Understanding Tower Light NOTAMs And Altitude Discrepancies

Hey guys! Ever scratched your head over a NOTAM that lists an obstacle altitude different from what you see on your sectional chart? You're not alone! Let's break down a common scenario involving tower light NOTAMs and altitude discrepancies. We'll dive deep into why these differences occur and how to interpret them correctly for safe flight planning.

Understanding the NOTAM

Let's look at a sample NOTAM. This NOTAM reports a tower obstruction with a light out of service:

!BDR 06/104 PUT OBST TOWER LGT (ASR UNKNOWN) 421735.59N0713401.40W
(10.3NM SSW 6B6) 711FT (202FT AGL) U/S 202506130715-202507280400

This NOTAM tells us a few crucial things:

• !BDR 06/104: This is the NOTAM identifier, indicating the airport or facility affected (BDR) and the NOTAM number.
• PUT OBST TOWER LGT: This signifies that a Obstruction Tower Light is Put Out of Service.
• (ASR UNKNOWN): This means the FAA's Obstruction database Automated Structure Reporting status is unknown for this particular obstacle.
• 421735.59N0713401.40W: These are the geographic coordinates (latitude and longitude) of the tower.
• (10.3NM SSW 6B6): This indicates the tower's location relative to the 6B6 identifier, in this case, 10.3 nautical miles South-Southwest.
• 711FT (202FT AGL): Here's where the potential confusion kicks in! This tells us the tower's height is 711 feet Mean Sea Level (MSL) and 202 feet Above Ground Level (AGL).
• U/S: This is short for inoperative or out of service, signaling that the tower light is not functioning.
• 202506130715-202507280400: This is the effective period for the NOTAM, indicating the start and end dates and times (in UTC) when the light is expected to be out of service. In this case, the light is out from June 13, 2025, at 07:15 UTC to July 28, 2025, at 04:00 UTC.

The core of the issue is the altitude listing: 711 feet MSL versus 202 feet AGL. Why do we see these two different numbers, and why might they not match what's on your sectional chart? This discrepancy is the heart of our discussion, so let's explore the reasons behind it.

Deciphering Altitude Discrepancies

So, why the difference between the MSL altitude listed in the NOTAM (711 feet) and what you might see depicted on a sectional chart? This often boils down to the datum used for measurement and the specific purpose of the information.

Understanding MSL and AGL

Before diving deeper, let's solidify the basics. Mean Sea Level (MSL) is the average altitude of the sea surface and serves as a standard reference point for altitudes depicted on aeronautical charts and used in aviation. It’s essentially the “zero” point for aviation altitude. On the other hand, Above Ground Level (AGL) is the height of an object or aircraft above the terrain directly beneath it. Think of AGL as the relative height, while MSL is the absolute height relative to sea level.

Datum Differences

The key reason for discrepancies is often the difference in datums. A datum is a reference point or surface against which position measurements are made. Sectional charts are typically based on a specific geodetic datum, such as the North American Datum of 1983 (NAD83), to provide horizontal positioning. The vertical datum, used for altitude, is typically the North American Vertical Datum of 1988 (NAVD88). However, the altitude of the terrain itself can have localized variations and might not perfectly align with the datum used for the tower's MSL altitude.

Essentially, the MSL altitude in the NOTAM is the actual height of the top of the tower above Mean Sea Level, as precisely surveyed and recorded. The AGL height is the height of the tower above the ground immediately surrounding its base. Sectional charts show terrain elevation, which is an approximation based on survey data and may not perfectly reflect the exact ground level at the tower's base. That’s why the AGL provided in the NOTAM is crucial, as it gives the pilot a real-world height reference.

Here's the thing guys: Sectionals are fantastic for overall situational awareness, but they're generalized representations of terrain. They can't capture every single undulation and variation. Think of it like a zoomed-out map – you get the big picture, but not every tiny detail.

The Role of Surveying

Towers are often surveyed with high precision when they're erected. This precise survey determines the tower's MSL altitude, which is then reported in databases and NOTAMs. The sectional chart, on the other hand, relies on terrain data that might have been collected using different methods or at a different time. It's a little like comparing a laser-measured distance to a distance estimated from a map – the laser measurement will likely be more accurate.

Obstruction Standards and Terrain Variations

Another factor is the FAA’s obstruction standards. These standards dictate when an object needs to be marked and lighted as an obstruction to air navigation. The AGL height is crucial in determining whether an object penetrates a specific obstruction clearance surface. If a tower is 200 feet AGL, it may need to be marked and lighted, regardless of its MSL altitude. This AGL height dictates the tower's visibility to pilots, which is a primary safety consideration.

Terrain variations also play a role. A tower situated on a hill will have a higher MSL altitude than the surrounding terrain, but its AGL height might be relatively moderate. Conversely, a tower in a valley might have a lower MSL altitude but a significant AGL height due to the surrounding terrain rising sharply. Sectional charts try to depict these terrain variations, but as mentioned, they're a generalized representation.

