quoted text – My 12 year old son is in 7th grade, learning about slope (y = mx + b). A question in his math book was, "How long will it take a plane to descend from 30,000 feet at 1500 ft/min?" I told him that the question could not be answered because (1) you don’t know how high the plane is, and (2) you don’t know where the ground is (MSL). (1) is easy and obvious, but for (2) I don’t have a copy of the FARs handy, so I need some cites… If you are above 18,000 MSL, you use 29.92" on your altimeter according to what reg? Thanks Sandy
Thanks to you all (and to those who sent me private e-mail, as requested) for spending the time to read my post. The correct answer to the only question which was raised, i.e., "What is the cite to the FAR?", was provided by James Knox. The answer being Section 91.121. I was quite surprised by the number of people who felt the need to answer the question raised by the math problem — 20 minutes. I was somewhat less surprised by the number of responses which I got for even raising the question… Yes, I agree that the intent of the problem was to have the student divide 30,000 ft by 1500 ft/min to obtain 20 minutes. Unmentioned in my posting was the fact that the teacher had already made it a point to "teach" the class that all aircraft altitudes are always measured AGL. While that was true on the radar altimeter in my Piper Lance, it ain’t true in general. So to all, I say, "Thank you" for your time. To Jim Knox, a special "Thanks." To those who I may have (privately) responded to in a manner found to be objectionable (by pointing out that they were answering a question which was not raised and not answering the one which was raised), I apologize; and in general, I suggest that since this particular news group is intended to be used by people with knowledge of IFR, I think that the questions raised here should be responded to in that manner. By the way, I just saw a photo of the crashed "Cessna 117" in a Time Magazine cover article. If those of us with some knowledge of aviation don’t make any effort to educate those who are writing about aviation, and otherwise teaching others, who will? Remember, often history is little more than lies recorded — Bryant Gumble, NBC Today Show. Sandy
Response:
As everyone has said, the authors of the math textbook almost certainly did not want the problem to include the pressure/altitude issues. They just want to see if he can conceptually relate a simple equation to a solution space and plot a straight-line curve. Once you’re sure your son really really understands that, and the homework is done, and if he seems interested, maybe you could make up some extra-credit work with him. Don’t do it unless he thinks it’s fun. But sometimes word problems are more fun if they’re more real, and teaching him how to make one up might teach him how to do them better. Also bear in mind that a common trick of word problems is to include irrelevant information, and the general trick of doing word problems is to figure out what the person writing the question was really trying to ask. But make sure he understands y=mx+b before bringing up any other jazz. Personally, I hated math, and was consistently terrible at it.
Response:
: My 12 year old son is in 7th grade, learning about slope (y = mx + b). : A question in his math book was, "How long will it take a plane to : descend from 30,000 feet at 1500 ft/min?" I told him that the question : could not be answered because (1) you don’t know how high the plane is, : and (2) you don’t know where the ground is (MSL). : (1) is easy and obvious, but for (2) I don’t have a copy of the FARs : handy, so I need some cites… If you are above 18,000 MSL, you use : 29.92" on your altimeter according to what reg? SANDY! What are you doing!?! Your confusing your son about stupid airplane issues when you should be helping him with his math! I’m sure the authors of his math book had no concept of MSL vs. AGL and standard pressure, etc. Just help him with his damn math! – Jay _/ Jay Masino and Teresa Larson `67 Piper Cherokee _/ _/ digital H/W design Database Design N4269J _/ _/ systems engineering Sybase and Oracle RDBMS __!__ _/ _/ UNIX and network admin UNIX and VAX _____(_)_____ _/ _/ C programming C programming ! ! ! _/ _/ Visit our homepage… http://www2.ari.net:80/home3/jmasino/ _/
Response:
: My 12 year old son is in 7th grade, learning about slope (y = mx + b). : A question in his math book was, "How long will it take a plane to : descend from 30,000 feet at 1500 ft/min?" I told him that the question : could not be answered because (1) you don’t know how high the plane is, : and (2) you don’t know where the ground is (MSL). … Poor kid… -Ade
Response:
My 12 year old son is in 7th grade, learning about slope (y = mx + b). A question in his math book was, "How long will it take a plane to descend from 30,000 feet at 1500 ft/min?" I told him that the question could not be answered because (1) you don’t know how high the plane is, and (2) you don’t know where the ground is (MSL).
With all due respect, I think #1 is a really stupid answer. The plane is at 30,000 feet. Those of us who make a study of flying might ask "Is that MSL, AGL, or PA?", but the point of the exercise is to learn math, not to argue FARs. The kid is trying to learn a fundamental mathematical concept, and the question applies that concept to a real-life problem. To clutter up the answer with trivial details such as the difference between MSL and Pressure Altitude is absurd. The correct answer, in the context of a 12 year old kid learning how y=mx+b applies to the real world, is "30000 ft / (1500 ft/min) = 20 min". You could even take advantage of the problem to work in a short discussion of dimensional analysis (i.e. why "ft/(ft/min)" gives you units of "min"), which will give him a head start for when he takes physics in a couple of years. If you want to suggest that the landing airport might not be at 0 altitude, that’s fine, you now have a non-zero "b" term in the equation; "How long will it take a plane to descend from 30,000 feet to Denver International Airport, which is at an altitude of 5000 feet, at 1500 ft/min?" is a reasonable question. Wait until he’s taking his PP-ASEL ground school to introduce the concept of altimeter settings. — Hippocrates Project, Department of Microbiology, Coles 202 NYU School of Medicine, 550 First Avenue, New York, NY 10016 "This never happened to Bart Simpson."
