Beyond the MEA: getting as low as possible while remaining IFR can mean a non-stop, light or getting in on a visual. Asking ATC often is the key.
Here's the hard part: Getting and staying low when operating on an IFR clearance often is incompatible with what ATC wants. Many instrument-rated pilots are convinced they have little choice in the matter, but we do have some options for getting and staying low. Using one depends on where you are and where you're going. You don't always have to remain at or above the MEA.
WHY STAY LOW?
The reasons for flying IFR as low as you can are all over the map. They can include headwinds, icing at higher altitudes, an inoperative cabin heating system, lack of oxygen or some other operational consideration. It also can pay big dividends when trying to get into a destination under cloud cover. The trick, of course, is getting the lowest altitude at which we can retain our IFR clearance, for whatever reason. When approaching an airport, getting lower is better every time.
One reason is a lower altitude increases our chance of making the approach, which also likely will be more stable than when bombing in from a couple thousand feet high. Plus, once we spot the airport and can remain in visual contact, we can request clearance for a visual approach, allowing us to get even lower, terrain and obstructions permitting.
The contact approach is another reason to get low when approaching an airport. And depending on the reference you use, this procedure doesn't require ground contact. The FAA Pilot/Controller Glossary's contact approach definition states, in part, the pilot "may deviate from the instrument approach procedure and proceed to the destination airport by visual reference to the surface." Meanwhile, the Aeronautical Information Manual, at paragraph 5-4-25, notes pilots "operating in accordance with an IFR flight plan, provided they are clear of clouds and have at least 1 mile flight visibility and can reasonably expect to continue to the destination airport in those conditions, may request ATC authorization for a contact approach."
Both references agree the procedure is only available at airports reporting ground visibility of one statute mile or more and having a standard (published) or special instrument approach procedure. A contact approach, however, does work best when we can see the ground, since we're now responsible for terrain and obstruction avoidance.
But everyone's old favorite, the visual approach, might not be available at some of the altitudes we fly to the final approach fix while transitioning from en route to terminal environments: We might simply be too high to spot the airport, necessitating a full procedure. Depending on the weather and terrain, getting lower sooner might save us 10 or more minutes, plus the fuel required.
In terminal airspace, minimum altitudes generally are published values associated with an instrument approach procedure or arrival. As with all parameters of terminal procedures, they're determined by applying FAA Order 8260.3B, United States Standard for Terminal Instrument Procedures, commonly known as TERPS.
The TERPS is used to establish, among other values of a terminal procedure, the minimum altitudes at which routes and route segments may be flown. The TERPS "criteria indirectly addresses issues of flyability and efficient use of navaids, the major safety contribution is the provision of obstacle clearance standards. This facet of TERPS allows aeronautical navigation in instrument meteorological conditions (IMC) without fear of collision with unseen obstacles." Under TERPS, required obstacle clearance (ROC) typically mean minimum altitude "values are: for en route procedure segments, 1000 feet (2000 over designated mountainous terrain); and for initial segments, 1000 feet, 500 feet in intermediate segments, and 350/300/250 feet in final segments." You generally will not be cleared along any terminal route or route segment at an altitude lower than these values, with an exception we'll get to in a moment.
But what about the minimum safe altitude (MSA) published on approach charts? According to the PICG, there are two types. One is the minimum sector altitude, which provides at least 1000 feet of obstacle clearance within a 25-mile radius of the procedure's associated navigation facility or initial approach fix. It's an emergency altitude and does not assure navigational signal coverage. The other is the emergency safe altitude--also depicted on approach charts--which normally is used only in military procedures.
In many areas, however, choosing a low cruising altitude can pay big time. If the winds at your normal altitude are howlingly had and on the nose, you've got little to lose by staying low. In many cases over the U.S. eastern seaboard, published minimum en route altitudes (MEAs) below 3000 feet msl can be found. In the U.S. Midwest, 3000 feet is common and some minimum obstruction clearance altitudes (MOCAs) are lower still.
The MEA is defined in the P/CG as the "lowest published altitude between radio fixes which assures acceptable navigational signal coverage and meets obstacle clearance requirements between those fixes." In fact, since a Victor airway's MEA is determined, in part, on VOR reception (remember it?) as well as obstacles within four miles of its center, a lower altitude may be available.
Meanwhile, the MOCA is "the lowest published altitude in effect between radio fixes on VOR airways, off-airway routes, or route segments which meets obstacle clearance requirements for the entire route segment and which assures acceptable navigational signal coverage only within 25 statute (2 2 nautical) miles of a VOR."
Both the MEA and the MOCA--along with the minimum reception altitude (MRA) and minimum crossing altitude (MCA)--are predicated on VOR navigation along published routes, something not all that common anymore. Enter the MVA, or minimum vectoring altitude.
