A Southwest Airlines 737 MAX 8 purportedly came within 150 feet of the ground during its approach into Tampa Bay. But are we reading the altitude correctly or is this potentially a case of improper instrument settings and therefore an incorrect altitude reading?
Close Call? Southwest Airlines 737 Purportedly Drops To 150 Feet Above Ground Four Miles From Tampa Bay
I don’t usually write about “close calls” because they do not interest me as much as other aviation or travel issues and because, admittedly, some of the technical issues surrounding them are not clear to me. That’s the case here, so see this post as an attempt to understand these types of situations and this particular situation better.
The incident occurred on Southwest Airlines flight 425 on Sunday, July 14, 2024, a flight from Columbus (CMH) to Tampa (TPA). The flight was operated by a Boeing 737 MAX 8 (registration code N8841L).
One Mile At A Time does a great job of laying out what happened as a Southwest 737 MAX 8 approached Tampa (TPA) airport in Florida. I won’t regurgitate it. A visual depiction posted on YouTube is also well done:
The 150-foot reading is based on ADS-B data. ADS-B stands for “Automatic Dependent Surveillance-Broadcast,” an aviation surveillance technology in which an aircraft’s position is determined via satellite navigation or other sensors (and the ADS-B-equipped aircraft periodically broadcasts its position and other related data, enabling it to be tracked). That broadcast shares the GPS-tracked position of the aircraft to the ground, where it is displayed to air traffic controllers.
But is that 150-foot measure correct? Put another way, were the pilots really that clueless or disoriented or are there some instrument issues at play that may have led to such a low altitude reading?
A key part of the exchange:
ATC: “Southwest 425, low altitude alert, check your altitude. Tampa’s altimeter is 30.14.
Pilot: “30.14, thank you, Southwest 425.”
The 150-foot altitude reading comes from FlightRadar24, which explains that its displayed altitudes are based on the standard atmosphere setting (29.92).
ADS-B only reports altitude values based on the standard pressure of 1013 hectopascals; this is why altitude values near the ground can sometimes appear unrealistic.
Take Denver for example, the “mile-high” airport that sits at 5,433 feet above sea level. Using ADS-B, the altitude reading can shift from 5,400 feet on final approach to 0 feet upon landing…all within seconds.
Pressure is measured in hectoPascals (hPa), also called millibars. Standard pressure at sea level is defined as 1013hPa, but we see large areas of either high or low pressure. Areas of high and low pressure are caused by ascending and descending air. The lower the pressure, “The fewer air molecules surround your airplane. In fact, decreasing atmospheric pressure by one inch of Mercury (inches Hg) increases your pressure and density altitudes by 1,000 feet. Your airplane performs like it’s 1,000 feet higher than the field elevation.”
Ok, so what does that mean? If I understand this correctly, if the actual setting was 30.14, it seems that about 250 feet should be added, so the plane’s lowest point was actually 400 feet.
Arguably, whether it was 400 feet or 150 feet doesn’t matter…both seem far too low…but there’s a big difference between 400 feet and 150 feet…
The truth is, it is not clear to me what happened. Would a radio altimeter (which “uses a pulse of radio frequency energy transmitted towards the earth to measure the absolute height above the terrain immediately underneath the aircraft”) have also indicated a flight level of 150 feet?
Can someone help me (and everyone) understand this rather technical issue in layman’s terms?
image: Southwest Airlines
Yes there is a big difference between 400 and 150 at that distance didn’t realize this possibility
Not only were they that low. They missed a condo building by 200 feet. This is the 4th incident they have had being too low. FAA needs to step in now.