The Mysteries Of Altimetry
There's more to understanding the significance of the information than reading needles and numbers.
At one a.m. local time on November 11, 1995, an MD-83 was making a VOR instrument approach to RWY 15 at Bradley Field (KBDL) in Windsor Locks, Connecticut, when it struck trees on a ridge line about 2.65 miles from the runway threshold. Before we continue, let’s review a few altimeter terms.
There are three types of “altimeter settings” and we’ll refer to them using ICAO terms based on the Morse “Q” codes:
- “QNH” is the altimeter setting that we get from ATIS and in METARs.
- “QNE” refers to altimeter setting of 29.92 IN of Hg above 18,000 feet MSL (in the US) for flight levels rather than altitudes.
- “QFE” is an altimeter setting that allows the altimeter to read zero feet on the airport surface.
Today, typically only gliders set the altimeter to zero feet AGL on the ground. Previously it was common for airlines (including the one involved in the Bradley incident) to have one of two altimeters set to QNH and the other to QFE. With QFE, most ILS CAT I approaches would indicate 200 feet at DA/H.
It was a dark night at Bradley; winds were gusty with heavy rain. The relevant weather issue in this incident was the atmospheric pressure that was falling rapidly. The flight was using the altimeter with the QFE setting and it was set 76 feet too high—meaning that the airplane was actually 76 feet lower when it read zero feet.
The NTSB report cited descending below the MDA (actually the MDH) without adequate visual runway references as the probable cause. A contributing factor was the “failure of the BDL approach controller to furnish the flight crew with a current altimeter setting and the flight crew’s failure to ask for a more current setting.”
Even though using the QFE altimeter did not directly result in the accident, the airline involved abandoned the practice of using an altimeter set to QNH and another one set to QFE. Only QNH would be used below 18,000 feet MSL.
The MD-83 successfully landed at Bradley but suffered extensive damage. One of the 72 passengers received minor injuries during deplaning; none of the crew members were injured. (The VOR approach is no longer available at Bradley).
Value Of The Altimeter
When in IMC, out of the six-pack instruments or its equivalent in glass, the altimeter is perhaps the most important one followed closely by the attitude indicator. The altimeter provides vertical clearance from the ground when flying at charted and assigned altitudes and horizontal clearance with other airplanes.
Also, single-engine airplanes typically don’t have a backup for an altimeter unless the GPS-derived altitude is used, which is not accurate. In other countries, usually two altimeters are required for IFR.
Luckily, altimeter failures are not common; the most likely failure mode is a clogged static port. Some airplanes have two static ports, one on each side of the fuselage to equalize the static pressure during uncoordinated flight. Most planes also have an alternate static source vented to the cabin or to an unpressurized part of the airplane. (Not to be confused with an alternate air source for the engine intake.)
Aneroid (pressure) altimeters are essentially barometers. It is an indirect measurement instrument since it senses atmospheric pressure but provides the units in feet rather than pressure.
The case is vented to the “outside” through the static port; as pressure in the case changes, the aneroid wafers expand or contract, and through a series of gears, moves the pointers on the face. The primary factor impacting the accuracy of an altimeter is atmospheric pressure. It was not until 1928 that Paul Kollsman developed the “sensitive” altimeter, which could be adjusted to reflect atmospheric pressure—what we call “altimeter setting.” Today we honor him with the Kollsman window found on all altimeters.
For IFR operations, the well-known FAR 91.411 applies: “Within the preceding 24 calendar months, each static pressure system, each altimeter instrument, and each automatic pressure altitude reporting system has been tested and inspected and found to comply with appendices E and F of part 43 of this chapter;” (Note Appendix E of Part 43 goes in great detail of how to do the inspection by an approved shop).
Of course, the inspection and results need to be documented in the airplane’s logbook. Since our instrument student days, we know that when on the ground we set the current altimeter setting in the Kollsman window: it should be plus-or-minus 75 feet from the field elevation. If it is, we are good to go, but we don’t make any correction to the altimeter setting. If it isn’t, then are we grounded for IFR.
The 75-foot discrepancy is not in the regulations, it is in AIM 7-2-3. In case of a mishap involving an altimeter, let’s not forget one of the catch all regulations: FAR 91.7: the need to “operate a civil aircraft unless it is in an airworthy condition.” If the altimeter is outside the acceptable 75-foot range, this might be interpreted as not being airworthy.
To test if we are complying with the 75-foot tolerance, we need to know the airport elevation. Of course, we can find the elevation on the airport diagram or on an approach chart. The airport elevation is the highest point of an airport’s usable runways. But be careful. A few years ago, runways 10R and 28L of Fort Lauderdale/ Hollywood International (KFLL) were extended and, due to space limitations, runway 28L threshold area goes over highway US 1; cars now pass beneath the runway. An upslope of the runway was required. Suddenly, the airport elevation went from about 8 feet to 65 feet. Well, 65 feet is the elevation of the threshold area of runway 28L.
Unless you are at the threshold of runway 28L, use the elevation of about eight feet as reference for the altimeter check: -67 to 83 feet. Be sure you have a good idea of the correct elevation on the airport where you are making the check.
Types Of Altitudes
What adds to the complexity of altimetry are different “types” of altitudes:
Indicated Altitude is altitude shown on the altimeter using the local altimeter setting.
True Altitude is the actual height above MSL. Elevations of airports, obstructions, mountain tops are true altitudes.
Pressure Altitude is what the altimeter shows when set to 29.92 inches of Hg. It defines flight levels and is used in the US when flying above 18,000 feet MSL. It is also an input in determining density altitude and true airspeed.
Density Altitude is pressure altitude corrected for non-standard temperature and it is the altitude the airplane “feels”—engines and airfoils. It also determines performance such as take-off distance and climb rate. It also corrects indicated airspeed taking in consideration temperature and pressure altitude to compute true airspeed.
Absolute Altitude is really a height, an AGL value. Examples are: Height Above Touchdown (HAT) and Threshold Crossing Altitude (TCH).
Altimeters have several errors but we will focus on two potential errors: non-standard pressure and temperatures. When we fly below 18,000 feet MSL, we fly using indicated altitudes. The safety problem arises when flying from an area of higher pressure to an area of lower pressure without adjusting the altimeter setting. The well-known “high to low, hot to cold, watch out below” applies. FAR 91.121 states that below 18,000 feet MSL, the altimeter setting needs to be set to a station along the route “within 100 NM of the aircraft.” When pressure is changing rapidly upwards and downwards (for example when crossing a front), the altimeter should be reset more frequently and to the nearest reporting station.
When going to a lower-pressure area from a higher-pressure area without updating the altimeter setting, the indicated altitude will result in a lower true altitude. It could impact ground clearance and create opposing traffic conflict.
Part 2 (coming soon) will address the impact of altimetry during approaches to certain airports when the temperatures are extremely cold.
NTSB reference to BDL incident: DCA96MA008
Luca Bencini-Tibo ATP/CFII, is a FAASTeam Lead Rep, aircraft owner and is a graduate of MIT with an MBA from Harvard.
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