Maintain 3,000 Until Established: A Premature Descent on the ILS
You are giving instrument instruction in a Cessna 172 on an IFR flight plan. ATC issues: 'Cessna 123AB, 4 miles from ZILKE, turn right heading 310, maintain 3,000 until established on the localizer, cleared ILS runway 27 approach.' The published glideslope intercept altitude is 2,400 MSL. The minimum safe altitude in the area is 3,200 MSL, but the segment inside ZILKE is 2,400. The student turns to 310, sees the glideslope pointer move toward center, and immediately lowers the nose toward 2,400. The localizer needle is still three-quarters full scale left. You are the instructor; the aircraft is in VMC and the student is under the hood. The airport is 8 miles ahead, and terrain rises to 2,900 MSL south of the final approach course. The student says, 'I'm established-the glideslope is centered.'
1. What is wrong with the student's interpretation of 'established,' and what should you do immediately?
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The student has confused glideslope indication with localizer establishment. 'Maintain 3,000 until established on the localizer' means do not descend until the aircraft is on the localizer course and tracking it. A centered glideslope needle does not establish the aircraft on the localizer; at that position it may be a false or transient indication, and the localizer is still three-quarters full scale left. Descending to 2,400 while still well left of course puts the aircraft below the area minimum safe altitude and near terrain that rises to 2,900 MSL.
Immediately take control or direct a climb back to 3,000, level off, and continue the intercept. Aviate first, then navigate, then communicate. If the deviation was significant or ATC needs to know, advise the controller. Do not continue the approach until the localizer is within normal tracking limits and the aircraft is established inbound. Then descend to 2,400 and intercept the glideslope from below. In the debrief, have the student state the clearance in his own words: 'I am cleared for the ILS, but I must stay at 3,000 until the localizer is alive and I am established.' Set the altitude bug at 3,000 and use a verbal callout before any descent.
2. After you correct the altitude, ATC says 'cleared for the approach.' How does that change your clearance, and what minimums apply?
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A subsequent 'cleared for the approach' does not cancel the specific ILS clearance already issued. You remain cleared for the ILS runway 27 approach, and the earlier altitude restriction remains part of the clearance until you are established on the localizer. If the controller's phrase is ambiguous and you are not certain which approach is authorized, ask for clarification rather than assuming. The controller must specify the approach when more than one is available, but a generic 'cleared for the approach' following an ILS clearance is normally understood as continuing the same approach.
Once established on the localizer and glideslope, you may descend on the glideslope to the published decision altitude. Do not descend below DA unless the requirements of 14 CFR 91.175 are met, including the required visual references and the ability to make a normal landing. The published minima for the ILS apply, not the visual glideslope indication alone. The instructor should use this exchange to teach the student to read back the full clearance and to distinguish 'cleared ILS' from 'cleared for the approach.'
3. What training technique would you use on the next approach to prevent this error?
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Use a clearance-briefing and callout technique that forces the student to separate the lateral and vertical tasks. Before takeoff or before the approach, have him write the clearance in shorthand: 'HDG 310, 3,000 until LOC established, cleared ILS 27.' Have him set the altitude preselector or bug to 3,000 and point to the localizer needle when he says 'localizer alive.' Require the callout sequence: 'Localizer alive; correcting; established; descending to 2,400; glideslope alive.' Do not let him say 'established' until the localizer is within one-half scale and the aircraft is tracking inbound.
On the next approach, act as a realistic ATC and issue the clearance with a distraction. If he starts to descend early, prompt him with 'What does your clearance say?' rather than 'Don't descend.' The goal is for him to catch the error from the clearance, not from your command. Then repeat the approach with no prompting and evaluate whether he stays at 3,000 until established. This builds the habit of interpreting the clearance before responding to the glideslope.