14 CFR 91.129(e)(2)&(3) provide that you must remain at or above the electronic glideslope between the outer marker and the middle marker, and at or above the visual approach slope indicator, if available.
It isn't either/or. You are obligated to remain above each, according to the respective phase of the approach. If you're outside the middle marker, then remain above the electronic glide slope; if you can see the visual slope indicator, then you need to remain above that, too.
Once inside the middle marker, you are obligated at a minimum to remain above the visual indicator.
A good rule of thumb in such a case is to maintain the electronic glide slope until minimums, and then remain at or above the visual indicator beyond that. Ideally, you should remain at or above the highest of the visual or electronic glide slopes, but the wording of the regulation requires that you remain at or above the electronic glide slope until the middle marker (as applicable), and after that you need only remain at or above the visual slope indications. If the electronic indicator is higher than the visual indicator then stick with that.
I wouldn't destabilize your approach in order to climb or desced to intercept the other slope indicator, and you're always best off flying the electronic glide slope if you have it as a matter of standardization.
Currently this requirement only applies to large or turbine aircraft (when flying in class D airspace to runways served by an electronic glideslope), and this is an error on the part of those who drafted the current revision of the CFR. Formerly, this requirement applied to all aircraft, and the current requirement as ammended to apply only to large aircraft, was inadvertant.
The requirement to remain at or above the visual indicator applies to all aircraft, not only large and turbine powered equipment.
Note also that these requirements apply to operations in Class D airspace.
I have included two legal interpretations below, the first of which is partially applicable (albeit somewhat bizarre), covering this subject. The regulation references have changed due to changes in codification in the CFR, but the ruling itself is still valid. The second more directly addresses the issue and is also applicable.
14 CFR 91.129(e)(2)(&(3):
(e) Minimum Altitudes. When operating to an airport in Class D airspace, each pilot of -
(2) A large or turbine-powered airplane approaching to land on a runway served by an instrument landing system (ILS), if the airplane is ILS equipped, shall fly that airplane at an altitude at or above the glide slope between the outer marker (or point of interception of glide slope, if compliance with the applicable distance from clouds criteria requires interception closer in) and the middle marker; and
(3) An airplane approaching to land on a runway served by a visual approach slope indicator shall maintain an altitude at or above the glide slope until a lower altitude is necessary for a safe landing.
Paragraphs (e)(2) and (e)(3) of this section do not prohibit normal bracketing maneuvers above or below the glide slope that are conducted for the purpose of remaining on the glide slope.
Legal Interpretation:
August 24, 1977
Trevor C. Spencer, Esq.
Dear Mr. Spencer:
This is in reply to your letter requesting an interpretation of FAR Section 91.87(d)(3). We regret the delay in responding to your inquiry. You state that an aircraft in which you have a partnership interest was struck by a golf ball when 100 feet AGL and 700 feet from the threshold of runway 1L on making an approach to Buchanan Field, Concord, California. This runway is equipped with VASI lights. On page 1 of exhibit 3 of your letter, it is stated that the VASI glide slope is 140 feet AGL at the point of impact.
You ask specifically: (1) whether the lower altitude reference in Section 91.87(d)(3) is a matter of pilot discretionary judgment, provided no known person or property is endangered; (2) for an explanation of the pilots discretionary values with regard to "normal bracketing"; and (3) the intent of Section 91.87(d)(3) and the identity of the NPRM on which it is based.
Referring to your first question, the pilot does not have discretion to go below the glide slope, regardless of what may or may not be endangered by such lower flight path, until it is necessary to do so to make a safe landing. Section 60.18(b)(6)(ii) of the Civil Air Regulations from which Section 91.87(d)(3) was recodified in 19163, with no substantive change intended, was more explicit in locating the point where the pilot, at his discretion, may go below the glide slope. That regulation provided that fixed wing aircraft when approaching to land on a runway served by visual glide slope devices shall be flown so as to remain at or above the glide slope until arrival at the runway threshold. This change of wording appearing in Section 91.87(d)(3) was not intended to alter substantially that location.
Referring tot he second question, "normal bracketing maneuvers" means maneuvers which remain within the limits of the higher and lower glide slope signals; and the pilot who remains within the envelope formed by the higher and lower signals, which constitute the glide slope, will not be in violation of Section 91.87(d)(3).
As previously stated, Section 91.87(d)(3) was recodified from Section 60.18(b)(6)(ii) of the Civil Air Regulations. That section was based on an NPRM published in Draft Release 60-17, appearing in the FEDERAL REGISTER on October 14, 1960 (25 FR 9868). As stated in both the notice and the final rule, the purpose of that regulatory action was to enhance both the safety of airport flight operations and the abatement of the airport noise problem as it affected adjacent communities.
Since you consider the 1L approach to Buchanan Field to be a hazardous condition, we are advising Flight Standards Service of this incident for their review.
If we can be of any further service to you, do not hesitate to call on us.
Sincerely,
J.P. ZIMMERMAN
for RICHARD W. DANFORTH
Chief, Airspace, Air Traffic &
Environmental Quality Branch
Office of the Chief Counsel
Second Legal Interpretation:
February 18, 1975
Air Line Pilots Association
Gentlemen:
Your letter of January 30, 1975, to the Administrator very properly questions our delay in responding to Mr. Linnert's letter of December 16, 1974, requesting an interpretation of Section 91.87 of the Federal Aviation Regulations. Please accept my regrets and my assurance of full and rapid response to your future inquiries.
Your December 16th letter cites the provisions of Section 91.87(d) that require the pilot of a turbine powered airplane, in certain circumstances, to fly the airplane at or above the glide slope during portions of an ILS approach. Your letter then cites provisions of the flush paragraph following Section 91.87(d) that state that the regulation does not prohibit "normal bracketing maneuvers," above or below the glide slope, that are conducted for the purpose of remaining on the glide slope. The letter then requests our advice on the following question:
What is the minimum deviation below glide slope, as indicated on the pilot's glide slope indicator, which would constitute a violation of this FAR when operating between the outer and middle marker on an approach to an airport with an operating control tower?
The term "normal bracketing maneuvers," as it appears in Section 91.87(d), involves maneuvers conducted for the purpose of remaining within the higher and lower limits of the glide slope scale as displayed on the pilot's flight instruments, that is, within 150 microamperes of the center or null position of the glide slope. A pilot who remains within these limits, which constitute the glide slope "envelope," is not in violation of Section 91.87(d).
The principal purposes of Section 91.87 are to standardize flight procedures at controlled airports and, to the extent practicable, provide for the uniform application of rules that enhance both the safety of airport flight operations and the abatement of aircraft noise. The regulation is applicable to all aircraft operating to, from, or on an airport with an operating control tower. It is our judgment that turbine powered or large aircraft remaining within the above described "envelope" of the glide slope while approaching to land meet the objectives of the regulation. In answer to your specific question, concerning deviation below the glide slope, please be advised that any excursion below the lower limit of the glide slope scale as displayed on the pilot's flight instruments (that is, beyond full scale deflection in the direction indicating flight below the glide slope), constitutes a prima facie violation of Section 91.87(d). It is recognized that factors beyond the pilot's control, such as equipment malfunction or sever turbulence, must be considered to determine the particular application of the regulation in specific cases.
Thank you for your patience in this matter. I hope this answer provides the guidance that you require. If more information is needed, please let us know.
Sincerely,
(Signed) Gerard J. Turner
GERARD J. TURNER
Chief Counsel
