Altimeter

“ The results of the various phases of laboratory research showed that the three-pointer altimeter consistently ranked poorest. The pilot preference in the flight tests was overwhelmingly in favor of one of the other types of altimeter. In all objective measurements taken, the three-pointer altimeter ranked the poorest among the instruments tested. The finding that the three-pointer was poorest proved to be statistically reliable in every study in which tests of statistical significance were applied. ”

“ Furthermore, all altimeter settings given the crew were within one or two hundreths of the 29.92 setting and it is possible the crew did not reset the altimeter because the maximum difference in indicated altitude would be approximately 30 feet which would not be significant during a VFR approach and landing, particularly with an ILS glide slope available for the approach. ”

“ It can be seen that in reading this altimeter the pilot must first look at the number below the index on the "drum" to identify the thousand-foot level, and then to the radial pointer to determine the hundred feet indication. Two separate readings are required to assess altitude. At stabilized altitudes, or at low rates of climb/descent where the drum is moving slowly or not at all, the pilot must be certain to associate the proper thousand feet indication in relationship to the hundred feet reading. ”

“ The two altimeters are served by separate static sources and there is no way in which the information on one altimeter can be displayed on the other. Any error that occurs in one system cannot be displayed on the other instrument thus providing a continuing cross-check capability for the crew. A review of the aircraft records revealed no history of altimeter malfunctions or problems. ”

“ Pressure decreases with increasing altitude. The altimeter is an aneroid barometer graduated in increments of altitude in the standard atmosphere instead of units of pressure. Temperature greatly affects the rate of pressure decrease with height; therefore, it influences altimeter readings. Temperature also determines the density of air at a given pressure (density altitude) . Density altitude is an index to aircraft performance. Always be alert for departures of pressure and temperature from normals and compensate for these abnormalities. ”

“ Station barometric pressure which when set in the altimeter allows for an altimeter reading of zero feet at the airport elevation. ↑ QNH – Local atmospheric pressure which when set in the altimeter allows for the appropriate mean sea level altitude reading at the airport elevation. ↑ The NASA VGH program is a continuing research project whereby recorded airspeed – acceleration – altitude data is collected for many different aircraft models and for different types of operation. ”

“ It is doubtful that the center (No. 3) altimeter would have been used on the approach as it would not have been indication absolute altitude over the airport as were the Nos. 1 and 2 altimeters.
It is reasonable to assume that the flight engineer would have been engaged in duties attendant to completing the landing checklist and would not have had time to devote his full attention to the monitoring of flight instruments during the descent along the base leg. ”

“ On Eastern aircraft barometric pressure of one altimeter is kept at sea level pressure which allows the pilot to fly at indicated altitudes above sea level. Prior to landing the barometric scale of the other altimeter is set to correspond with a duplicate instrument on the airport which is kept set to show zero elevation. This allows the pilot to land to a zero reading at that particular airport. ↑ Trip 19 had landed a few minutes prior to Trip 21's arrival over the Stone Mountain fan marker. ”

“ The next morning, at atmospheric speed, we settled down swiftly over the larger of the two continents, Correy giving orders to the navigating room while I divided my attention between the television disk and the altimeter, with a glance every few seconds at the surface temperature gauge. In unknown atmospheres, it is not difficult to run up a considerable surface temperature, and that is always uncomfortable and sometimes dangerous. ”

“ THE WITNESS: All crews are supposed to monitor the altimeter while the altitude is changing. If for some reason such as mechanical failure or just anomalies that the airplane fails to capture an altitude, it is expected that the first crew to notice this with call it out.
MR. BERMAN: Okay. I understand.
Referring to the approach chart for the ILS approach to runway 6-left executing the approach with the glide slope inoperative, what would the crew do if the outer marker were not received in this approach? ”

“ At the end of a fifteen-minute wait, basing his estimate on their former rate of ascension, when the altimeter was still registering accurately, Robert calculated that they had reached a distance of approximately 150 miles above the Earth. At this distance the atmosphere should be sufficiently thinned to eliminate it as a factor of interference with their course or danger of air friction. He could now safely utilize the disk’s full magnetic power. ”

“ All events leading up to impact, including the final radio transmissions from the flight, indicate that the crew was not concerned with and totally unaware of a dangerously low altitude situation. In examining the many potential ramifications involved in the aircraft's deviation from the desired altitude profile one obvious possibility is a misinterpretation or misreading of the information being presented on the altimeters. It is noted that the drum altimeter has been in operation for many years and is considered by all segments of the industry to be an accurate, highly reliable instrument. ”

“ He realized that he was falling. A glance at his altimeter confirmed the impression. The needle had dropped four hundred feet and was slowly moving toward sea-level.
ith an exclamation of alarm, Purdy advanced his throttle until the three motors of his plane roared at full capacity. For a moment his air-speed picked up, but the gain was only momentary. As he watched, the meter dropped to zero, although the propellers still whirled at top speed. His altimeter showed that he was gradually losing elevation. ”

“ He broke out of his reverie and tossed a glance at the altimeter. The needle was moving much faster now, climbing steadily toward seventy thousand feet.
"It's about time to go now, isn't it?" he asked Borion. ”

“ Beware of the low pressure-bad weather, high pressure-good weather rule of thumb. It frequently fails. Always get the complete weather picture. When flying from high pressure to low pressure at constant indicated altitude and without adjusting the altimeter, you are losing true altitude. ”