Obstruction Databases

The FAA maintains databases of obstructions to air navigation, and these databases are used to generate NOTAMs and update sectional charts. However, there can be a time lag between when an obstruction is surveyed and when it's reflected on a sectional chart. This is where NOTAMs become critical – they provide the most up-to-date information about obstructions, including changes in their status (like a light being out of service) and any discrepancies in altitude data. The NOTAM supplements the chart and gives a current operational perspective.

In summary, the MSL altitude in a NOTAM is a precise measurement, while the terrain depicted on a sectional chart is a generalized representation. The AGL height provides the crucial information about the tower's height relative to the surrounding terrain, which is vital for pilots. This brings us to the critical question of how to effectively use this information in flight planning and decision-making.

Practical Applications for Pilots

Alright, so we've established why these altitude discrepancies exist. Now, let's talk about how you, as a pilot, can use this information to make safe and informed decisions. It's not enough to just know the difference; you need to apply that knowledge.

Cross-Referencing Information

The first step is always to cross-reference your information. Don't rely solely on one source. Compare the NOTAM data with your sectional chart and any other available resources, such as airport/facility directories (AF/Ds) or electronic flight bag (EFB) applications. Look for consistency. If you spot a discrepancy, dig deeper. Is the NOTAM newer than the chart's publication date? Is there a known issue with the terrain data in the area?

For instance, if the sectional shows a terrain elevation of 500 feet MSL near the tower, but the NOTAM lists the tower at 711 feet MSL (202 feet AGL), you know the tower extends significantly above the surrounding terrain. This is crucial for obstacle clearance, especially in low-visibility conditions or at night.

Understanding Obstacle Clearance Requirements

Familiarize yourselves, guys, with obstacle clearance requirements for different phases of flight. Regulations dictate minimum altitudes for enroute flight, approaches, and circling maneuvers. Understanding these requirements is paramount for safety. The AGL height in the NOTAM is your key metric for these calculations. If you're flying a night flight or in instrument meteorological conditions (IMC), the absence of a tower light significantly increases the risk. The NOTAM's information about the light's status (U/S) is directly tied to your decision-making process.

Adjusting Flight Planning and Routes

Let's consider a scenario: You're planning a low-level night flight near the tower described in our NOTAM. The light is out of service. The 202-foot AGL height is now a critical factor. You might need to adjust your planned altitude or route to ensure adequate clearance. This might involve flying at a higher MSL altitude or deviating from your direct course to avoid the tower altogether. Always remember: Safety first. It's better to add a few minutes to your flight time than to risk a collision.

Utilizing EFBs and GPS Navigation

Electronic Flight Bags (EFBs) and GPS navigation systems can be incredibly helpful, but they're not foolproof. These tools often display obstacle data, but that data is only as good as its source. Always verify the information displayed on your EFB with official sources like NOTAMs and sectional charts. Pay close attention to any alerts or warnings generated by your EFB regarding obstacles. Use the AGL height from the NOTAM to assess the risk accurately. It's best practice not to blindly trust the technology but to integrate the tech's information with your understanding and reasoning.

Pro Tip: Configure your EFB to display obstacle heights AGL. This makes it much easier to assess potential hazards, especially in areas with varying terrain.

Night Flight Considerations

Night flying introduces unique challenges. The absence of visual references makes obstacle identification much more difficult. A tower light that's out of service (as indicated in the NOTAM) poses a significant threat. During night flights, maintaining situational awareness is paramount, and thoroughly reviewing NOTAMs for obstacle lighting outages is an essential part of preflight preparation. Make sure your aircraft lighting is fully operational, and consider using supplemental lighting (like landing lights) when appropriate.

Communication with ATC

Don't hesitate to communicate with Air Traffic Control (ATC). If you have any concerns about obstacle clearance, request assistance from ATC. They can provide radar vectors, altitude assignments, and other services to help you avoid potential hazards. They also have access to real-time information that might not be available through other sources. Effective communication is a cornerstone of aviation safety.

Scenario-Based Training

Consider incorporating scenario-based training into your flight preparation. Simulate situations where you encounter altitude discrepancies in NOTAMs and have to make decisions about routing and altitude. This kind of practice helps you develop the critical thinking skills necessary to handle real-world situations effectively. The more you practice, the more instinctive safe decision-making becomes.

In short, guys, understanding and applying information from tower light NOTAMs is about more than just reading the numbers. It's about integrating that data into your overall flight plan, assessing risks, and making sound decisions. Cross-referencing information, understanding obstacle clearance requirements, adjusting your route, using technology wisely, considering night flying factors, communicating with ATC, and scenario-based training are all crucial components of safe flying practices.

Conclusion

Discrepancies between tower light NOTAMs and sectional chart altitudes can seem puzzling at first, but understanding the reasons behind them – differences in datums, surveying methods, and the purpose of MSL versus AGL heights – is key to safe flight planning. The MSL altitude provides a precise height above sea level, while the AGL height offers crucial information about the tower's height relative to the surrounding terrain. It is the AGL value that is most important to pilots in terms of obstacle clearance.

By cross-referencing information, understanding obstacle clearance requirements, adjusting flight plans, utilizing EFBs wisely, and being especially cautious during night flights, pilots can effectively use NOTAM data to mitigate risks. Remember, NOTAMs provide the most up-to-date information and should always be considered a primary source for flight planning. Stay safe out there, and happy flying!