Is a stand-alone magnetic compass required for an IFR fight when the aircraft has a glass panel with a Horizontal Situation Indicator?
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$\begingroup$ possible duplicate of Is a magnetic compass required for VFR? $\endgroup$– rbpJan 12, 2015 at 6:02
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2$\begingroup$ @rbp Um, not to be rude but... Unless IFR and VFR became the same thing, these are not the same question. $\endgroup$– Jae CarrJan 12, 2015 at 7:40
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2$\begingroup$ @Jay rbp is correct, the answer to the other question anwers this question though, as the text further down states that all items required for VFR flights are also required for IFR flights. $\endgroup$– SentryRavenJan 12, 2015 at 8:37
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1$\begingroup$ @SentryRaven Ah, fair enough. I guess I'm more looking at it from a "search" perspective. People looking for an answer to an IFR question aren't likely to look at a VFR question, that's all I'm saying. $\endgroup$– Jae CarrJan 12, 2015 at 8:58
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1$\begingroup$ I think it's worthwhile leaving this as a standalone question: Although the other includes mention of IFR flight as part of the context of the answer, it is still a better fit for the SE Q&A format to keep this as a separate question, unless we also modify the Question of the other post to better fit a generic IFR/VFR answer. $\endgroup$– Jon StoryJan 12, 2015 at 9:48
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2 Answers
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A magnetic heading indicator is required, but not necessarily a wet compass, like this one:
Image Source: Wikipedia - Author: Chopper
Digital devices or combined devices are also possible:
Image Source: WikiPedia - Author: Mysid
The required equipment for VFR flights states, as already posted in this question:
§91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.
(a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.
(b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:
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(3) Magnetic direction indicator.
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The text further states for IFR:
(d) Instrument flight rules. For IFR flight, the following instruments and equipment are required:
(1) Instruments and equipment specified in paragraph (b) of this section, and, for night flight, instruments and equipment specified in paragraph (c) of this section.
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$\begingroup$ Note that as @rbp points out the magnetic direction indicator is not required to be a separate piece of hardware (such as a wet compass). My understanding is that for glass panels it's usually integrated into the HSI by slaving that to a magnetometer (which has the benefit of eliminating the need to correct the DG/HSI for gyroscopic precession as well). $\endgroup$– voretaq7Jan 12, 2015 at 16:28
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$\begingroup$ @voretaq7 I edited my answer to incorporate that. $\endgroup$ Jan 12, 2015 at 18:24
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$\begingroup$ The device depicted is not a magnetic direction indicator. It is a HSI containing a heading indictor which works on gyroscopes. $\endgroup$ Jan 13, 2015 at 15:50
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$\begingroup$ @PaulOgilvie, but it has no correction knob, so it must be slaved to a magnetic device. A heading indicator would be this: en.wikipedia.org/wiki/File:C172_heading_indicator.jpg $\endgroup$ Jan 13, 2015 at 16:58
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$\begingroup$ @SentryRaven, how would the deviation be set? (Difference between magnetic and true North.) The pilot will want the true North heading to be displayed on the device in front of him. $\endgroup$ Jan 14, 2015 at 10:41
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If you mean a "whiskey compass," like the thing that sits above the center of the panel, then the answer is no.
For aircraft certified under Part 23, as described in AC 23.1311-1C:
Section 23.1303(c), Amendment 23-62, amended the requirement from “A direction indicator (non-stabilized magnetic compass)” to “A magnetic direction indicator.” As new technology becomes more affordable for part 23 airplanes, many electronic flight instrument systems will use magnetically stabilized direction indicators (or electric compass systems) to measure and indicate the airplane heading to provide better performance.
A fluxgate compass, which can be remotely mounted in the wing or the tail would also satisfy the regs.
Also, MEMS fluxgates are being developed right now, which could be integrated directly into an AHRS. And there are a number of other technologies being developed to sense the earth's magnetic field.
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$\begingroup$ I do wonder though, what do they do in case of an electrical failure? No mechanical backup required? $\endgroup$ Jan 12, 2015 at 15:22
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1$\begingroup$ This still effectively requires a magnetic compass ("magnetic direction indicator") - a device which uses magnetism to determine your current heading, it just doesn't have to be a traditional wet compass. You could not, for example, legally fly a glass panel aircraft which only had GPS and MEMS or laser gyros in order to determine and display heading. $\endgroup$– voretaq7Jan 12, 2015 at 16:20
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$\begingroup$ @voretaq7 but you could with a solid-state fluxgate compass $\endgroup$– rbpJan 12, 2015 at 17:40
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$\begingroup$ Yes - a fluxgate (or other electric magnetometer) is still a "magnetic direction indicator" :) $\endgroup$– voretaq7Jan 12, 2015 at 18:12
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$\begingroup$ Does the definition "magnetic direction indicator" allow the pick-up device to be separated from the display device? Or is one of the aspects of the requirement that, when everything else fails, there is still a reliable (magnetic) heading indicator available in the cockpit? $\endgroup$ Jan 13, 2015 at 15:47