Response:
: My 12 year old son is in 7th grade, learning about slope (y = mx + b). : A question in his math book was, "How long will it take a plane to : descend from 30,000 feet at 1500 ft/min?" I told him that the question : could not be answered because (1) you don’t know how high the plane is, : and (2) you don’t know where the ground is (MSL). come on! this is a seven grade math book! they assume the ground is 0 M.S.L and that 30,000 feet is in M.S.L also. lets not drive the kids nuts, just cause we want to be cool as pilots! ;-) Certified Flight Instructor – Airplane, Instrument Ground Instructor – Advanced, Instrument Commercial Pilot – Single Engine Land, Instrument what good is it to fly … eh, a quoi bon l’enfant qui vient de naitre?
Response:
My 12 year old son is in 7th grade, learning about slope (y = mx + b). A question in his math book was, "How long will it take a plane to descend from 30,000 feet at 1500 ft/min?" I told him that the question could not be answered because (1) you don’t know how high the plane is, and (2) you don’t know where the ground is (MSL). (1) is easy and obvious, but for (2) I don’t have a copy of the FARs handy, so I need some cites… If you are above 18,000 MSL, you use 29.92" on your altimeter according to what reg?
You could answer it if you assume the author of the problem is a non- pilot. Most difficulties in working story problems come from making the problem more difficult than it is by introducing "real-world" factors such as the already mentioned pressure vs real altitude dichotomy and the elevation of ground relative to sea level. No 7th grader is expected to know these things so there’s no reason why they should be factored into the solution. It’s like the classic case of a physics story problem which sets a box of given dimension and weight into the back of a moving vehicle (pickup truck in the one I’ve seen), moves the vehicle at a given speed and then stops it and asks at which speed the vehicle had to be moving to make the box tip over. Asking whether the bed of the truck was rusty and therefore flexible thus affecting the answer was neither expected nor desired… 8^) — Mike My opinions, not Argonne’s…
Response:
My 12 year old son is in 7th grade, learning about slope (y = mx + b). A question in his math book was, "How long will it take a plane to descend from 30,000 feet at 1500 ft/min?" I told him that the question could not be answered because (1) you don’t know how high the plane is, and (2) you don’t know where the ground is (MSL). (1) is easy and obvious, but for (2) I don’t have a copy of the FARs handy, so I need some cites… If you are above 18,000 MSL, you use 29.92" on your altimeter according to what reg? Thanks Sandy
actually, sandy, the problem doesn’t state whether the 30,000 feet is pressure altitude, msl, or agl. you are making the _assumption_ that it is p. a. based on your piloting knowledge. however, if you switch to the _assumption_ that it is agl, the problem is easily solvable (soluble?). that is the assumption _i_ would make, given the problem, supported by two further assumptions: 1) the writers of the textbook would probably not give a seventh grader a ‘trick’ unsolvable problem, and 2) the writers of the textbooks are most likely non-pilots. (even if they were pilots, they were writing to assumed non-pilots, seventh graders. if they added ‘agl’ to the end of the 30,000 feet without explanation, it would confuse more people than it would to leave it off, and the explanation is beyond the scope of the lesson — they are teaching math, not piloting). put the problem in the proper context. mho, john — ‘this place is full of yahoos’ — jonathon swift
Response:
My 12 year old son is in 7th grade, learning about slope (y = mx + b). A question in his math book was, "How long will it take a plane to descend from 30,000 feet at 1500 ft/min?" I told him that the question could not be answered because (1) you don’t know how high the plane is, and (2) you don’t know where the ground is (MSL). (1) is easy and obvious, but for (2) I don’t have a copy of the FARs handy, so I need some cites… If you are above 18,000 MSL, you use 29.92" on your altimeter according to what reg? Thanks Sandy
Response:
My 12 year old son is in 7th grade, learning about slope (y = mx + b). A question in his math book was, "How long will it take a plane to descend from 30,000 feet at 1500 ft/min?" I told him that the question could not be answered because (1) you don’t know how high the plane is, and (2) you don’t know where the ground is (MSL).
{:) I suspect you already know a **LOT** more about it than did the person who wrote the problem, which means that they were probably just looking for "20 minutes." As you point out, the first BIG question is "descent to what?" 20 minutes of descent over Denver CO will produce a lot of noise during the last couple of minutes! <G The question you raise about 29.92 is correct, but we have to assume it is not relevant. Below 18,000 the altimeter is set to a correction pressure so that it will indicate MSL (altitude relative to mean sea level). Obviously all it *really* reads is atmospheric pressure. As you fly along the local pressure changes and you keep getting (and setting) new correction numbers (in the Kollsman window). Above 18000 the planes are assumed flying so fast and far that the corrections would come too fast to be practical. Accuracy is not important relative to the ground (no ground up that high <G), but relative to the other aircraft accuracy *is* important. So everyone sets their altimeter to 29.92 and reads altitude based on the same "assumed" pressure. [And hopefully remembers to set to local when descending below 18,000.] The point is, I doubt whoever wrote the book knew about this. A plane above 18,000 *indicating* 30,000 might be at 30,000 — or might not. So we have to assume they mean the plane is REALLY at 30,000 feet MSL, no matter what is indicated. Some typical city elevations in the US: Austin, TX AUS 632 feet Dallas-Ft Worth DFW 603 feet Denver CO DEN 5431 feet Houston Int IAH 98 feet Los Angeles LAX 126 feet Cheyanne WY CYS 6156 feet James Knox