The MVA is defined as the "lowest MSL altitude at which an IFR aircraft will be vectored by a radar controller, except as otherwise authorized for radar approaches, departures, and missed approaches." The altitude meets IFR obstacle clearance criteria. It may be lower than the published MEA along an airway or J-route segment." In a radar environment, this is the lowest altitude you're likely to get.
It's used by ATC for radar vectoring only upon the controller's determination that an adequate radar return is being received. The biggest trick with the MVA is it isn't published: "Charts depicting minimum vectoring altitudes are normally available only to the controllers and not to pilots," according to the P/CG. But it's usually the lowest altitude you can get until receiving a clearance for a visual or contact approach. Even then, such a clearance may include an altitude restriction, especially if there's a traffic conflict.
The MVA isn't only for terminal airspace; it also may be available in the en route environment. When flying long distances, however, it may not pay to descend that far. By definition, the MVA depends on radar accuracy, which translates to distance from the antenna. Too, tall, widespread obstacles may generate many different MVAs over a route, leading to constant climbs and descents while en route. It wouldn't be our first choice, but we can see where it could be an operational necessity. We'd also suggest if you need the MVA in the en route environment, you likely have a whole 'nuther set of problems.
Yes, it's relatively easy to get below the published MEA in most parts of the U.S. airspace system, but it depends on how much lower you want to get and where you're going. Usually, trying for a lower altitude, to enhance the likelihood of spotting the airport, mitigate a headwind or for some other operational reason is only a radio call away, at least in the radar environment. When not in radar contact, ATC isn't likely to send you on your way at anything less than the MEA, unless you're in Class G airspace and then all bets are off anyway.
The question you really need to ask yourself, however, is if a lower altitude really is necessary. There certainly is such a thing as "too low," and these kinds of options only should be used when a clear, well-defined operational need exists.
RELATED ARTICLE: Defining Altitude
The value we read off the cockpit-mounted altimeter or our glass panel's rendition may or may not be the actual height of the aircraft above sea level. The Pilot's Handbook of Aeronautical Knowledge (PHAK), FAA-H-8083-25A, includes definitions for five different types of altitude:
The value read directly from the altimeter (uncorrected) when it is set to the current altimeter setting. This value is subject to instrument error.
The vertical distance of the aircraft above sea level--its actual altitude. Airport, terrain and obstacle elevations on aeronautical charts are true altitudes.
The vertical distance of an aircraft above the terrain, or above ground level (AGL).
The altitude indicated when the altimeter setting window (barometric scale) is adjusted to 29.92 in. Hg. This is the altitude above the standard datum plane, which is a theoretical plane where air pressure (corrected to 15 deg.
C) equals 29.92 in. Hg. Pressure altitude is used to compute density altitude, true altitude, true airspeed (TAS) and other performance data.
Pressure altitude corrected for variations from standard temperature. When conditions are standard, pressure altitude and density altitude are the same. If the temperature is above standard, the density altitude is higher than pressure altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. This is an important altitude because it is directly related to the aircraft's performance.
RELATED ARTICLE: So, Do You Really Want to Get Low?
What if you're not on an airway, terminal procedure's published route segment or in radar contact to obtain a clearance down to the MVA? Is there some other altitude you can use to get lower? Turns out there is, but getting cleared to it is the challenge. It's found in FAR 91.177, Minimum altitudes for (FR operations. It's paraphrased below.
* If no applicable minimum altitude (MEA/MOCA/published termi-nal procedure route segment) is prescribed in FAR Parts 95 (IFR Altitudes) and 97 (Standard Instrument Procedures), then the lowest IFR altitude is:
* 2000 feet above the highest obstacle within a horizontal distance of four nm from the course to be flown when in a designated moun-tainous area, or
* when not in a designated mountainous area, 1000 feet above the highest obstacle within a horizontal distance of four nautical miles from the course to be flown.
Good luck 'getting.: cleared to this altitude. Your friendly local ATC facility has neither the nor incentive to determine what this altitude would be for your route of flight. You Certainly. could pull out the necessary charts and plot your route, noting the proximity of terrain and published obstacles, and request it, but you probably never will be cleared anywhere at that altitude. Instead, it exists. mainly as a relic and as a rule to be employed when IFR in Class G (i.e., uncontrilled) airspace. In other words, you're clearly on your own in determining what this altitude is for any given route or position along it.
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|Title Annotation:||INSTRUMENT FLIGHT; minimum en route altitudes; air traffic controllers; instrument flight rules|
|Author:||Burnside, Joseph E.|
|Date:||Jun 1, 2012|
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