“ The procedure for either a missed-approach or a rejected landing was the same:
Set takeoff power. Rotate immediately to stop descent (minimum 10°) and simultaneously call flaps 15°. Continue as in normal takeoff. Do not raise gear until climb is established. The radio altimeter system was described in Southern Airways DC-9 Operating Manual in general terms, including the following, "Two separate radio altimeter systems on the (Dash 31) ... are provided to obtain precise altitude information above the ground at the minimum decision (sic) altitude (MDA) . ”

“ The knob also positions an index on the periphery of the instrument face which is used for setting field pressure altitude (QFE) . [12] The procedures for setting altimeters, as applicable in this case, are set forth in the AA Flight Manual as follows:
" (3) Descent Approach:
" (a) Aircraft descending through FL 180 (15,000' in Mexico) shall have the No. 1 and 2 altimeters change to appropriate QNH [13] settings. Once again, all three altimeters will be set to the same setting and the No. 1 altimeter becomes the master. ”

“ It would require the utmost skill and vigilance combined, thus to keep going over about the same line of travel, with naught to depend upon save his reliable instruments, aided by the deductions he must be continually making, with his eyes on the compass, the speed indicator, altimeter, and kindred apparatus by means of which, in conjunction, a clever pilot may cut circles around an objective at will. ”

“ The altimetre "begins at the ground"; it registers your height above the altitude from which you started. Now, since all ground is more or less irregular, you may be coming down on a spot lower—or, much worse, higher—than that from which you started. Besides, when a fog comes up the atmospheric pressure changes and, as the altimetre is a barometer, it becomes from that moment unregulated. Then, of course, on landing you may strike bad ground—houses or shrubbery or fences, all of which adds to the uncertainty and risk. ”

“ If, when you and the sun are between the equinoctial and the antarctic pole, the shadow falls towards the north, you will add the altitude you have taken to the sun's declination that day, and act according to the second rule. 5. When you have an altitude of 90 deg., you will be so many degrees distant from the equator as there are degrees of the sun's declination, and if the sun has no declination you will be under the equator. ”

“ The latitude was accurately determined every day by measuring the altitude of the sun as it passed the meridian. To ascertain the longitude was a more difficult matter. They were obliged to rely mainly on their dead reckoning; that is, to make a calculation of the course and distance run daily, from the points steered by the compass and the rate as indicated by the log-line and half-glass. A reckoning on such a basis, where unknown currents prevail, where a vessel is steered wildly, or where the rate of sailing may be inaccurately recorded, is liable to many errors ”

“ Besides, to observe the altitude on both sides of the meridian is just as expeditious as to observe on one side only and at the same time to find the elevation of the pole. And he who can take one altitude by the instrument can also take another; but if the one altitude be uncertain, then all the labour with the globe, numbers, sines and triangles is lost; nevertheless those exercises of ingenious mathematicians are to be commended. It is easy for anyone, if he stand on land, to learn the variation by accurate observations and suitable instruments, especially in a nearly upright sphere ”

“ In calculating a meridian altitude, an error in the dip directly affects the result by a corresponding amount, so extra care should be exercised in this respect. In this instance, we endeavor to locate the body relative to our zenith and anything that affects its altitude directly affects the latitude. In a time sight, a different principle is involved. Here the position of the body as defined by the latitude locates the apex of one angle of the astronomical triangle and hence a small error in the altitude will very likely cause a greater effect on the longitude. ”

“ The south part of the reef was thus ascertained to be in latitude 13° 35' S.; and by the sun's meridian altitude, brought forward by account, 13° 35'; and by the mean of the meridian altitudes of several stars observed between 6h 40' and 9 P.M., and carried back by account 13° 35' 25". And from the care thus bestowed, I think the latitude may be considered pretty correct. The longitude by chronometer, corrected by lunar observation, is 125° 35' 45". The reef trended to the N.E. as far as the eye could distinctly survey [4] . ”

“ The declination and the correction of the observed altitude are picked out of the Almanac and Bowditch tables in a somewhat different manner peculiar to each body.
It is found by many navigators to be more convenient to observe a body for meridian altitude by time than in waiting for the “dip.” The altitude is taken at exactly [Pg 81] local apparent noon in case of the sun and the time of meridian passage in the cases of other bodies. This expedient is especially desirable in observing stars, as the horizon is not as distinct and the “dip” not so easily detected as with the sun. ”

“ The altitude of a body at any selected time for an assumed position can be readily calculated. If this altitude does not agree (and it seldom does) with the altitude measured simultaneously with the sextant, corrected for the usual errors, the assumed position is not coincident with the actual position of the vessel. The navigator now proceeds to lay off from the assumed position, the line of azimuth of the body taken from the azimuth tables, Weir’s Azimuth Diagram, or determined by observation. ”

“ But they can scarcely have been able to take observations of this kind on board ship during their long voyages in early times, and they still less understood how to compute the latitude from such measurements except perhaps at the equinoxes and solstices. It is true that from the narrative, to be mentioned later, of a voyage in the north of Baffin’s Bay, about 1267, it appears that at sea also they attempted to get an idea of the sun’s altitude by observing where the shadow of the gunwale, on the side nearest the sun, fell on a man lying athwartships when the sun was in the south. ”

“ At sea level a column of air I square inch in cross-section, presses with a weight of 14.7 lbs., but at a height of 19,000 feet the weight of the column is only half as great. For the first few hundred feet above sea level the pressure decreases at the rate of 0.1 inch for each 90 feet of ascent; at an altitude of 3000 feet it is at the rate of 0.1 inch for 100 feet of ascent. The greater number of weather stations in the United States are 1000 feet or more above sea level, and many of those west of the Denver meridian are more than 5000 feet above sea level. ”